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# Location mobile phone tracker

An indoor location tracker map on a mobile phone
is a process for identifying the location of a mobile phone, whether stationary or moving. Localization may be effected by a number of technologies, such as using multilateration of radio signals between (several) cell towers of the network and the phone, or simply using GPS. To locate a mobile phone using multilateration of mobile radio signals, it must emit at least the idle signal to contact nearby antenna towers, but the process does not require an active call. The Global System for Mobile Communications (GSM) is based on the phone’s signal strength to nearby antenna masts.
Mobile positioning may be used for location-based services that disclose the actual coordinates of a mobile phone. Telecommunication companies use this to approximate the location of a mobile phone, and there by also its user.

## Contents

Technology
Network-based
Handset-based
SIM-based
Wi-Fi
Hybrid positioning system
Operational purpose
Consumer applications
Privacy
China
Europe
United States
Technology

Tracker a mobile phone can be determined in a number of ways.
Network-based of tracking

The location of a mobile phone can be determined using the service provider’s network infrastructure. The advantage of network-based techniques, from a service provider’s point of view, is that they can be implemented non-intrusively without affecting handsets. Network-based techniques were developed many years prior to the widespread availability of GPS on handsets. (See US 5519760, issued 21 May 1996 for one of the first works relating to this.

The technology of locating is based on measuring power levels and antenna patterns and uses the concept that a powered mobile phone always communicates wirelessly with one of the closest base stations, so knowledge of the location of the base station implies the cell phone is nearby.

Advanced systems determine the sector in which the mobile phone is located and roughly estimate also the distance to the base station. Further approximation can be done by interpolating signals between adjacent antenna towers. Qualified services may achieve a precision of down to 50 meters in urban areas where mobile traffic and density of antenna towers (base stations) is sufficiently high. Rural and desolate areas may see miles between base stations and therefore determine locations less precisely.

GSM localization uses multilateration to determine the location of GSM mobile phones, or dedicated trackers, usually with the intent to locate the user.

The accuracy of network-based techniques varies, with cell identification as the least accurate, due to differential signals transposing between towers, otherwise known as “bouncing signals”. Triangulation is moderately accurate, and the newer “advanced forward link trilateration” timing methods are the most accurate. The accuracy of network-based techniques depends on the concentration of cell base stations, with urban environments achieving the highest accuracy because of the high density of cell towers. Their accuracy also depends on the implementation of the most current timing methods.

One of the key challenges of network-based techniques is the requirement to work closely with the service provider, as it entails the installation of hardware and software within the operator’s infrastructure. Frequently the compulsion associated with a legislative framework, such as Enhanced 9-1-1, is required before a service provider will deploy a solution.

In December 2020, it emerged that the Israeli surveillance company Rayzone Group may have gained access, in 2018, to the SS7 signaling system via cellular network provider Sure Guernsey, thereby being able to track the location of any cellphone globally.

## Handset-based

The location of a mobile phone can be determined using client software installed on the handset. This technique determines the location of the handset by putting its location by cell identification, signal strengths of the home and neighboring cells, which is continuously sent to the carrier. In addition, if the handset is also equipped with GPS then significantly more precise location information can be then sent from the handset to the carrier.

Another approach is to use a fingerprinting-based technique,[8][9][10] where the “signature” of the home and neighboring cells signal strengths at different points in the area of interest is recorded by war-driving and matched in real-time to determine the handset location. This is usually performed independent from the carrier.

The key disadvantage of handset-based techniques, from service provider’s point of view, is the necessity of installing software on the handset. It requires the active cooperation of the mobile subscriber as well as software that must be able to handle the different operating systems of the handsets. Typically, smartphones, such as one based on Symbian, Windows Mobile, Windows Phone, BlackBerry OS, iOS, or Android, would be able to run such software, e.g. Google Maps.

One proposed work-around is the installation of embedded hardware or software on the handset by the manufacturers, e.g., Enhanced Observed Time Difference (E-OTD). This avenue has not made significant headway, due to the difficulty of convincing different manufacturers to cooperate on a common mechanism and to address the cost issue. Another difficulty would be to address the issue of foreign handsets that are roaming in the network.

## SIM-based

Using the subscriber identity module (SIM) in GSM and Universal Mobile Telecommunications System (UMTS) handsets, it is possible to obtain raw radio measurements from the handset. Available measurements include the serving Cell ID, round-trip time, and signal strength. The type of information obtained via the SIM can differ from that which is available from the handset. For example, it may not be possible to obtain any raw measurements from the handset directly, yet still obtain measurements via the SIM.

## Wi-Fi

Crowdsourced Wi-Fi data can also be used to identify a handset’s location. The poor performance of the GPS-based methods in indoor environment and the increasing popularity of Wi-Fi have encouraged companies to design new and feasible methods to carry out Wi-Fi-based indoor positioning. Most smartphones combine Global Navigation Satellite Systems (GNSS), such as GPS and GLONASS, with Wi-Fi positioning systems.

## Hybrid positioning system

Hybrid positioning systems use a combination of network-based and handset-based technologies for location determination. One example would be some modes of Assisted GPS, which can both use GPS and network information to compute the location. Both types of data are thus used by the telephone to make the location more accurate (i.e., A-GPS). Alternatively tracking with both systems can also occur by having the phone attain its GPS-location directly from the satellites, and then having the information sent via the network to the person that is trying to locate the telephone. Such systems include Google Maps, as well as, LTE’s OTDOA and E-CellID.

There are also hybrid positioning systems which combine several different location approaches to position mobile devices by Wi-Fi, WiMAX, GSM, LTE, IP addresses, and network environment data.

## Operational purpose

In order to route calls to a phone, cell towers listen for a signal sent from the phone and negotiate which tower is best able to communicate with the phone. As the phone changes location, the antenna towers monitor the signal, and the phone is “roamed” to an adjacent tower as appropriate. By comparing the relative signal strength from multiple antenna towers, a general location of a phone can be roughly determined. Other means make use of the antenna pattern, which supports angular determination and phase discrimination.

Newer phones may also allow the tracking of the phone even when turned on but not active in a telephone call. This results from the roaming procedures that perform hand-over of the phone from one base station to another.

## Consumer applications

A phone’s location can be shared with friends and family, posted to a public website, recorded locally, or shared with other users of a smartphone app. The inclusion of GPS receivers on smartphones has made geographical apps nearly ubiquitous on these devices. Specific applications include:

Locator apps like Find My Friends
Dating apps like Grindr
Recording a journey, for example to show a hiking accomplishment
For quantified self purposes such as fitness tracking
GPS drawing
In January 2019, the location of her iPhone as determined by her sister helped Boston police find kidnapping victim Olivia Ambrose.

## Privacy

Locating or positioning touches upon delicate privacy issues, since it enables someone to check where a person is without the person’s consent.[17] Strict ethics and security measures are strongly recommended for services that employ positioning.
In 2012 Malte Spitz held a TED talk[18] on the issue of mobile phone privacy in which he showcased his own stored data that he received from Deutsche Telekom after suing the company. He described the data, which consists of 35,830 lines of data collected during the span of Germany’s data retention at the time, saying, “This is six months of my life […] You can see where I am, when I sleep at night, what I’m doing.” He partnered up with ZEIT Online and made his information publicly available in an interactive map which allows users to watch his entire movements during that time in fast-forward. Spitz concluded that technology consumers are the key to challenging privacy norms in today’s society who “have to fight for self determination in the digital age.”

## China

Chinese government has proposed using this technology to track commuting patterns of Beijing city residents. Aggregate presence of mobile phone users could be tracked in a privacy-preserving fashion.

## Europe

In Europe most countries have a constitutional guarantee on the secrecy of correspondence, and location data obtained from mobile phone networks is usually given the same protection as the communication itself.

## United States

In the United States, there is a limited constitutional guarantee on the privacy of telecommunications through the Fourth Amendment. The use of location data is further limited by statutory, administrative, and case law. Police access of seven days of a citizen’s location data is unquestionably enough to be a fourth amendment search requiring both probable cause and a warrant.

In November 2017, the United States Supreme Court ruled in Carpenter v. United States that the government violates the Fourth Amendment by accessing historical records containing the physical locations of cellphones without a search warrant.[36]

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# Types of Fish Finder

A fishfinder or sounder (Australia) is an instrument used to locate
fish underwater by detecting reflected pulses of sound energy, as in sonar. A modern fishfinder displays measurements of reflected sound on a graphical display, allowing an operator to interpret information to locate schools of fish, underwater debris, and the bottom of body of water. Fishfinder instruments are used both by sport and commercial fishermen. Modern electronics allows a high degree of integration between the fishfinder system, marine radar, compass and GPS navigation systems.

Regular 2D fish finder
DownScan fish finder
Sidescan fish

## Let’s take a little of content about fish finder

Fathometer

Operating theory

General interpretation

Fish arches

General history in sporting and fishing

Commercial and naval units

Fathometer
Fish finders were derived from fathometers, active
sonar instruments used for navigation and safety to determine the depth of water.[1] The fathom is a unit of water depth, from which the instrument gets its name. The fathometer is an echo sounding system for measurement of water depth. A fathometer will display water depth and can make an automatic permanent record of measurements. Since both fathometers and fishfinders work the same way, and use similar frequencies and can detect both the bottom and fish, the instruments have merged.[2]

Operating theory
In operation, an electrical impulse from a transmitter is converted into a sound wave by an underwater transducer, called a hydrophone, and sent into the water.[3] When the wave strikes something such as a fish, it is reflected back and displays size, composition, and shape of the object. The exact extent of what can be discerned depends on the frequency and power of the pulse transmitted. Knowing the speed of the wave in the water, the distance to the object that reflected the wave can be determined. The speed of sound through the water column depends on the temperature, salinity and pressure (depth). This is approximately c = 1404.85 + 4.618T – 0.0523T2 + 1.25S + 0.017D (where c = sound speed (m/s), T = temperature (degrees Celsius), S = salinity (per mille) and D = depth).[4] Typical values used by commercial fish finders are 4921 ft/s (1500 m/s) in seawater and 4800 ft/s (1463 m/s) in freshwater.

The process can be repeated up to 40 times per second and eventually results in the bottom of the ocean being displayed versus time (the fathometer function that eventually spawned the sporting use of fishfinding.

The temperature and pressure sensitivity capability of fish finder units allow one to identify the exact location of the fish in the water by the use of a temperature gauge. Functionality present in many modern fish finders also have track back capabilities in order to check the changes in movement in order to switch position and location whilst fishing.

It is easy to get more detail at screen when the frequency of fish finder is high. Deep-sea trawlers and commercial fishermen normally use low-frequency which is in between 50-200 kHz where modern fish finders have multiple frequencies to view split screen results.

General interpretation

Display of a consumer type fishfinder

Sonar image of a white bass feeding frenzy
The image above, at right, clearly shows the bottom structure—plants, sediments and hard bottom are discernible on sonar plots of sufficiently high power and appropriate frequency. Slightly more than halfway up from the bottom to the left of the screen centre and about a third away from the left side, this image is also displaying a fish – a light spot just to the right of a ‘glare’ splash from the camera’s flashbulb. The X-axis of the image represents time, oldest (and behind the soundhead) to the left, most recent bottom (and current location) on the right; thus the fish is now well behind the transducer, and the vessel is now passing over a dip in the ocean floor or has just left it behind. The resulting distortion depends on both the speed of the vessel and how often the image is updated by the echo sounder.

Fish arches
With the Fish Symbol feature disabled, an angler can learn to distinguish between fish, vegetation, schools of baitfish or forage fish, debris, etc. Fish will usually appear on the screen as an arch. This is because the distance between the fish and the transducer changes as the boat passes over the fish (or the fish swims under the boat). When the fish enters the leading edge of the sonar beam, a display pixel is turned on. As the fish swims toward the centre of the beam, the distance to the fish decreases, turning on pixels at shallower depths. When the fish swims directly under the transducer, it is closer to the boat so the stronger signal shows a thicker line. As the fish swims away from the transducer, the distance increases, which shows as progressively deeper pixels.

The image to the right shows a school of white bass aggressively feeding on a school of threadfin shad. Note the school of baitfish near the bottom. When threatened, baitfish form a tightly packed school, as the individuals seek safety in the center of the school. This typically looks like an irregularly shaped ball or thumbprint on the fishfinder screen. When no predators are nearby, a school of baitfish frequently appears as a thin horizontal line across the screen, at the depth where the temperature and oxygen levels are optimal. The nearly-vertical lines near the right edge of the screen show the path of fishing lures falling to the bottom.

General history in sporting and fishing
By the early 1970’s, a common pattern of depth finder used an ultrasonic transducer immersed in water, and an electromechanical readout device. A neon lamp mounted on the end of an arm was rotated around a circular scale at a fixed speed by a small electric motor. The circular scale was calibrated in terms of depth of water. The instrument was arranged to send out a pulse of ultrasonic waves as the lamp passed the zero point of the scale. The transducer was then arranged to detect any reflected ultrasound impulses; the lamp would flash when an echo returned to the transducer, and by its position on the scale would indicate the elapsed time and therefore the depth of the water. [5] These also gave a small flickering flash for echos off of fish. Like today’s low-end digital fathometers, they kept no record of the depth over time and provided no information about bottom structure. They had poor accuracy, especially in rough water, and were hard to read in bright light. Despite the limitations, they were still usable for rough estimates of depth, such as for verifying that the boat had not drifted into an unsafe area.

Eventually, CRTs were married with a fathometer for commercial fishing and the fishfinder was born. With the advent of large LCD arrays, the high power requirements of a CRT gave way to the LCD in the early 1990s and fishfinding fathometers reached the sporting markets. Nowadays, many fishfinders available for hobby fishers have color LCD screens, built-in GPS, charting capabilities, and come bundled with transducers. Today, sporting fishfinders lack only the permanent record of the big ship navigational fathometer, and that is available in high end units that can use the ubiquitous computer to store that record as well.

Fishfinders may use higher frequencies to improve the image of underwater objects.[6] Side-looking transducers provide additional visibility of underwater objects on either side of the boat’s path. [7]

Commercial and naval units
Commercial and naval fathometers of yesteryear used a strip chart recorder where an advancing roll of paper was marked by a stylus to make a permanent copy of the depth, usually with some means of also recording time (Each mark or time ‘tic’ is proportional to distance traveled) so that the strip charts could be readily compared to navigation charts and maneuvering logs (speed changes). Much of the world’s ocean depths have been mapped using such recording strips. Fathometers of this type usually offered multiple (chart advance) speed settings, and sometimes, multiple frequencies as well. (Deep Ocean—Low Frequency carries better, Shallows—high frequency shows smaller structures (like fish, submerged reefs, wrecks, or other bottom composition features of interest.) At high frequency settings, high chart speeds, such fathometers give a picture of the bottom and any intervening large or schooling fish that can be related to position. Fathometers of the constant recording type are still mandated for all large vessels (100+ tons displacement) in restricted waters (i.e. generally, within 15 miles (24 km) of land).

Source: Wikipedia

# Global Positioning System GPS

“GPS” redirects here. For GPS devices, see Satellite navigation device. For other uses, see GPS (disambiguation).

## About the GPS functionality and extended use

The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radio navigation system owned by the United States government and operated by the United States Space Force.[2] It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.[3] Obstacles such as mountains and buildings block the relatively weak GPS signals.

The GPS does not require the user to transmit any data, and it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.[4]

The GPS project was started by the U.S. Department of Defense in 1973, with the first prototype spacecraft launched in 1978 and the full constellation of 24 satellites operational in 1993. Originally limited to use by the United States military, civilian use was allowed from the 1980s following an executive order from President Ronald Reagan after the Korean Air Lines Flight 007 incident.[5] Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX).[6] Announcements from Vice President Al Gore and the Clinton Administration in 1998 initiated these changes, which were authorized by the U.S. Congress in 2000.

During the 1990s, GPS quality was degraded by the United States government in a program called “Selective Availability”; this was discontinued on May 1, 2000 by a law signed by President Bill Clinton.[7]

The GPS service is provided by the United States government, which can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time.[8] As a result, several countries have developed or are in the process of setting up other global or regional satellite navigation systems. The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.[9] GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more quickly and accurately, to within two meters (6.6 ft).[10] China’s BeiDou Navigation Satellite System began global services in 2018, and finished its full deployment in 2020.[11] There are also the European Union Galileo positioning system, and India’s NavIC. Japan’s Quasi-Zenith Satellite System (QZSS) is a GPS satellite-based augmentation system to enhance GPS’s accuracy in Asia-Oceania, with satellite navigation independent of GPS scheduled for 2023.[12]

When selective availability was lifted in 2000, GPS had about a five-meter (16 ft) accuracy. GPS receivers that use the L5 band can have much higher accuracy, pinpointing to within 30 centimeters (11.8 in).[13][14] As of May 2021, 16 GPS satellites are broadcasting L5 signals, and the signals are considered pre-operational, scheduled to reach 24 satellites by approximately 2027.

Contents
1 History
1.1 Predecessors
1.2 Development
1.3 Timeline and modernization
1.4 Awards
2 Basic concept
2.1 Fundamentals
2.2 More detailed description
2.3 User-satellite geometry
3 Structure
3.1 Space segment
3.2 Control segment
3.3 User segment
4 Applications
4.1 Civilian
4.1.1 Restrictions on civilian use
4.2 Military
4.3 Timekeeping
4.3.1 Leap seconds
4.3.2 Accuracy
4.3.3 Format
5 Communication
5.1 Message format
5.2 Satellite frequencies
5.3 Demodulation and decoding
6.1 Problem description
6.2 Geometric interpretation
6.2.1 Spheres

History
Crystal Project video camera.png
Air Force film introducing the Navstar Global Positioning System, circa 1977
File:AFSC Film, NAVSTAR GPS-Circa 1977.ogv
The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems,[15] integrating ideas from several predecessors, including classified engineering design studies from the 1960s. The U.S. Department of Defense developed the system, which originally used 24 satellites. It was initially developed for use by the United States military and became fully operational in 1995. Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it.[16] The work of Gladys West is credited as instrumental in the development of computational techniques for detecting satellite positions with the precision needed for GPS.[17]

The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s.

In 1955, Friedwardt Winterberg proposed a test of general relativity – detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth’s observers to run 38 microseconds faster per day than the clocks on the Earth. The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day (6 mi/d). This was corrected for in the design of GPS.[18]

Predecessors
When the Soviet Union launched the first artificial satellite (Sputnik 1) in 1957, two American physicists, William Guier and George Weiffenbach, at Johns Hopkins University’s Applied Physics Laboratory (APL) decided to monitor its radio transmissions.[19] Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC to do the heavy calculations required.

Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem—pinpointing the user’s location, given the satellite’s. (At the time, the Navy was developing the submarine-launched Polaris missile, which required them to know the submarine’s location.) This led them and APL to develop the TRANSIT system.[20] In 1959, ARPA (renamed DARPA in 1972) also played a role in TRANSIT.[21][22][23]

TRANSIT was first successfully tested in 1960.[24] It used a constellation of five satellites and could provide a navigational fix approximately once per hour.

In 1967, the U.S. Navy developed the Timation satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.

In the 1970s, the ground-based OMEGA navigation system, based on phase comparison of signal transmission from pairs of stations,[25] became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.

Although there were wide needs for accurate navigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation of a constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded. It is also the reason for the ultra-secrecy at that time. The nuclear triad consisted of the United States Navy’s submarine-launched ballistic missiles (SLBMs) along with United States Air Force (USAF) strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier.

Precise navigation would enable United States ballistic missile submarines to get an accurate fix of their positions before they launched their SLBMs.[26] The USAF, with two thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The U.S. Navy and U.S. Air Force were developing their own technologies in parallel to solve what was essentially the same problem.

To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (comparable to the Soviet SS-24 and SS-25) and so the need to fix the launch position had similarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study, Project 57, was worked in 1963 and it was “in this study that the GPS concept was born”. That same year, the concept was pursued as Project 621B, which had “many of the attributes that you now see in GPS”[27] and promised increased accuracy for Air Force bombers as well as ICBMs.

Updates from the Navy TRANSIT system were too slow for the high speeds of Air Force operation. The Naval Research Laboratory (NRL) continued making advances with their Timation (Time Navigation) satellites, first launched in 1967, second launched in 1969, with the third in 1974 carrying the first atomic clock into orbit and the fourth launched in 1977.[28]

Another important predecessor to GPS came from a different branch of the United States military. In 1964, the United States Army orbited its first Sequential Collation of Range (SECOR) satellite used for geodetic surveying.[29] The SECOR system included three ground-based transmitters at known locations that would send signals to the satellite transponder in orbit. A fourth ground-based station, at an undetermined position, could then use those signals to fix its location precisely. The last SECOR satellite was launched in 1969.[30]

Development
With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program. Satellite orbital position errors, induced by variations in the gravity field and radar refraction among others, had to be resolved. A team led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970–1973, used real-time data assimilation and recursive estimation to do so, reducing systematic and residual errors to a manageable level to permit accurate navigation.[31]

During Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that the real synthesis that became GPS was created. Later that year, the DNSS program was named Navstar.[32] Navstar is often erroneously considered an acronym for “NAVigation System Using Timing and Ranging” but was never considered as such by the GPS Joint Program Office (TRW may have once advocated for a different navigational system that used that acronym).[33] With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites, Navstar-GPS.[34] Ten “Block I” prototype satellites were launched between 1978 and 1985 (an additional unit was destroyed in a launch failure).[35]

The effect of the ionosphere on radio transmission was investigated in a geophysics laboratory of Air Force Cambridge Research Laboratory, renamed to Air Force Geophysical Research Lab (AFGRL) in 1974. AFGRL developed the Klobuchar model for computing ionospheric corrections to GPS location.[36] Of note is work done by Australian space scientist Elizabeth Essex-Cohen at AFGRL in 1974. She was concerned with the curving of the paths of radio waves (atmospheric refraction) traversing the ionosphere from NavSTAR satellites.[37]

After Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down in 1983 after straying into the USSR’s prohibited airspace,[38] in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good.[39] The first Block II satellite was launched on February 14, 1989,[40] and the 24th satellite was launched in 1994. The GPS program cost at this point, not including the cost of the user equipment but including the costs of the satellite launches, has been estimated at US5 billion (then-year dollars).[41] Initially, the highest-quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded, in a policy known as Selective Availability. This changed with President Bill Clinton signing on May 1, 2000 a policy directive to turn off Selective Availability to provide the same accuracy to civilians that was afforded to the military. The directive was proposed by the U.S. Secretary of Defense, William Perry, in view of the widespread growth of differential GPS services by private industry to improve civilian accuracy. Moreover, the U.S. military was actively developing technologies to deny GPS service to potential adversaries on a regional basis.[42] Since its deployment, the U.S. has implemented several improvements to the GPS service, including new signals for civil use and increased accuracy and integrity for all users, all the while maintaining compatibility with existing GPS equipment. Modernization of the satellite system has been an ongoing initiative by the U.S. Department of Defense through a series of satellite acquisitions to meet the growing needs of the military, civilians, and the commercial market. As of early 2015, high-quality, FAA grade, Standard Positioning Service (SPS) GPS receivers provided horizontal accuracy of better than 3.5 meters (11 ft),[43] although many factors such as receiver and antenna quality and atmospheric issues can affect this accuracy. GPS is owned and operated by the United States government as a national resource. The Department of Defense is the steward of GPS. The Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996 to 2004. After that, the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems.[44] The executive committee is chaired jointly by the Deputy Secretaries of Defense and Transportation. Its membership includes equivalent-level officials from the Departments of State, Commerce, and Homeland Security, the Joint Chiefs of Staff and NASA. Components of the executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison. The U.S. Department of Defense is required by law to “maintain a Standard Positioning Service (as defined in the federal radio navigation plan and the standard positioning service signal specification) that will be available on a continuous, worldwide basis,” and “develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses.” Timeline and modernization Summary of satellites[45][46][47] Block Launch period Satellite launches Currently in orbit and healthy Suc- cess Fail- ure In prep- aration Plan- ned I 1978–1985 10 1 0 0 0 II 1989–1990 9 0 0 0 0 IIA 1990–1997 19 0 0 0 0 IIR 1997–2004 12 1 0 0 7 IIR-M 2005–2009 8 0 0 0 7 IIF 2010–2016 12 0 0 0 12 IIIA 2018– 5 0 5 0 5 IIIF — 0 0 0 22 0 Total 75 2 5 22 31 (Last update: 08 July 2021) USA-203 from Block IIR-M is unhealthy [48] For a more complete list, see List of GPS satellites In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of four prototype GPS receivers in a Y configuration over White Sands Missile Range, using ground-based pseudo-satellites.[49] In 1978, the first experimental Block-I GPS satellite was launched.[35] In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that GPS would be made available for civilian uses once it was completed,[50][51] although it had been previously published [in Navigation magazine], and that the CA code (Coarse/Acquisition code) would be available to civilian users.[citation needed] By 1985, ten more experimental Block-I satellites had been launched to validate the concept. Beginning in 1988, command and control of these satellites was moved from Onizuka AFS, California to the 2nd Satellite Control Squadron (2SCS) located at Falcon Air Force Station in Colorado Springs, Colorado.[52][53] On February 14, 1989, the first modern Block-II satellite was launched. The Gulf War from 1990 to 1991 was the first conflict in which the military widely used GPS.[54] In 1991, a project to create a miniature GPS receiver successfully ended, replacing the previous 16 kg (35 lb) military receivers with a 1.25 kg (2.8 lb) handheld receiver.[22] In 1992, the 2nd Space Wing, which originally managed the system, was inactivated and replaced by the 50th Space Wing. Emblem of the 50th Space Wing By December 1993, GPS achieved initial operational capability (IOC), with a full constellation (24 satellites) available and providing the Standard Positioning Service (SPS).[55] Full Operational Capability (FOC) was declared by Air Force Space Command (AFSPC) in April 1995, signifying full availability of the military’s secure Precise Positioning Service (PPS).[55] In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[56] declaring GPS a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset. In 1998, United States Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety, and in 2000 the United States Congress authorized the effort, referring to it as GPS III. On May 2, 2000 “Selective Availability” was discontinued as a result of the 1996 executive order, allowing civilian users to receive a non-degraded signal globally. In 2004, the United States government signed an agreement with the European Community establishing cooperation related to GPS and Europe’s Galileo system. In 2004, United States President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.[57] November 2004, Qualcomm announced successful tests of assisted GPS for mobile phones.[58] In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.[59] On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan.[60] On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.[61] On May 21, 2009, the Air Force Space Command allayed fears of GPS failure, saying “There’s only a small risk we will not continue to exceed our performance standard.”[62] On January 11, 2010, an update of ground control systems caused a software incompatibility with 8,000 to 10,000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.[63] On February 25, 2010,[64] the U.S. Air Force awarded the contract to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization. Awards Air Force Space Commander presents Dr. Gladys West with an award as she is inducted into the Air Force Space and Missile Pioneers Hall of Fame for her GPS work on Dec. 6, 2018. Air Force Space Commander presents Gladys West with an award as she is inducted into the Air Force Space and Missile Pioneers Hall of Fame for her GPS work on Dec. 6, 2018. On February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the US’s most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the USAF, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation honors them “for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago.” Two GPS developers received the National Academy of Engineering Charles Stark Draper Prize for 2003: Ivan Getting, emeritus president of The Aerospace Corporation and an engineer at MIT, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation). Bradford Parkinson, professor of aeronautics and astronautics at Stanford University, conceived the present satellite-based system in the early 1960s and developed it in conjunction with the U.S. Air Force. Parkinson served twenty-one years in the Air Force, from 1957 to 1978, and retired with the rank of colonel. GPS developer Roger L. Easton received the National Medal of Technology on February 13, 2006.[65] Francis X. Kane (Col. USAF, ret.) was inducted into the U.S. Air Force Space and Missile Pioneers Hall of Fame at Lackland A.F.B., San Antonio, Texas, March 2, 2010 for his role in space technology development and the engineering design concept of GPS conducted as part of Project 621B. In 1998, GPS technology was inducted into the Space Foundation Space Technology Hall of Fame.[66] On October 4, 2011, the International Astronautical Federation (IAF) awarded the Global Positioning System (GPS) its 60th Anniversary Award, nominated by IAF member, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognized the uniqueness of the GPS program and the exemplary role it has played in building international collaboration for the benefit of humanity.[67] Gladys West was inducted into the Air Force Space and Missile Pioneers Hall of Fame in 2018 for recognition of her computational work which led to breakthroughs for GPS technology.[68] On February 12, 2019, four founding members of the project were awarded the Queen Elizabeth Prize for Engineering with the chair of the awarding board stating “Engineering is the foundation of civilisation; there is no other foundation; it makes things happen. And that’s exactly what today’s Laureates have done – they’ve made things happen. They’ve re-written, in a major way, the infrastructure of our world.”[69] Basic concept This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2015) (Learn how and when to remove this template message) Fundamentals The GPS receiver calculates its own position and time based on data received from multiple GPS satellites. Each satellite carries an accurate record of its position and time, and transmits that data to the receiver. The satellites carry very stable atomic clocks that are synchronized with one another and with ground clocks. Any drift from time maintained on the ground is corrected daily. In the same manner, the satellite locations are known with great precision. GPS receivers have clocks as well, but they are less stable and less precise. Since the speed of radio waves is constant and independent of the satellite speed, the time delay between when the satellite transmits a signal and the receiver receives it is proportional to the distance from the satellite to the receiver. At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time). More detailed description Each GPS satellite continually broadcasts a signal (carrier wave with modulation) that includes: A pseudorandom code (sequence of ones and zeros) that is known to the receiver. By time-aligning a receiver-generated version and the receiver-measured version of the code, the time of arrival (TOA) of a defined point in the code sequence, called an epoch, can be found in the receiver clock time scale A message that includes the time of transmission (TOT) of the code epoch (in GPS time scale) and the satellite position at that time Conceptually, the receiver measures the TOAs (according to its own clock) of four satellite signals. From the TOAs and the TOTs, the receiver forms four time of flight (TOF) values, which are (given the speed of light) approximately equivalent to receiver-satellite ranges plus time difference between the receiver and GPS satellites multiplied by speed of light, which are called as pseudo-ranges. The receiver then computes its three-dimensional position and clock deviation from the four TOFs. In practice the receiver position (in three dimensional Cartesian coordinates with origin at the Earth’s center) and the offset of the receiver clock relative to the GPS time are computed simultaneously, using the navigation equations to process the TOFs. The receiver’s Earth-centered solution location is usually converted to latitude, longitude and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to the geoid, which is essentially mean sea level. These coordinates may be displayed, such as on a moving map display, or recorded or used by some other system, such as a vehicle guidance system. User-satellite geometry Further information: § Geometric interpretation Although usually not formed explicitly in the receiver processing, the conceptual time differences of arrival (TDOAs) define the measurement geometry. Each TDOA corresponds to a hyperboloid of revolution (see Multilateration). The line connecting the two satellites involved (and its extensions) forms the axis of the hyperboloid. The receiver is located at the point where three hyperboloids intersect.[70][71] It is sometimes incorrectly said that the user location is at the intersection of three spheres. While simpler to visualize, this is the case only if the receiver has a clock synchronized with the satellite clocks (i.e., the receiver measures true ranges to the satellites rather than range differences). There are marked performance benefits to the user carrying a clock synchronized with the satellites. Foremost is that only three satellites are needed to compute a position solution. If it were an essential part of the GPS concept that all users needed to carry a synchronized clock, a smaller number of satellites could be deployed, but the cost and complexity of the user equipment would increase. Receiver in continuous operation The description above is representative of a receiver start-up situation. Most receivers have a track algorithm, sometimes called a tracker, that combines sets of satellite measurements collected at different times—in effect, taking advantage of the fact that successive receiver positions are usually close to each other. After a set of measurements are processed, the tracker predicts the receiver location corresponding to the next set of satellite measurements. When the new measurements are collected, the receiver uses a weighting scheme to combine the new measurements with the tracker prediction. In general, a tracker can (a) improve receiver position and time accuracy, (b) reject bad measurements, and (c) estimate receiver speed and direction. The disadvantage of a tracker is that changes in speed or direction can be computed only with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the random error of position measurement. GPS units can use measurements of the Doppler shift of the signals received to compute velocity accurately.[72] More advanced navigation systems use additional sensors like a compass or an inertial navigation system to complement GPS. Non-navigation applications For a list of applications, see § Applications. GPS requires four or more satellites to be visible for accurate navigation. The solution of the navigation equations gives the position of the receiver along with the difference between the time kept by the receiver’s on-board clock and the true time-of-day, thereby eliminating the need for a more precise and possibly impractical receiver based clock. Applications for GPS such as time transfer, traffic signal timing, and synchronization of cell phone base stations, make use of this cheap and highly accurate timing. Some GPS applications use this time for display, or, other than for the basic position calculations, do not use it at all. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.[73][74][75] Structure This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2015) (Learn how and when to remove this template message) The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment.[76] The U.S. Space Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.[77] Space segment See also: GPS satellite blocks and List of GPS satellites Unlaunched GPS block II-A satellite on display at the San Diego Air & Space Museum A visual example of a 24 satellite GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth’s surface changes with time. The point in this example is in Golden, Colorado, USA (39.7469°N 105.2108°W). The space segment (SS) is composed of 24 to 32 satellites, or Space Vehicles (SV), in medium Earth orbit, and also includes the payload adapters to the boosters required to launch them into orbit. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits,[78] but this was modified to six orbital planes with four satellites each.[79] The six orbit planes have approximately 55° inclination (tilt relative to the Earth’s equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit’s intersection).[80] The orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locations[81] or almost the same locations[82] every day. The orbits are arranged so that at least six satellites are always within line of sight from everywhere on the Earth’s surface (see animation at right).[83] The result of this objective is that the four satellites are not evenly spaced (90°) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30°, 105°, 120°, and 105° apart, which sum to 360°.[84] Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi),[85] each SV makes two complete orbits each sidereal day, repeating the same ground track each day.[86] This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones. As of February 2019,[87] there are 31 satellites in the GPS constellation, 27 of which are in use at a given time with the rest allocated as stand-bys. A 32nd was launched in 2018, but as of July 2019 is still in evaluation. More decommissioned satellites are in orbit and available as spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve accuracy but also improves reliability and availability of the system, relative to a uniform system, when multiple satellites fail.[88] With the expanded constellation, nine satellites are usually visible from any point on the ground at any time, ensuring considerable redundancy over the minimum four satellites needed for a position. Control segment Ground monitor station used from 1984 to 2007, on display at the Air Force Space and Missile Museum. The control segment (CS) is composed of: a master control station (MCS), an alternative master control station, four dedicated ground antennas, and six dedicated monitor stations. The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.[89] The tracking information is sent to the MCS at Schriever Air Force Base 25 km (16 mi) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Space Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite’s internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.[90] Satellite maneuvers are not precise by GPS standards—so to change a satellite’s orbit, the satellite must be marked unhealthy, so receivers don’t use it. After the satellite maneuver, engineers track the new orbit from the ground, upload the new ephemeris, and mark the satellite healthy again. The operation control segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports GPS users and keeps the GPS operational and performing within specification. OCS successfully replaced the legacy 1970s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported U.S. armed forces. OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System[6] (OCX), is fully developed and functional. The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, enabling[91] U.S. Space Force to greatly enhance GPS operational services to U.S. combat forces, civil partners and myriad domestic and international users. The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50%[92] sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX is expected to cost millions less than the cost to upgrade OCS while providing four times the capability. The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program. OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals. Built on a flexible architecture that can rapidly adapt to the changing needs of today’s and future GPS users allowing immediate access to GPS data and constellation status through secure, accurate and reliable information. Provides the warfighter with more secure, actionable and predictive information to enhance situational awareness. Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do. Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks. Supports higher volume near real-time command and control capabilities and abilities. On September 14, 2011,[93] the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development. The GPS OCX program has missed major milestones and is pushing its launch into 2021, 5 years past the original deadline. According to the Government Accounting Office, even this new deadline looks shaky.[94] User segment Further information: GPS navigation device GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as these. The first portable GPS unit, a Leica WM 101, displayed at the Irish National Science Museum at Maynooth. The user segment (US) is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels. Though there are many receiver manufacturers, they almost all use one of the chipsets produced for this purpose.[citation needed] A typical OEM GPS receiver module measuring 15 mm × 17 mm (0.6 in × 0.7 in) GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.[citation needed] Receivers with internal DGPS receivers can outperform those using external RTCM data.[citation needed] As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers. A typical GPS receiver with integrated antenna. Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA),[95] references to this protocol have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws.[clarification needed] Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth. Applications This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2015) (Learn how and when to remove this template message) Main article: GNSS applications While originally a military project, GPS is considered a dual-use technology, meaning it has significant civilian applications as well. GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. GPS’s accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching.[77] Civilian This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing. Many civilian applications use one or more of GPS’s three basic components: absolute location, relative movement, and time transfer. Atmosphere: studying the troposphere delays (recovery of the water varpour content) and ionosphere delays (recovery of the number of free electrons).[96] Recovery of Earth surface displacements due to the atmospheric pressure loading.[97] Astronomy: both positional and clock synchronization data is used in astrometry and celestial mechanics and precise orbit determination.[98] GPS is also used in both amateur astronomy with small telescopes as well as by professional observatories for finding extrasolar planets. Automated vehicle: applying location and routes for cars and trucks to function without a human driver. Cartography: both civilian and military cartographers use GPS extensively. Cellular telephony: clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter. Clock synchronization: the accuracy of GPS time signals (±10 ns)[99] is second only to the atomic clocks they are based on, and is used in applications such as GPS disciplined oscillators. Disaster relief/emergency services: many emergency services depend upon GPS for location and timing capabilities. GPS-equipped radiosondes and dropsondes: measure and calculate the atmospheric pressure, wind speed and direction up to 27 km (89,000 ft) from the Earth’s surface. Radio occultation for weather and atmospheric science applications.[100] Fleet tracking: used to identify, locate and maintain contact reports with one or more fleet vehicles in real-time. Geodesy: determination of Earth orientation parameters including the daily and sub-daily polar motion,[101] and length-of-day variabilities,[102] Earth’s center-of-mass – geocenter motion,[103] and low-degree gravity field parameters.[104] Geofencing: vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate devices that are attached to or carried by a person, vehicle, or pet. The application can provide continuous tracking and send notifications if the target leaves a designated (or “fenced-in”) area.[105] Geotagging: applies location coordinates to digital objects such as photographs (in Exif data) and other documents for purposes such as creating map overlays with devices like Nikon GP-1 GPS aircraft tracking GPS for mining: the use of RTK GPS has significantly improved several mining operations such as drilling, shoveling, vehicle tracking, and surveying. RTK GPS provides centimeter-level positioning accuracy. GPS data mining: It is possible to aggregate GPS data from multiple users to understand movement patterns, common trajectories and interesting locations.[106] GPS tours: location determines what content to display; for instance, information about an approaching point of interest. Navigation: navigators value digitally precise velocity and orientation measurements, as well as precise positions in real-time with a support of orbit and clock corrections.[107] Orbit determination of low-orbiting satellites with GPS receiver installed onboard, such as GOCE,[108] GRACE, Jason-1, Jason-2, TerraSAR-X, TanDEM-X, CHAMP, Sentinel-3,[109] and some cubesats, e.g., CubETH. Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to compute phasors. Recreation: for example, Geocaching, Geodashing, GPS drawing, waymarking, and other kinds of location based mobile games such as Pokémon Go. Reference frames: realization and densification of the terrestrial reference frames[110] in the framework of Global Geodetic Observing System. Co-location in space between Satellite laser ranging[111] and microwave observations[112] for deriving global geodetic parameters.[113][114] Robotics: self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading. Sport: used in football and rugby for the control and analysis of the training load.[115] Surveying: surveyors use absolute locations to make maps and determine property boundaries. Tectonics: GPS enables direct fault motion measurement of earthquakes. Between earthquakes GPS can be used to measure crustal motion and deformation[116] to estimate seismic strain buildup for creating seismic hazard maps. Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems. Restrictions on civilian use The U.S. government controls the export of some civilian receivers. All GPS receivers capable of functioning above 60,000 ft (18 km) above sea level and 1,000 kn (500 m/s; 2,000 km/h; 1,000 mph), or designed or modified for use with unmanned missiles and aircraft, are classified as munitions (weapons)—which means they require State Department export licenses.[117] This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse/Acquisition) code. Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule refers to operation at both the target altitude and speed, but some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches that regularly reach 30 km (100,000 feet). These limits only apply to units or components exported from the United States. A growing trade in various components exists, including GPS units from other countries. These are expressly sold as ITAR-free. Military Attaching a GPS guidance kit to a dumb bomb, March 2003. M982 Excalibur GPS-guided artillery shell. As of 2009, military GPS applications include: Navigation: Soldiers use GPS to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commander’s Digital Assistant and lower ranks use the Soldier Digital Assistant.[118] Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile.[citation needed] These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, use GPS to find targets. Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and artillery shells. Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s2 (260,000 mph/s) have been developed for use in 155-millimeter (6.1 in) howitzer shells.[119] Search and rescue. Reconnaissance: Patrol movement can be managed more closely. GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor called a bhangmeter, an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System.[120][121] General William Shelton has stated that future satellites may drop this feature to save money.[122] GPS type navigation was first used in war in the 1991 Persian Gulf War, before GPS was fully developed in 1995, to assist Coalition Forces to navigate and perform maneuvers in the war. The war also demonstrated the vulnerability of GPS to being jammed, when Iraqi forces installed jamming devices on likely targets that emitted radio noise, disrupting reception of the weak GPS signal.[123] GPS’s vulnerability to jamming is a threat that continues to grow as jamming equipment and experience grows.[124][125] GPS signals have been reported to have been jammed many times over the years for military purposes. Russia seems to have several objectives for this behavior, such as intimidating neighbors while undermining confidence in their reliance on American systems, promoting their GLONASS alternative, disrupting Western military exercises, and protecting assets from drones.[126] China uses jamming to discourage US surveillance aircraft near the contested Spratly Islands.[127] North Korea has mounted several major jamming operations near its border with South Korea and offshore, disrupting flights, shipping and fishing operations.[128] Iranian Armed Forces disrupted the civilian airliner plane Flight PS752’s GPS when it shot down the aircraft.[129][130] Timekeeping Leap seconds While most clocks derive their time from Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to “GPS time”. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks.[131] The GPS navigation message includes the difference between GPS time and UTC. As of January 2017, GPS time is 18 seconds ahead of UTC because of the leap second added to UTC on December 31, 2016.[132] Receivers subtract this offset from GPS time to calculate UTC and specific time zone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits). Accuracy GPS time is theoretically accurate to about 14 nanoseconds, due to the clock drift that atomic clocks experience in GPS transmitters, relative to International Atomic Time.[133] Most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 nanoseconds.[134][135] Format Further information: GPS Week Number Rollover As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). It happened the second time at 23:59:42 UTC on April 6, 2019. To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern in the future the modernized GPS civil navigation (CNAV) message will use a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until 2137 (157 years after GPS week zero). Communication Main article: GPS signals The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military. Message format GPS message format Subframes Description 1 Satellite clock, GPS time relationship 2–3 Ephemeris (precise satellite orbit) 4–5 Almanac component (satellite network synopsis, error correction) Each GPS satellite continuously broadcasts a navigation message on L1 (C/A and P/Y) and L2 (P/Y) frequencies at a rate of 50 bits per second (see bitrate). Each complete message takes 750 seconds (12+1⁄2 minutes) to complete. The message structure has a basic format of a 1500-bit-long frame made up of five subframes, each subframe being 300 bits (6 seconds) long. Subframes 4 and 5 are subcommutated 25 times each, so that a complete data message requires the transmission of 25 full frames. Each subframe consists of ten words, each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes in a frame times 25 frames in a message, each message is 37,500 bits long. At a transmission rate of 50-bit/s, this gives 750 seconds to transmit an entire almanac message (GPS). Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.[136] The first subframe of each frame encodes the week number and the time within the week,[137] as well as the data about the health of the satellite. The second and the third subframes contain the ephemeris – the precise orbit for the satellite. The fourth and fifth subframes contain the almanac, which contains coarse orbit and status information for up to 32 satellites in the constellation as well as data related to error correction. Thus, to obtain an accurate satellite location from this transmitted message, the receiver must demodulate the message from each satellite it includes in its solution for 18 to 30 seconds. To collect all transmitted almanacs, the receiver must demodulate the message for 732 to 750 seconds or 12+1⁄2 minutes.[138] All satellites broadcast at the same frequencies, encoding signals using unique code-division multiple access (CDMA) so receivers can distinguish individual satellites from each other. The system uses two distinct CDMA encoding types: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P(Y)) code, which is encrypted so that only the U.S. military and other NATO nations who have been given access to the encryption code can access it.[139] The ephemeris is updated every 2 hours and is sufficiently stable for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The almanac is updated typically every 24 hours. Additionally, data for a few weeks following is uploaded in case of transmission updates that delay data upload.[citation needed] Satellite frequencies GPS frequency overview[140]:607 Band Frequency Description L1 1575.42 MHz Coarse-acquisition (C/A) and encrypted precision (P(Y)) codes, plus the L1 civilian (L1C) and military (M) codes on future Block III satellites. L2 1227.60 MHz P(Y) code, plus the L2C and military codes on the Block IIR-M and newer satellites. L3 1381.05 MHz Used for nuclear detonation (NUDET) detection. L4 1379.913 MHz Being studied for additional ionospheric correction. L5 1176.45 MHz Proposed for use as a civilian safety-of-life (SoL) signal. All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique[140]:607 where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The actual internal reference of the satellites is 10.22999999543 MHz to compensate for relativistic effects[141][142] that make observers on the Earth perceive a different time reference with respect to the transmitters in orbit. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[84] The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user. The L3 signal at a frequency of 1.38105 GHz is used to transmit data from the satellites to ground stations. This data is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth’s atmosphere and near space.[143] One usage is the enforcement of nuclear test ban treaties. The L4 band at 1.379913 GHz is being studied for additional ionospheric correction.[140]:607 The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that provides this signal was launched in May 2010.[144] On February 5th 2016, the 12th and final Block IIF satellite was launched.[145] The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. “L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy.”[146] Ambox current red.svg This section needs to be updated. Please update this article to reflect recent events or newly available information. (May 2021) In 2011, a conditional waiver was granted to LightSquared to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify issue that might arise due to the larger signal power from the LightSquared terrestrial network. The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, essentially repurposing what was supposed to be a “quiet neighborhood” for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the impact of the lower 10 MHz of spectrum is minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10 MHz intended for use by LightSquared may have some impact on GPS devices. There is some concern that this may seriously degrade the GPS signal for many consumer uses.[147][148] Aviation Week magazine reports that the latest testing (June 2011) confirms “significant jamming” of GPS by LightSquared’s system.[149] Demodulation and decoding Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code. Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary sequence known as a Gold code. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver.[150][151] If the almanac information has previously been acquired, the receiver picks the satellites to listen for by their PRNs, unique numbers in the range 1 through 32. If the almanac information is not in memory, the receiver enters a search mode until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite’s signal, it identifies it by its distinct C/A code pattern. There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data. Processing of the navigation message enables the determination of the time of transmission and the satellite position at this time. For more information see Demodulation and Decoding, Advanced. Navigation equations Further information: GNSS positioning calculation See also: Pseudorange Problem description The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent (s) are designated as [xi, yi, zi, si] where the subscript i denotes the satellite and has the value 1, 2, …, n, where n ≥ 4. When the time of message reception indicated by the on-board receiver clock is t̃i, the true reception time is ti = t̃i − b, where b is the receiver’s clock bias from the much more accurate GPS clocks employed by the satellites. The receiver clock bias is the same for all received satellite signals (assuming the satellite clocks are all perfectly synchronized). The message’s transit time is t̃i − b − si, where si is the satellite time. Assuming the message traveled at the speed of light, c, the distance traveled is (t̃i − b − si) c. For n satellites, the equations to satisfy are: {\displaystyle d_{i}=\left({\tilde {t}}_{i}-b-s_{i}\right)c,\;i=1,2,\dots ,n}{\displaystyle d_{i}=\left({\tilde {t}}_{i}-b-s_{i}\right)c,\;i=1,2,\dots ,n} where di is the geometric distance or range between receiver and satellite i (the values without subscripts are the x, y, and z components of receiver position): {\displaystyle d_{i}={\sqrt {(x-x_{i})^{2}+(y-y_{i})^{2}+(z-z_{i})^{2}}}}{\displaystyle d_{i}={\sqrt {(x-x_{i})^{2}+(y-y_{i})^{2}+(z-z_{i})^{2}}}} Defining pseudoranges as {\displaystyle p_{i}=\left({\tilde {t}}_{i}-s_{i}\right)c}p_{i}=\left({\tilde {t}}_{i}-s_{i}\right)c, we see they are biased versions of the true range: {\displaystyle p_{i}=d_{i}+bc,\;i=1,2,…,n}{\displaystyle p_{i}=d_{i}+bc,\;i=1,2,…,n} .[152][153] Since the equations have four unknowns [x, y, z, b]—the three components of GPS receiver position and the clock bias—signals from at least four satellites are necessary to attempt solving these equations. They can be solved by algebraic or numerical methods. Existence and uniqueness of GPS solutions are discussed by Abell and Chaffee.[70] When n is greater than four, this system is overdetermined and a fitting method must be used. The amount of error in the results varies with the received satellites’ locations in the sky, since certain configurations (when the received satellites are close together in the sky) cause larger errors. Receivers usually calculate a running estimate of the error in the calculated position. This is done by multiplying the basic resolution of the receiver by quantities called the geometric dilution of position (GDOP) factors, calculated from the relative sky directions of the satellites used.[154] The receiver location is expressed in a specific coordinate system, such as latitude and longitude using the WGS 84 geodetic datum or a country-specific system.[155] Geometric interpretation The GPS equations can be solved by numerical and analytical methods. Geometrical interpretations can enhance the understanding of these solution methods. Spheres 2-D Cartesian true-range multilateration (trilateration) scenario. The measured ranges, called pseudoranges, contain clock errors. In a simplified idealization in which the ranges are synchronized, these true ranges represent the radii of spheres, each centered on one of the transmitting satellites. The solution for the position of the receiver is then at the intersection of the surfaces of these spheres; see trilateration (more generally, true-range multilateration). Signals from at minimum three satellites are required, and their three spheres would typically intersect at two points.[156] One of the points is the location of the receiver, and the other moves rapidly in successive measurements and would not usually be on Earth’s surface. In practice, there are many sources of inaccuracy besides clock bias, including random errors as well as the potential for precision loss from subtracting numbers close to each other if the centers of the spheres are relatively close together. This means that the position calculated from three satellites alone is unlikely to be accurate enough. Data from more satellites can help because of the tendency for random errors to cancel out and also by giving a larger spread between the sphere centers. But at the same time, more spheres will not generally intersect at one point. Therefore, a near intersection gets computed, typically via least squares. The more signals available, the better the approximation is likely to be. Hyperboloids Three satellites (labeled as “stations” A, B, C) have known locations. The true times it takes for a radio signal to travel from each satellite to the receiver are unknown, but the true time differences are known. Then, each time difference locates the receiver on a branch of a hyperbola focused on the satellites. The receiver is then located at one of the two intersections. If the pseudorange between the receiver and satellite i and the pseudorange between the receiver and satellite j are subtracted, pi − pj, the common receiver clock bias (b) cancels out, resulting in a difference of distances di − dj. The locus of points having a constant difference in distance to two points (here, two satellites) is a hyperbola on a plane and a hyperboloid of revolution (more specifically, a two-sheeted hyperboloid) in 3D space (see Multilateration). Thus, from four pseudorange measurements, the receiver can be placed at the intersection of the surfaces of three hyperboloids each with foci at a pair of satellites. With additional satellites, the multiple intersections are not necessarily unique, and a best-fitting solution is sought instead.[70][71][157][158][159][160] Inscribed sphere A smaller circle (red) inscribed and tangent to other circles (black), that need not necessarily be mutually tangent. The receiver position can be interpreted as the center of an inscribed sphere (insphere) of radius bc, given by the receiver clock bias b (scaled by the speed of light c). The insphere location is such that it touches other spheres. The circumscribing spheres are centered at the GPS satellites, whose radii equal the measured pseudoranges pi. This configuration is distinct from the one described above, in which the spheres’ radii were the unbiased or geometric ranges di.[159]:36–37[161] Hypercones The clock in the receiver is usually not of the same quality as the ones in the satellites and will not be accurately synchronized to them. This produces pseudoranges with large differences compared to the true distances to the satellites. Therefore, in practice, the time difference between the receiver clock and the satellite time is defined as an unknown clock bias b. The equations are then solved simultaneously for the receiver position and the clock bias. The solution space [x, y, z, b] can be seen as a four-dimensional spacetime, and signals from at minimum four satellites are needed. In that case each of the equations describes a hypercone (or spherical cone),[162] with the cusp located at the satellite, and the base a sphere around the satellite. The receiver is at the intersection of four or more of such hypercones. Solution methods Least squares When more than four satellites are available, the calculation can use the four best, or more than four simultaneously (up to all visible satellites), depending on the number of receiver channels, processing capability, and geometric dilution of precision (GDOP). Using more than four involves an over-determined system of equations with no unique solution; such a system can be solved by a least-squares or weighted least squares method.[152] {\displaystyle \left({\hat {x}},{\hat {y}},{\hat {z}},{\hat {b}}\right)={\underset {\left(x,y,z,b\right)}{\arg \min }}\sum _{i}\left({\sqrt {(x-x_{i})^{2}+(y-y_{i})^{2}+(z-z_{i})^{2}}}+bc-p_{i}\right)^{2}}\left({\hat {x}},{\hat {y}},{\hat {z}},{\hat {b}}\right)={\underset {\left(x,y,z,b\right)}{\arg \min }}\sum _{i}\left({\sqrt {(x-x_{i})^{2}+(y-y_{i})^{2}+(z-z_{i})^{2}}}+bc-p_{i}\right)^{2} Iterative Both the equations for four satellites, or the least squares equations for more than four, are non-linear and need special solution methods. A common approach is by iteration on a linearized form of the equations, such as the Gauss–Newton algorithm. The GPS was initially developed assuming use of a numerical least-squares solution method—i.e., before closed-form solutions were found. Closed-form One closed-form solution to the above set of equations was developed by S. Bancroft.[153][163] Its properties are well known;[70][71][164] in particular, proponents claim it is superior in low-GDOP situations, compared to iterative least squares methods.[163] Bancroft’s method is algebraic, as opposed to numerical, and can be used for four or more satellites. When four satellites are used, the key steps are inversion of a 4×4 matrix and solution of a single-variable quadratic equation. Bancroft’s method provides one or two solutions for the unknown quantities. When there are two (usually the case), only one is a near-Earth sensible solution.[153] When a receiver uses more than four satellites for a solution, Bancroft uses the generalized inverse (i.e., the pseudoinverse) to find a solution. A case has been made that iterative methods, such as the Gauss–Newton algorithm approach for solving over-determined non-linear least squares (NLLS) problems, generally provide more accurate solutions.[165] Leick et al. (2015) states that “Bancroft’s (1985) solution is a very early, if not the first, closed-form solution.”[166] Other closed-form solutions were published afterwards,[167][168] although their adoption in practice is unclear. Error sources and analysis Main article: Error analysis for the Global Positioning System GPS error analysis examines error sources in GPS results and the expected size of those errors. GPS makes corrections for receiver clock errors and other effects, but some residual errors remain uncorrected. Error sources include signal arrival time measurements, numerical calculations, atmospheric effects (ionospheric/tropospheric delays), ephemeris and clock data, multipath signals, and natural and artificial interference. Magnitude of residual errors from these sources depends on geometric dilution of precision. Artificial errors may result from jamming devices and threaten ships and aircraft[169] or from intentional signal degradation through selective availability, which limited accuracy to ≈ 6–12 m (20–40 ft), but has been switched off since May 1, 2000.[170][171] Accuracy enhancement and surveying Main article: GNSS enhancement Augmentation Integrating external information into the calculation process can materially improve accuracy. Such augmentation systems are generally named or described based on how the information arrives. Some systems transmit additional error information (such as clock drift, ephemera, or ionospheric delay), others characterize prior errors, while a third group provides additional navigational or vehicle information. Examples of augmentation systems include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Differential GPS (DGPS), inertial navigation systems (INS) and Assisted GPS. The standard accuracy of about 15 m (49 ft) can be augmented to 3–5 m (9.8–16.4 ft) with DGPS, and to about 3 m (9.8 ft) with WAAS.[172] Precise monitoring Accuracy can be improved through precise monitoring and measurement of existing GPS signals in additional or alternative ways. The largest remaining error is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but some errors remain. This is one reason GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency. Military receivers can decode the P(Y) code transmitted on both L1 and L2. Without decryption keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. This technique is slow, so it is currently available only on specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies. All users will then be able to perform dual-frequency measurements and directly compute ionospheric delay errors. A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). This corrects the error that arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite–receiver sequence matching) operation is imperfect. CPGPS uses the L1 carrier wave, which has a period of {\displaystyle {\frac {1\,\mathrm {s} }{1575.42\times 10^{6}}}=0.63475\,\mathrm {ns} \approx 1\,\mathrm {ns} \ }{\frac {1\,\mathrm {s} }{1575.42\times 10^{6}}}=0.63475\,\mathrm {ns} \approx 1\,\mathrm {ns} \ , which is about one-thousandth of the C/A Gold code bit period of {\displaystyle {\frac {1\,\mathrm {s} }{1023\times 10^{3}}}=977.5\,\mathrm {ns} \approx 1000\,\mathrm {ns} \ }{\frac {1\,\mathrm {s} }{1023\times 10^{3}}}=977.5\,\mathrm {ns} \approx 1000\,\mathrm {ns} \ , to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to 2–3 m (6 ft 7 in–9 ft 10 in) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 cm (1.2 in) of ambiguity. By eliminating this error source, CPGPS coupled with DGPS normally realizes between 20–30 cm (7.9–11.8 in) of absolute accuracy. Relative Kinematic Positioning (RKP) is a third alternative for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 cm (3.9 in). This is done by resolving the number of cycles that the signal is transmitted and received by the receiver by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK). Carrier phase tracking (surveying) Main article: GNSS enhancement § Carrier-phase tracking (surveying) Another method that is used in surveying applications is carrier phase tracking. The period of the carrier frequency multiplied by the speed of light gives the wavelength, which is about 0.19 m (7.5 in) for the L1 carrier. Accuracy within 1% of wavelength in detecting the leading edge reduces this component of pseudorange error to as little as 2 mm (0.079 in). This compares to 3 m (9.8 ft) for the C/A code and 0.3 m (1 ft 0 in) for the P code. 2 mm (0.079 in) accuracy requires measuring the total phase—the number of waves multiplied by the wavelength plus the fractional wavelength, which requires specially equipped receivers. This method has many surveying applications. It is accurate enough for real-time tracking of the very slow motions of tectonic plates, typically 0–100 mm (0.0–3.9 in) per year. Triple differencing followed by numerical root finding, and the least squares technique can estimate the position of one receiver given the position of another. First, compute the difference between satellites, then between receivers, and finally between epochs. Other orders of taking differences are equally valid. Detailed discussion of the errors is omitted. The satellite carrier total phase can be measured with ambiguity as to the number of cycles. Let {\displaystyle \ \phi (r_{i},s_{j},t_{k})}\ \phi (r_{i},s_{j},t_{k}) denote the phase of the carrier of satellite j measured by receiver i at time {\displaystyle \ \ t_{k}}\ \ t_{k}. This notation shows the meaning of the subscripts i, j, and k. The receiver (r), satellite (s), and time (t) come in alphabetical order as arguments of {\displaystyle \ \phi }\ \phi and to balance readability and conciseness, let {\displaystyle \ \phi _{i,j,k}=\phi (r_{i},s_{j},t_{k})}\ \phi _{i,j,k}=\phi (r_{i},s_{j},t_{k}) be a concise abbreviation. Also we define three functions, :{\displaystyle \ \Delta ^{r},\Delta ^{s},\Delta ^{t}}\ \Delta ^{r},\Delta ^{s},\Delta ^{t}, which return differences between receivers, satellites, and time points, respectively. Each function has variables with three subscripts as its arguments. These three functions are defined below. If {\displaystyle \ \alpha _{i,j,k}}\ \alpha _{i,j,k} is a function of the three integer arguments, i, j, and k then it is a valid argument for the functions, :{\displaystyle \ \Delta ^{r},\Delta ^{s},\Delta ^{t}}\ \Delta ^{r},\Delta ^{s},\Delta ^{t}, with the values defined as {\displaystyle \ \Delta ^{r}(\alpha _{i,j,k})=\alpha _{i+1,j,k}-\alpha _{i,j,k}}\ \Delta ^{r}(\alpha _{i,j,k})=\alpha _{i+1,j,k}-\alpha _{i,j,k}, {\displaystyle \ \Delta ^{s}(\alpha _{i,j,k})=\alpha _{i,j+1,k}-\alpha _{i,j,k}}\ \Delta ^{s}(\alpha _{i,j,k})=\alpha _{i,j+1,k}-\alpha _{i,j,k}, and {\displaystyle \ \Delta ^{t}(\alpha _{i,j,k})=\alpha _{i,j,k+1}-\alpha _{i,j,k}}\ \Delta ^{t}(\alpha _{i,j,k})=\alpha _{i,j,k+1}-\alpha _{i,j,k} . Also if {\displaystyle \ \alpha _{i,j,k}\ and\ \beta _{l,m,n}}\ \alpha _{i,j,k}\ and\ \beta _{l,m,n} are valid arguments for the three functions and a and b are constants then {\displaystyle \ (a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})}\ (a\ \alpha _{i,j,k}+b\ \beta _{l,m,n}) is a valid argument with values defined as {\displaystyle \ \Delta ^{r}(a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})=a\ \Delta ^{r}(\alpha _{i,j,k})+b\ \Delta ^{r}(\beta _{l,m,n})}\ \Delta ^{r}(a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})=a\ \Delta ^{r}(\alpha _{i,j,k})+b\ \Delta ^{r}(\beta _{l,m,n}), {\displaystyle \ \Delta ^{s}(a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})=a\ \Delta ^{s}(\alpha _{i,j,k})+b\ \Delta ^{s}(\beta _{l,m,n})}\ \Delta ^{s}(a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})=a\ \Delta ^{s}(\alpha _{i,j,k})+b\ \Delta ^{s}(\beta _{l,m,n}), and {\displaystyle \ \Delta ^{t}(a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})=a\ \Delta ^{t}(\alpha _{i,j,k})+b\ \Delta ^{t}(\beta _{l,m,n})}\ \Delta ^{t}(a\ \alpha _{i,j,k}+b\ \beta _{l,m,n})=a\ \Delta ^{t}(\alpha _{i,j,k})+b\ \Delta ^{t}(\beta _{l,m,n}) . Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch.[173] This difference is designated as {\displaystyle \ \Delta ^{s}(\phi _{1,1,1})=\phi _{1,2,1}-\phi _{1,1,1}}{\displaystyle \ \Delta ^{s}(\phi _{1,1,1})=\phi _{1,2,1}-\phi _{1,1,1}} Double differencing [174] computes the difference of receiver 1’s satellite difference from that of receiver 2. This approximately eliminates satellite clock errors. This double difference is: {\displaystyle {\begin{aligned}\Delta ^{r}(\Delta ^{s}(\phi _{1,1,1}))\,&=\,\Delta ^{r}(\phi _{1,2,1}-\phi _{1,1,1})&=\,\Delta ^{r}(\phi _{1,2,1})-\Delta ^{r}(\phi _{1,1,1})&=\,(\phi _{2,2,1}-\phi _{1,2,1})-(\phi _{2,1,1}-\phi _{1,1,1})\end{aligned}}}{\begin{aligned}\Delta ^{r}(\Delta ^{s}(\phi _{1,1,1}))\,&=\,\Delta ^{r}(\phi _{1,2,1}-\phi _{1,1,1})&=\,\Delta ^{r}(\phi _{1,2,1})-\Delta ^{r}(\phi _{1,1,1})&=\,(\phi _{2,2,1}-\phi _{1,2,1})-(\phi _{2,1,1}-\phi _{1,1,1})\end{aligned}} Triple differencing [175] subtracts the receiver difference from time 1 from that of time 2. This eliminates the ambiguity associated with the integral number of wavelengths in carrier phase provided this ambiguity does not change with time. Thus the triple difference result eliminates practically all clock bias errors and the integer ambiguity. Atmospheric delay and satellite ephemeris errors have been significantly reduced. This triple difference is: {\displaystyle \ \Delta ^{t}(\Delta ^{r}(\Delta ^{s}(\phi _{1,1,1})))}\ \Delta ^{t}(\Delta ^{r}(\Delta ^{s}(\phi _{1,1,1}))) Triple difference results can be used to estimate unknown variables. For example, if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs may be sufficient to solve for receiver 2’s three position components. This may require a numerical procedure.[176][177] An approximation of receiver 2’s position is required to use such a numerical method. This initial value can probably be provided from the navigation message and the intersection of sphere surfaces. Such a reasonable estimate can be key to successful multidimensional root finding. Iterating from three time pairs and a fairly good initial value produces one observed triple difference result for receiver 2’s position. Processing additional time pairs can improve accuracy, overdetermining the answer with multiple solutions. Least squares can estimate an overdetermined system. Least squares determines the position of receiver 2 that best fits the observed triple difference results for receiver 2 positions under the criterion of minimizing the sum of the squares. Regulatory spectrum issues concerning GPS receivers In the United States, GPS receivers are regulated under the Federal Communications Commission’s (FCC) Part 15 rules. As indicated in the manuals of GPS-enabled devices sold in the United States, as a Part 15 device, it “must accept any interference received, including interference that may cause undesired operation.”[178] With respect to GPS devices in particular, the FCC states that GPS receiver manufacturers, “must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum.”[179] For the last 30 years, GPS receivers have operated next to the Mobile Satellite Service band, and have discriminated against reception of mobile satellite services, such as Inmarsat, without any issue. The spectrum allocated for GPS L1 use by the FCC is 1559 to 1610 MHz, while the spectrum allocated for satellite-to-ground use owned by Lightsquared is the Mobile Satellite Service band.[180] Since 1996, the FCC has authorized licensed use of the spectrum neighboring the GPS band of 1525 to 1559 MHz to the Virginia company LightSquared. On March 1, 2001, the FCC received an application from LightSquared’s predecessor, Motient Services, to use their allocated frequencies for an integrated satellite-terrestrial service.[181] In 2002, the U.S. GPS Industry Council came to an out-of-band-emissions (OOBE) agreement with LightSquared to prevent transmissions from LightSquared’s ground-based stations from emitting transmissions into the neighboring GPS band of 1559 to 1610 MHz.[182] In 2004, the FCC adopted the OOBE agreement in its authorization for LightSquared to deploy a ground-based network ancillary to their satellite system – known as the Ancillary Tower Components (ATCs) – “We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service.”[183] This authorization was reviewed and approved by the U.S. Interdepartment Radio Advisory Committee, which includes the U.S. Department of Agriculture, U.S. Space Force, U.S. Army, U.S. Coast Guard, Federal Aviation Administration, National Aeronautics and Space Administration (NASA), U.S. Department of the Interior, and U.S. Department of Transportation.[184] In January 2011, the FCC conditionally authorized LightSquared’s wholesale customers—such as Best Buy, Sharp, and C Spire—to only purchase an integrated satellite-ground-based service from LightSquared and re-sell that integrated service on devices that are equipped to only use the ground-based signal using LightSquared’s allocated frequencies of 1525 to 1559 MHz.[185] In December 2010, GPS receiver manufacturers expressed concerns to the FCC that LightSquared’s signal would interfere with GPS receiver devices[186] although the FCC’s policy considerations leading up to the January 2011 order did not pertain to any proposed changes to the maximum number of ground-based LightSquared stations or the maximum power at which these stations could operate. The January 2011 order makes final authorization contingent upon studies of GPS interference issues carried out by a LightSquared led working group along with GPS industry and Federal agency participation. On February 14, 2012, the FCC initiated proceedings to vacate LightSquared’s Conditional Waiver Order based on the NTIA’s conclusion that there was currently no practical way to mitigate potential GPS interference. GPS receiver manufacturers design GPS receivers to use spectrum beyond the GPS-allocated band. In some cases, GPS receivers are designed to use up to 400 MHz of spectrum in either direction of the L1 frequency of 1575.42 MHz, because mobile satellite services in those regions are broadcasting from space to ground, and at power levels commensurate with mobile satellite services.[187] As regulated under the FCC’s Part 15 rules, GPS receivers are not warranted protection from signals outside GPS-allocated spectrum.[179] This is why GPS operates next to the Mobile Satellite Service band, and also why the Mobile Satellite Service band operates next to GPS. The symbiotic relationship of spectrum allocation ensures that users of both bands are able to operate cooperatively and freely. The FCC adopted rules in February 2003 that allowed Mobile Satellite Service (MSS) licensees such as LightSquared to construct a small number of ancillary ground-based towers in their licensed spectrum to “promote more efficient use of terrestrial wireless spectrum.”[188] In those 2003 rules, the FCC stated “As a preliminary matter, terrestrial [Commercial Mobile Radio Service (“CMRS”)] and MSS ATC are expected to have different prices, coverage, product acceptance and distribution; therefore, the two services appear, at best, to be imperfect substitutes for one another that would be operating in predominantly different market segments… MSS ATC is unlikely to compete directly with terrestrial CMRS for the same customer base…”. In 2004, the FCC clarified that the ground-based towers would be ancillary, noting that “We will authorize MSS ATC subject to conditions that ensure that the added terrestrial component remains ancillary to the principal MSS offering. We do not intend, nor will we permit, the terrestrial component to become a stand-alone service.”[183] In July 2010, the FCC stated that it expected LightSquared to use its authority to offer an integrated satellite-terrestrial service to “provide mobile broadband services similar to those provided by terrestrial mobile providers and enhance competition in the mobile broadband sector.”[189] GPS receiver manufacturers have argued that LightSquared’s licensed spectrum of 1525 to 1559 MHz was never envisioned as being used for high-speed wireless broadband based on the 2003 and 2004 FCC ATC rulings making clear that the Ancillary Tower Component (ATC) would be, in fact, ancillary to the primary satellite component.[190] To build public support of efforts to continue the 2004 FCC authorization of LightSquared’s ancillary terrestrial component vs. a simple ground-based LTE service in the Mobile Satellite Service band, GPS receiver manufacturer Trimble Navigation Ltd. formed the “Coalition To Save Our GPS.”[191] The FCC and LightSquared have each made public commitments to solve the GPS interference issue before the network is allowed to operate.[192][193] According to Chris Dancy of the Aircraft Owners and Pilots Association, airline pilots with the type of systems that would be affected “may go off course and not even realize it.”[194] The problems could also affect the Federal Aviation Administration upgrade to the air traffic control system, United States Defense Department guidance, and local emergency services including 911.[194] On February 14, 2012, the FCC moved to bar LightSquared’s planned national broadband network after being informed by the National Telecommunications and Information Administration (NTIA), the federal agency that coordinates spectrum uses for the military and other federal government entities, that “there is no practical way to mitigate potential interference at this time”.[195][196] LightSquared is challenging the FCC’s action.[needs update] Other systems Main article: Satellite navigation Orbit size comparison of GPS, GLONASS, Galileo, BeiDou-2, and Iridium constellations, the International Space Station, the Hubble Space Telescope, and geostationary orbit (and its graveyard orbit), with the Van Allen radiation belts and the Earth to scale.[a] The Moon’s orbit is around 9 times as large as geostationary orbit.[b] (In the SVG file, hover over an orbit or its label to highlight it; click to load its article.) Other notable satellite navigation systems in use or various states of development include: Beidou – system deployed and operated by the People’s Republic of China’s, initiating global services in 2019.[197][198] Galileo – a global system being developed by the European Union and other partner countries, which began operation in 2016,[199] and is expected to be fully deployed by 2020. GLONASS – Russia’s global navigation system. Fully operational worldwide. NavIC – A regional navigation system developed by the Indian Space Research Organisation. Michibiki – A regional navigation system receivable in the Asia-Oceania regions, with a focus on Japan Source: Wikipedia.- Facebook Comments # Portable Phone Charger Power Bank ### Compare similar products # Anker PowerCore 13000, Compact 13000mAh 2-Port Ultra-Portable Phone Charger Power Bank with PowerIQ and VoltageBoost Technology for iPhone, iPad, Samsung Galaxy (Black) • Amazon Choice • The Anker Advantage: Join the 30 million+ powered by our leading technology. • Upgraded Capacity: The successor to PowerCore 10400—identical compact size but packed with even more power. Charges an iPhone 8 almost 5 times, an iPhone X or Samsung Galaxy S9 3 times, or an iPad Air 2 once. • Light and Compact: A super-high 13000 mAh capacity with two high speed USB ports is all contained in a portable charger smaller than a wallet. • High-Speed Charging: PowerIQ and VoltageBoost combine to deliver a max speed charge to any device (up to 3A – Qualcomm Quick Charge not supported). Input: 5V / 2A. • What You Get: Anker PowerCore 13000 Portable Charger, Micro USB cable, travel pouch, welcome guide, our worry-free 18-month warranty and friendly customer service. ## PowerCore 13000 The successor to PowerCore 10400. Exactly the same small-size but with even more power squeezed into its robust casing. No larger than a wallet but able to charge an iPhone 4 times.Powered by Anker’s world-famous charging technology to provide a high-speed charge, no matter the device. li> High-Speed Charging • 30% Lighter Than Other Chargers • Easy to Use and Carry with You • Simultaneous Dual-Port Charging ## Super-High Capacity Enough power to keep you going for days. Charge an iPhone X over 3 times, a Galaxy S9 3 times, or an iPad mini 4 over 1-and-a-half times. ## PowerIQ Technology By intelligently identifying any connected device, PowerIQ delivers the optimum, high-speed charge to all devices. Includes Apple and Android phones and tablets as well as cameras, headphones, and more. ## Superior Portability PowerCore 13000’s huge power is squeezed into a pocket-sized body that is ready to go anywhere you do. It’s the perfect partner whenever, and wherever you need to charge. ## Reviews: Top positive review See all 5,735 positive reviews on amazon › Paulina 5.0 out of 5 stars MUST BUY June 24, 2017 This thing is so damn reliable. I bought it in preparation for an 8 day Hawaii vacation where I’d be out and about the entire day doing tours and activities. It was there when I needed it, especially when I used my phone to take lots and lots of pictures so that drained my battery very quickly. I could even charge my fiances phone too!!! Whenever I remembered that this thing had TWO connections I gave myself a pat on the back and made a mental note to write a five-star review for this when I got back from Hawaii. Now this is an essential item in my purse as well and I don’t go anywhere without it. Anker is so amazing and they have gained a loyal customer. Deborah Culp-Hook No more charging worries for me This is my second Anker PowerCore. I love having the security of all my mobile electronics being easily recharged regardless of where I am. My Galaxy phone has a limited battery life when I am actively on it and I have found myself out and about with no charging capabilities for the last time. I bought a second so that I would always have one in my purse. I keep one of the PowerCores charging overnight so I don’t ever have to be without. This new one is slightly larger and heavier so it is better for long travel. Can easily watch movies on my Kindle when traveling on a plane or waiting in an aurport…not always trying to jockey for positions in airports with recharging stations, and don’t worry that some evil person has installed something on public charging stations to rip off my info. Hotels that don’t have a plug by the bed? Not a problem any more. Safety in having my phone right by me at night when on the road is important to me. This was a great buy for increased peace of mind. PapaZhan Excellent Power Backup for a Great Price This is a very good power bank and an excellent value. Charging it is accomplished by a micro USB input on the top of the unit. There are 4 blue LED indicator lights on the unit that show the level of charge and they turn off when a full charge is reached. The reserve capacity is incredible for the physical size of the unit and should run any current smart phone for a couple days off almost non stop high draw use. A lot of us use our phones as camera and camcorder replacements when on the go and this unit provided ample power to take as much video and shoot as many pictures as a person could manage in a day. Highly recommended and very happy with purchase. myminimaliststyle Powerful Portable Charger! This is the best portable charger that I have used. This thing is a beast! I had a different brand that wasn’t as powerful and couldn’t full charge my iPad Pro 9.7″. So I thought I would give this Anker PowerCore a try. Well, I am glad that I did! This charger takes all night to charge up to its full capacity. I can charge my iPhone 6s Plus and my iPad Pro 9.7″ on this charger and make it through entire day and the charger will have no charge left. Which works out fine for me, since I usually heading back home. Then I charge it up again for the next usage. I love that this battery charger comes with its own drawstring bag! I keep my iPhone cable charger in the bag when I travel with my Anker PowerCore battery charger. This is a great product. I’m very pleased with this product. recordmaven Good solid unit – but be sure to allocate enough time to charge up this big battery So far, this is a very good battery. At 13000mAh, it has a lot of juice to provide multiple charges for an iPhone or other device. It’s not too big or heavy. Because of its capacity, it does take a while to charge – note comments from other reviewers. If all you have is one of those small iPhone chargers, it’s going to take a lot of hours to fill this up. Therefore, if you don’t already have some kind of high capacity USB charger with multiple USB outlets on it, I would recommend taking advantage of the combo offer that Amazon typically offers with this unit, to include a charger and a case. When I added a high-current charger to the picture, the charge time really dropped. Even with a high-current charger, it does take some hours to charge this up from a low charge, though – so be sure to allocate the time. On a medium-current charger, the initial charge-up took about 10 hours. Facebook Comments # Anti Theft for Iphone Tables with Security Case Holder ## Anti-Theft Tablet Security Case Holder – Metal Heavy Duty Vesa Wall Mount Tablet Kiosk, Mounts on Surface, Landscape/Portrait Mounting, Designed for iPad 2, 3, 4, Air, Air 2 Tablets – Pyle PSPADLKW06 ### Product description Safely and securely place your iPad on display. The Pyle iPad anti-theft wall mount Stand can be installed on any wall or flat surface. Rugged and durable steel construction provides device security where needed. Allow Convenient access to your iPad when installed in high traffic areas like in an office, lobby, front desk, customer counter or showroom floor. Choose between landscape and portrait iPad orientation to fit any displaying need. You can also use the included accessory Tab to either allow or block access to the home button. The PSPADLKW06 anti-theft iPad Stand comes with all the necessary mounting hardware and brackets to grant safe and Secure access to your device — all while adding a professional and clean look. Compatible with iPad 2nd, 3rd, 4th generations, and iPad Air, iPad Air 2. #### Product information • Product Dimensions 9 x 1 x 12.2 inches • Item Weight 2.97 pounds • Shipping Weight 3 pounds (View shipping rates and policies) • ASIN B01BH8TW7K • Item model number PSPADLKW06 • Customer Reviews 4.2 out of 5 stars 29 customer reviews 4.2 out of 5 stars #### Warranty & Support Manufacturer’s warranty can be requested from customer service. Click here to make a request to customer service. Feedback If you are a seller for this product, would you like to suggest updates through seller support? Would you like to tell us about a lower price? ### Customer Reviews 1-Great product. Works perfectly. Fits iPad and other tablets well. Good quality. Also if you want it in black, just buy a cheap can of spray paint and it’s still better deal than the alternatives that already come in black and customer service guarantee make this a great buy. 2-Perfect fit of my iPad Air 1. High quality case and very good looking. The case I installed on the wall look like a picture frame. Inside the case has four posts with soft plastic touched the edge of the iPad only. Very good protection and wouldn’t scratch the iPad. Highly recommended!. 3-Product does what it is suppost to do, it holds an Ipad for Wallmounting, all needed material is delivered in the box. Could have been a little nicer finish, f.e if the lid was going over the edges, you wouldn’t see the small seam. 4-Installed easily. 5-ordered it for the office. I liked everything, but there was only one screw in the kit (D in the manual) than to replace it and how to use it until we know. 6-My autistic child broke 6 ipads before we discovered this. It allows her to play freely while keeping it one secure location. 7-I got it streaight out of the box and I have a big yellow stain on the frame. Complete ripoff and disappointment for a product used in a professional environment!. 8-Works as intended, happy with my purchase. 9-Well design and flexable wall orientation worth every penny. 10-Well built and it provides great protection for a reasonable price. 11-Great product! This case was very easy to install and durable! 12-Looks very solid but the hole for the charger’s cable need a redesign. It pinch the cable. 13-Good but no longer available. 14-HOrrible. the case is made for the huge Ipad. it is not as described in information. 15-Just as described, if not better… IPad 2 fit perfectly and quality is good. No issues with cable pinching or routing. I have mine mounted as a weather station and cookbook in our kitchen. Love it. 16-Works as it should, solid metal construction, nice padded rubbery inside to keep ipad undamaged and secured within the mount. 17-It’s well constructed and looks nice. You have the option of mounting it to an arm mount or straight onto a wall. You could probably use it without using the screws to close it up, in case you wanted to pull it out often. The tamper proofing and all that works nice and overall spacing/alignment is spot on. 18-As shown in the pictures, the mount takes up quite a bit more space than the naked iPad would, so be aware of that when planning placement on a wall. 19-my fault, but I had to drill a hole for the audio jack. 20-I should have looked more closely… I had to modify it to fit the charging cable and a hole for an audio cable. However, if I didn’t need those things – this thing is well made and looks good. 21-ok but it will reduce the wifi signal. 22-Perfect fit for the iPad 4, thabks. 23-Installed for a friend in their workout studio, very professional look and extremely secure. 24-Good quality and fast shipping. 25-very good item, great. 26-fast, good price, love it!. 27-2.0 out of 5 starsDoesn’t really fit an iPad 3. 28-The construction is great and the metal is sturdy. However, it’s really hard to get the ipad locked in place and then have the cover attached because there are no latches or catches for the ipad to be held in place. Only four rubber knobs surround the frame of the ipad. My iPad 3 also did not fit properly. the sides all bulge out of the frame with a really big gap that can easily slide in a dime. It appears that the foam strip that protects the back from getting scratched by the mounting screws is too thick for the ipad 3. There are no adjustments possible to accommodate for the ipad 3’s thicker casing. There is a bit of pinching of the original Apple 30 pin charging cable but not so much that I would worry about it. The audio port is sadly inaccessible. The microphone works ok but looses a bit of sensitivity when I try “Hey Siri”. The audio is of course a bit muted but still audible. None of the buttons are accessible except for the home button. There is even a special cover to prevent the home button from being pressed if fits your purpose. Volume controls will be really hard to reach as the control centre that requires swiping up is almost impossible, as the frame is a perfect fit for the screen (along with the cutout for the front facing camera). Sadly, the frame that has too little tolerance for the ipad 3’s thicker profile ruins an otherwise perfectly made product. Works well for what we needed. Facebook Comments # Shark Rocket Ultralight Vacuum Cleaner + Car Detail Kit (Certified Refurbished) ## Shark Rocket Ultralight ### Product description The Certified Factory Refurbished Shark Rocket Vacuum provides versatility for floor to ceiling cleaning. The advanced swivel steering allows you to get into hard to reach places, on both carpets and bare floors, while weighing under 8 pounds. ### About Amazon Renewed Amazon Renewed is your trusted destination for pre-owned and refurbished products that are inspected and tested to work and look like new. A Renewed supplier who is Amazon-qualified and performance managed, performs a full diagnostic test, replaces any defective parts, and thoroughly cleans the product. The products will have minimal to no signs of wear, no visible cosmetic imperfections when held 12 inches away, and may arrive in a brown or white box with relevant accessories that may be generic. This product comes with a minimum 90-day supplier-backed warranty An Amazon qualified supplier will provide a replacement or refund within 90 days of your receipt if the product does not work as expected. The warranty is in addition to Amazon’s standard return policy. Learn more Shop for smartphones, computers, laptops, tablets, home and kitchen appliances, game consoles, office products, and more on Amazon Renewed. ### Product information • Product Dimensions 10.7 x 12.1 x 32.3 inches • Item Weight 13.5 pounds • Shipping Weight 13.5 pounds (View shipping rates and policies) • Manufacturer Shark|Ninja • ASIN B01GHELINU • Customer Reviews 4.3 out of 5 stars 33 customer reviews • 4.3 out of 5 stars • Best Sellers Rank #121,184 in Home & Kitchen (See Top 100 in Home & Kitchen) #93 in Home & Kitchen > Vacuums & Floor Care > Vacuums > Stick Vacuums & Electric Brooms Warranty & Support • Product Warranty: For warranty information about this product, please click here Feedback • If you are a seller for this product, would you like to suggest updates through seller support? • Would you like to tell us about a lower price? #### Amazon reviews • Bought mine after using this product at a house I stayed at for extended period. Love it. My only complaint is refurbished item doesn’t include wall hanger. When using vacuum, you have to lean it against a wall if you need both hands. It’s top heavy & doesn’t stand upright by itself. Easy empty of dust canister. Overall light weight & easy to maneuver. • It’s a little top-heavy, fix up pretty good that just not that impressed with it. The best thing I could say about is it was affordable. Usually like Shark products, but, not this one. • So far sooooo good. I am DumbFounded at the amount of dirt it has sucked out of my carpets!!!. • Good as new. • Design enables easy stow away so that its not taking up a lot of space in closet or sitting out. Sleek design doesn’t sacrifice power, sucks hard. • Love it. However, I do wish that there was a way to attach/store the attachments to the vacuum itself. • Product is terrible. Vacuum picks up lots, but the collection bucket does not stay closed. So dirt and junk fly all over the room I’m trying to clean! Very unhappy. Did anyone test this?. • I like it working great! , accepted the cup holding dust broke out on me, when I try to empty the dust every thing falling apart. • Best vacuum I have ever owned. • Good product, light for carry and do the job very well. • How light it is for vacuuming steps and furniture. Good suction as well. • Great vacuum like everything about it. • Worked as expected. Good suction. • My friend’s mother cleans houses for a living and told me that this was the inexpensive vacuum that all the rich people had. I got one and I really could not believe how well this thing works. I really can’t say enough good things about this light and effective tool. This is the vacuum a Dyson wishes it could be. • I am very happy with my purchase. No complaints. • It is definitely a used product. Despite the scratched surfaces, the vacuum works great. So easy to turn with just a flick of you wrist. It’s light enough for me to do stairs with and strong enough for me to clean the whole house. • Good vacuum. Due to the heavy canister up top the vacuum does not stand up right without leaning against something. • I LOVE this vacuum! The cord is extra long. There are so many configurations with all of the attachments, it’s perfect for all my cleaning needs (stairs, mini blinds, nooks & cranies, carpet, bare floor and even cleaning our vehicles). There is one down fall and that is the size of the dirt canister. I just bring a small waste basket in each room. We have a very large dog that sheds a lot as well as a lot of high traffic areas which require me to empty the canister frequently. I still give this 5 stars as it has done the best job out of any product I have ever owned. It is very versatile and light weight. We even brought it along in our camper this past holiday weekend. I would 100% purchase this again. And, I purchased this as a refurbished item. • This vacuum is simply awesome. I’ve had the350 simplicity and a $600 Dyson. This shark does everything those will do and everything they couldn’t do. Superlight supersmart very navigable. Great suction and ample attachments included. The powerhead is somewhat narrow compared to others but with all the advantages that’s easily overlooked. Works great on hardwood and laminate Floors. As well as rugs and carpet. The dirt cup is somewhat small but easy to empty. Pics at pet hair nicely as I have a husky and a pitbull. Love the fact that he has no bags and washable filters. However if there is only one thing I could change it would be that the brush roll has an on off feature. It does have a high and low speed but no off. Having said that,Shark does make a model that does have both Highlow and off.I bought a refurbished unit that look brand new as well as functions like new for under$100. I’ve had mine for about three months now and I am still loving it. Would definitely recommend you will not be disappointed.
• Unfortunately, this unit was not refurbished as advertised. The hinge that held the dust cup on was broken and partially missing. The dust cup would not stay on and dumped itself on my rugs and furniture a couple of times until I got smarter and found a phone number for Shark on the item directions. They could not have been nicer, and they said that even though they did not sell the item, the company would make good on the purchase. They sent me a new motor with the hinged dust cup intact. I feel the seller was not thorough in “refurbishing” the unit, however I do have a working unit now due to the Shark Company’s response.
• I love this! I only wished I’d gotten he cordless version. I have a very small house so if it runs for 20 minutes that would have been fine with me.
I will say it is top heavy BUT if you relax your wrist and stop trying to force it to turn it will turn more easily with less pressure on your wrist. It’s also so easy to clean. Just pop the top off, hold it over the trash can, and press the button. If you miss the trash can you have the vacuum right in your hand to vacuum it up again and dump it again. It goes smoothly from floor to carpet and does an excellent job on my wool rug. I would buy it again and would buy it as a gift.
• Great! does everything I need it to do.
• One of the best vacuums I’ve ever purchased, though the hinge did break and I’m still trying to figure out how to repair. Not sure why that would break so easily; vacuum isn’t that old.
• Works great on bare floors also! No bending over to get under things
• Cup is kinda small so I have to empty it once or twice per vacuuming session but impressed with how much it picks up. Doesn’t stand up independently if left put together but it comes apart very easily so I just stand the bottom part in the corner and put the main part up on a shelf.
• I have one of these already and was pleased with it. Got this one for my sister in law. Price very good, and quality is fine.
• it works great. I really like that it is light weight and no big bag to empty
• Did not come with the wall mount other than that works great.
• It is so light perfect
• Lightweight but powerful.
• A outstanding value and even better vac!.

# Whatever It Takes-From The Album Evolve

## Whatever it takes

Imagine Dragons
From the álbum Evolve
June 23, 2017

### Product details

• Original Release Date: June 23, 2017
• Release Date: May 9, 2017
• Label: KIDinaKORNER/Interscope Records
• Duration: 3:21 minutes
• Genres:Alternative Rock
• ASIN: B071L3H1GT
• Average Customer Review: 4.9 out of 5 stars 61 customer reviews
• Amazon Best Sellers Rank: #3 Paid in Songs (See Top 100 Paid in Songs) #1 in Digital Music > Songs > Alternative Rock