The Global Positioning System
Atomic Clocks, Relativity, and Knowing Where You Are
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This guide explains how GPS works through chapters covering the space segment, control segment, and user segment. It details message formats, satellite frequencies, and demodulation processes. The book covers pre-GPS navigation methods, the system's development history, and why selective availability was removed in 2000.
Beyond traditional navigation, GPS now supports timing, surveying, and even smartphone location services. Readers will find practical information about receiver operation, least squares calculations, and regulatory issues affecting civilian use. Anyone working with positioning technology or interested in how satellites enable modern navigation will find this worth their time.
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The GPS system began in 1973 when the U.S. Department of Defense started working on it. A prototype satellite launched in 1978, and by 1993 the full system of 24 satellites was up and running. Then, after Korean Air Lines Flight 007 was shot down in 1983 for entering Soviet airspace, President Ronald Reagan decided to open GPS to civilian use in 1988. But at first, civilian accuracy was limited to about 100 meters because of a feature called Selective Availability, which added intentional errors into the signals—errors that only military receivers could fix.
As GPS use by civilians grew, the need to fix the system's errors became clear. During the Gulf War, the military disabled the Selective Availability system temporarily because there weren’t enough GPS units for soldiers, so many used civilian ones sent from home. In the 1990s, systems like those from the US Coast Guard and Federal Aviation Administration began offering local corrections, cutting down on both SA issues and atmospheric interference. The U.S. military also developed ways to jam signals locally, removing the need for global degradation. Because of this, President Bill Clinton signed a bill in 2000 ordering that Selective Availability be turned off by May 1. Then, in 2007, the U.S. government announced future GPS satellites wouldn’t include that feature either.
The Global Positioning System has undergone continuous upgrades, with efforts to modernize it leading to the development of GPS Block III satellites and the Next Generation Operational Control System, or OCX, which was authorized by U.S. Congress in 2000. That system was later canceled in April 2026. When Selective Availability was turned off, GPS accuracy improved to about five meters. Receivers using the L5 band can now achieve thirty centimeters of accuracy, and high-end tools used in engineering or surveying reach two centimeters or even less with extended measurements. Smartphones, with support from services like Wi-Fi positioning, can deliver accuracy as good as four point nine meters or better.
As of March 2026, 21 GPS satellites are already broadcasting L5 signals, though those signals are still labeled as pre-operational. The full network is expected to include 24 satellites, which will be fully operational by 2027. That’s when the system will reach its intended capacity and provide complete coverage for positioning and timing services around the globe.
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The GPS system began in 1973, when the U.S. Department of Defense started working on a new navigation tool to fix problems with older systems. It drew on ideas from earlier projects, including secret studies from the 1960s. The system was built using 24 satellites and was meant for military use at first, becoming fully operational in 1993. Civilian access began in 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 recognized as the inventors. Gladys West’s work at the Ballistic Sciences Branch in Dahlgren Naval Proving Ground was key too—her contributions helped create the mathematical models needed to accurately track satellites.
The Global Positioning System builds on earlier radio-navigation methods like LORAN and the Decca Navigator System, which were developed in the 1940s. In 1955, Friedwardt Winterberg suggested using atomic clocks aboard satellites to test Einstein’s theory of general relativity. According to both special and general relativity, time moves faster in weaker gravitational fields, so clocks on GPS satellites would appear to run 38 microseconds fast each day compared to those on Earth. The system’s design includes corrections for this effect to ensure accuracy.
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When the Soviet Union launched Sputnik 1 in 1957, two physicists at Johns Hopkins University’s Applied Physics Laboratory, William Guier and George Weiffenbach, tracked its radio signals and recognized quickly that the Doppler effect could be used to follow its path through space. The lab's director allowed them access to their UNIVAC I computer to handle the complicated calculations. The next year, Frank McClure, who was deputy director of the APL, tasked Guier and Weiffenbach with tackling the reverse problem: figuring out where a user was located based on the satellite’s position. This challenge came from the Navy’s need to know submarine locations for the Polaris missile program. That work eventually led to the creation of the TRANSIT system. In 1959, ARPA became involved in the project, which would later be renamed DARPA in 1972.
In 1960, the TRANSIT system was first tested successfully, using a group of five satellites and offering navigation fixes about once every hour. By 1967, the U.S. Navy had developed the Timation satellite, which showed that precise clocks could work in space—a key step needed for GPS. During the 1970s, the OMEGA navigation system, which relied on comparing signals from pairs of ground stations, became the first global radio navigation network. But because of its limits, there was a growing need for a better, more accurate way to find locations anywhere on Earth.
Although accurate navigation was needed by both military and civilian users, few saw it as worth the billions required for satellite development and operation. During the Cold War, the nuclear threat to the U.S. was the one issue that Congress deemed justification for the expense. That’s why GPS was funded, and why it remained highly classified at the time. The nuclear triad included submarine-launched ballistic missiles (SLBMs), strategic bombers, and intercontinental ballistic missiles (ICBMs). The Navy needed precise positioning to launch SLBMs from submarines, while the Air Force, which controlled two-thirds of the triad, also required a better navigation system. Both branches worked separately on similar problems. There was even a proposal to use mobile launch platforms for ICBMs, which would require the same kind of location accuracy as SLBMs.
In 1960, the Air Force proposed a system called MOSAIC, or Mobile System for Accurate ICBM Control, which was basically a three-dimensional version of LORAN. A follow-up study in 1963, known as Project 57, is where the idea that would become GPS first took shape. That same year, the concept moved forward under Project 621B, which had many features similar to what we now recognize as GPS and promised better accuracy for U.S. Air Force bombers and ICBMs. The Navy’s TRANSIT system wasn’t fast enough for those needs, so the Naval Research Laboratory kept improving their Timation satellites, launching the first in 1967, the second in 1969, the third in 1974 with the first atomic clock in space, and the fourth in 1977.
In 1964, the United States Army launched its first Sequential Collation of Range satellite, known as SECOR, which was used for geodetic surveying. The system relied on three ground-based transmitters at fixed locations sending signals to a satellite transponder in orbit. A fourth ground station, located somewhere unknown, could then determine its own position by using those signals. This process allowed for very precise location fixing. The final SECOR satellite was sent into space in 1969.
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In the 1960s, engineers merged technologies from systems like 621B, Transit, Timation, and SECOR into a new multi-service program that would evolve into the Global Positioning System. A key issue was minimizing errors from gravitational variations and radar refraction. From 1970 to 1973, Harold L. Jury’s team at Pan Am Aerospace Division in Florida worked on this problem using real-time data assimilation and recursive estimation. Their efforts reduced both systematic and residual errors to a point where accurate navigation became feasible.
In 1973, a group of about twelve military officers met at the Pentagon during Labor Day weekend to plan what would become the Defense Navigation Satellite System, or DNSS. That meeting marked the moment when the idea truly came together. The program was later named Navstar. The name wasn’t an acronym but just a sound that worked well. As the satellites were called Navstar, the full system of them became known as Navstar-GPS. Between 1978 and 1985, ten Block I prototype satellites were launched, with one lost during launch.
In 1974, work at the Air Force Geophysical Research Lab, formerly known as the Air Force Cambridge Research Laboratory, focused on how the ionosphere affects radio signals. That year, Australian space scientist Elizabeth Essex-Cohen studied how radio waves from Navstar satellites bend as they pass through the ionosphere. Her research contributed to the development of the Klobuchar model, which helps correct GPS locations for ionospheric delays. This model was crucial for improving the accuracy of positioning data provided by the Global Positioning System.
After a tragic incident in 1983, when Korean Air Lines Flight 007, a Boeing 747 with 269 people on board, was shot down by a Soviet interceptor after going off course due to navigation errors near the Sakhalin and Moneron Islands, President Ronald Reagan ordered that GPS be made available for civilian use once it was developed enough. The first Block II satellite launched February 14, 1989, and the final satellite went up in 1994. At that point, the program’s cost, not including user equipment but covering launches, had reached about $5 billion—roughly $11 billion in today’s money.
Initially, the most accurate GPS signals were saved for military use, while civilians received deliberately weakened ones through a policy called Selective Availability. That changed on May 1, 2000, when President Bill Clinton signed a directive to end Selective Availability and give civilians the same level of accuracy as the military. The idea came from U.S. Secretary of Defense William Perry, who noted that private companies were already improving civilian access through differential GPS services. At the same time, the military was working on systems to block GPS signals in specific regions to deny them to enemies. Selective Availability was officially removed from the GPS system with the launch of GPS-III satellites.
Since its deployment, the U.S. has been improving GPS, adding new signals for civilian use and making the system more accurate and reliable for everyone, while still working with older equipment. The U.S. Department of Defense has kept upgrading the satellite system through a series of new launches to meet needs from the military, civilians, and businesses. By early 2015, the Standard Positioning Service gave horizontal accuracy better than 3.5 meters, though conditions like antenna quality and weather could change that.
The GPS system is managed by the U.S. government, with the Department of Defense in charge. From 1996 through 2004, the Interagency GPS Executive Board handled policy decisions. In 2004, a new body called the National Space-Based Positioning, Navigation and Timing Executive Committee was formed by presidential order to guide federal agencies on GPS and related technologies. This committee is co-chaired by the deputy secretaries of Defense and Transportation. Its members include senior officials from the Departments of State, Commerce, and Homeland Security, along with the Joint Chiefs of Staff and NASA. The executive office of the president attends as observers, and the FCC chairman joins as a liaison.
The U.S. Department of Defense must guarantee that a Standard Positioning Service is always available worldwide, as required by law. This service has specific definitions laid out in official documents, and it must operate continuously. The Department also needs to protect GPS from being used in harmful ways by enemies, all while ensuring civilian users aren’t unnecessarily disturbed or damaged by such efforts.
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On February 10, 1993, the National Aeronautic Association awarded the 1992 Robert J. Collier Trophy to the GPS Team, the most prestigious aviation honor in the United States. The team was made up of experts from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. They were recognized for creating the most important advance in safe and efficient air and spacecraft navigation and surveillance since radio navigation began fifty years earlier.
Ivan Getting, who served as emeritus president of The Aerospace Corporation and worked as an engineer at Massachusetts Institute of Technology, laid the groundwork for what would become the Global Positioning System. He built upon the technology developed during World War II, specifically improving on the land-based radio navigation system known as LORAN, which stood for Long-Range Radio Aid to Navigation. His contributions were essential in moving navigation from fixed ground stations to the satellite-based system we rely on today.
Bradford Parkinson, who worked as a professor of aeronautics and astronautics at Stanford University, developed the concept for the satellite-based navigation system in the early 1960s. He collaborated with the U.S. Air Force on this project. Parkinson was part of the Air Force for two decades, serving from 1957 until 1978, and he retired with the rank of colonel.
Roger L. Easton, one of the people who helped create GPS, received the National Medal of Technology in 2006. Francis X. Kane, a retired colonel in the U.S. Air Force, was honored at Lackland A.F.B. in San Antonio, Texas, in 2010 for his work on space technology and the engineering design of GPS as part of Project 621B. The system was also recognized in 1998 when it was inducted into the Space Foundation Space Technology Hall of Fame.
In 2011, the Global Positioning System received the International Astronautical Federation’s 60th Anniversary Award, nominated by the American Institute for Aeronautics and Astronautics. The IAF recognized GPS for its unique role in fostering international cooperation. In 2018, Gladys West was honored in the Air Force Space and Missile Pioneers Hall of Fame for her work on a precise Earth model used in GPS orbit calculations. Then, in 2019, four of GPS’s original project leaders were awarded the Queen Elizabeth Prize for Engineering, with the board chair stating that their work had fundamentally rewritten the world’s engineering infrastructure.
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The GPS system relies on satellites equipped with highly stable atomic clocks, which stay synchronized with reference clocks at ground control stations. Any deviation in the satellite clocks is regularly corrected to maintain accuracy. Since radio waves travel at a constant speed, the time it takes for a signal to travel from a satellite to a receiver on the ground corresponds directly to the distance between them. By gathering this timing data from multiple ground stations, the precise location of each satellite can be determined at any moment.
Each GPS satellite keeps precise track of where it is and what time it is, sending that information out all the time. A GPS receiver on the ground picks up data from several satellites to figure out exactly where it is in space and time. To do this, it needs signals from at least four satellites. With those signals, the receiver can determine its position in three dimensions and also correct for any differences between its own clock and the satellite clocks. That’s how you get accurate location data down to the meter.
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A pseudorandom code, a sequence of ones and zeros known to the receiver, plays a key role in determining location. The receiver compares its own version of the code with the one it measures from a satellite signal. By aligning these two versions in time, the receiver can pinpoint exactly when a specific part of the code—called an epoch—arrived. This arrival time is measured using the receiver’s own clock, allowing the system to calculate where the receiver is based on that timing data.
A GPS message contains the time of transmission, marked as the code epoch in GPS time scale, and includes where the satellite was at that exact moment. This information allows users to calculate their location based on how long the signal took to reach them. The system relies on precise timing from atomic clocks aboard each satellite, which must account for Einstein’s theory of relativity. Without these adjustments, errors would build up quickly, making positioning inaccurate. Each message sent includes both the time and the satellite's position at that time, ensuring a reliable and accurate fix anywhere on Earth.
The receiver in your GPS device measures the time it takes for signals from four satellites to reach you, using its own internal clock. These measurements give what are called times of arrival, or TOAs. From those and the times the satellites sent their signals—called time of transmission, or TOTs—the receiver calculates four values known as time of flight, or TOFs. Each TOF corresponds roughly to the distance between you and each satellite, adjusted for the difference in time between your clock and the satellite clocks, multiplied by the speed of light. These are called pseudo-ranges. Using these four pseudo-ranges, the receiver then figures out your exact location in three-dimensional space and also corrects for any difference between your clock and the atomic clocks aboard the satellites.
In GPS, the receiver figures out its exact location in three-dimensional space and adjusts for any clock difference between itself and the system's time, all at once. It uses something called navigation equations to process the time it took for signals to travel from satellites, known as TOFs. This method allows the receiver to calculate both its position and its time offset simultaneously, making the positioning accurate and reliable.
When a GPS receiver calculates where you are, it starts with a location centered on Earth and then changes that into latitude, longitude, and height using a mathematical model of our planet's shape. That height can then be adjusted to show how far above sea level you are. These coordinates might appear on a map or be fed into another system, like one that guides a car or plane.
As of 2025, the GPS system is being upgraded with new satellites called GPS III and GPS IIIF. These satellites have better atomic clocks that improve timekeeping accuracy. They also send out stronger, more secure signals like L1C, L2C, and L5. These changes make the timing measurements more precise and help reduce interference. All users benefit from more reliable position calculations thanks to these advancements in the GPS constellation.
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A receiver in continuous operation uses a track algorithm, or tracker, to improve accuracy by combining satellite measurements taken at different times. Since positions are usually close together, the tracker predicts where the receiver will be next and compares that with new data. It then blends the prediction with fresh measurements using a weighting scheme. This method helps refine location and time, filter out bad data, and even estimate speed and direction.
A tracker can only figure out changes in speed or direction after the fact, and it gets unreliable when the distance between position measurements is close to the random error of the GPS signal. That’s because GPS units measure where you are, not how fast you're going. But they can use the Doppler shift of the signals to calculate velocity more accurately. More advanced systems add other tools like a compass or an inertial navigation system to improve the results.
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GPS needs at least four satellites visible to figure out where you are and correct for time differences between your receiver’s clock and accurate time. This method removes the need for a super precise, possibly impractical clock in the receiver. Many GPS uses go beyond just navigation—like sharing time signals, controlling traffic lights, or syncing cell phone towers—and take advantage of the cheap, highly accurate timing that GPS provides. Some applications even display that time, though in most cases, it's only used for positioning and not for the actual timekeeping itself.
Even though GPS normally needs four satellites to work properly, there are situations where fewer will do. If you already know one piece of information—like your height above sea level—a receiver can figure out where you are with just three satellites. For instance, a ship floating on the ocean usually has an elevation close to zero meters, and planes often have their altitude recorded. Some GPS devices use extra tricks or guesses, like remembering the last known height, estimating movement based on previous data, or pulling in info from a vehicle’s computer, so they can still give you a location—even if it's not perfect—when only a few satellites are in view.
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The space segment of GPS includes 24 to 32 satellites, called Space Vehicles, orbiting in medium Earth orbit, along with payload adapters needed for launch. Originally designed with 24 satellites in three circular orbits, the system was updated to six orbital planes, four satellites each. These planes have an inclination of about 55° and are spaced 60° apart along the equator. Each satellite takes roughly 11 hours and 58 minutes to complete an orbit, passing over the same spots on Earth daily. The arrangement ensures that at least six satellites are always visible from any point on the planet. Within each orbit, the satellites aren’t evenly spaced; instead, their angular separation is 30°, 105°, 120°, and 105°—adding up to a full 360°.
Each satellite in the GPS constellation orbits about 20,200 kilometers above Earth, following a path that takes them around the planet at a radius of roughly 26,600 kilometers. They complete two full orbits every sidereal day, meaning they pass over the same spots on the ground at the same time each day. This regular pattern made early development easier—just four satellites were enough so that from any single location, all four would be visible together for a few hours each day. The predictable ground tracks also helped military planners ensure consistent coverage in key areas.
As of February 2019, the GPS system consists of thirty-one satellites in orbit, with twenty-seven currently in use and the rest serving as backups. A thirty-second satellite was launched in 2018, but by July of that year it was still being evaluated. There are also several decommissioned satellites still in space, available as spare parts. This expanded number of satellites improves GPS accuracy by offering more redundant measurements. The constellation has been rearranged into a nonuniform pattern, which increases both accuracy and system reliability when some satellites fail. With this setup, nine satellites are typically visible from any location on Earth at once, far exceeding the minimum four needed for positioning, ensuring strong redundancy.
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The Control Segment of GPS is managed by the Missile Control System (MCS) at Schriever Space Force Base near Colorado Springs, coordinating with U.S. Space Force monitoring stations in Hawaii, Kwajalein Atoll, Cape Canaveral, and shared NGA stations in England, Argentina, and other locations. These stations track satellite paths and send data to the MCS, which operates through the 2nd Space Operations Squadron (2 SOPS) using dedicated or shared ground antennas to contact each GPS satellite. Updates keep atomic clocks synchronized and adjust orbital information called ephemeris, calculated by a Kalman filter using ground station data, space weather, and other sources. When satellites are maneuvered, they're marked unhealthy until engineers retrack their new orbit, upload updated ephemeris, and declare them healthy again. The Operation Control Segment (OCS) ensures GPS stays operational and accurate for users.
In September 2007, the Ground Control System, or OCS, replaced an older mainframe computer at Schriever Air Force Base. This update helped support new security improvements for U.S. armed forces. The OCS system will remain in use until the newer Next Generation GPS Operation Control System, or OCX, is ready. The U.S. Department of Defense says OCX will improve GPS mission capabilities and support U.S. Space Force operations. It's also expected to cut costs and reduce risks through better software design and performance-based logistics. According to the department, OCX should cost millions less than upgrading OCS while delivering four times the capability.
The GPS OCX program is a key part of updating the GPS system, offering better information security than the older GPS OCS program. It will control and manage both the existing legacy satellites and the new GPS III satellites, while also supporting all the military signals the system can broadcast.
The Global Positioning System operates with a flexible design that quickly adjusts to user needs, ensuring fast access to GPS data and satellite status through secure, accurate, and reliable information. It offers major improvements in information assurance compared to the current system, including the ability to detect and prevent cyber attacks, as well as isolate, contain, and keep operating during such incidents.
On September 14, 2011, the U.S. Air Force said the GPS OCX program had finished its preliminary design review and was moving to the next stage of development. But the project had already missed major deadlines, pushing its launch back to 2021—five years past the original schedule. By 2019, the Government Accounting Office suggested that 2021 deadline was looking uncertain. The delays continued through 2023, and as of June of that year, the program was 73% over budget. Later in 2023, Assistant Secretary Frank Calvelli said the system might become operational sometime during the summer of 2024.
The US Space Force received OCX Blocks I and II from RTX on July 1, 2025, more than eight years past schedule and four billion dollars over budget, because of a rigid development approach and ongoing changes during the project. If the Government Accountability Office estimates are accurate, the updated system will become operational in December 2025. Work is now underway on OCX Block 3F, meant to manage GPS IIIF satellites that are expected to begin launching in 2027.
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The user segment of GPS reaches into the military and civilian realms, with hundreds of thousands of users from the U.S. and allied forces accessing the secure Precise Positioning Service, while also encompassing a much larger group of civil, commercial, and scientific users of the Standard Positioning Service. These users depend on GPS receivers, which generally include an antenna designed to pick up satellite signals, receiver-processors, and a stable timekeeping component—often a crystal oscillator. Some models also come with a display to show location and speed information directly to the user.
GPS receivers can take in differential corrections using a format called RTCM SC-104, usually through an RS-232 port running at 4,800 bits per second. The actual data transfer happens much slower than that, which limits how accurate the signal can be. Some receivers have built-in DGPS capabilities and can perform better than those relying on external RTCM data. By 2006, even inexpensive GPS units often came with WAAS receivers built in.
Many GPS receivers can send their location data to a computer or another device using a standard called NMEA 0183. This protocol was officially created by the National Marine Electronics Association, but since public records describe how it works, tools like gpsd can read it without breaking any laws. Other protocols exist too, such as SiRF and MTK, which are used by different manufacturers. These receivers connect to other devices in several ways—through a serial connection, USB, or Bluetooth.
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Each GPS satellite sends out a navigation message continuously, using two frequencies—L1 and L2—and the data flows at 50 bits per second. It takes 750 seconds, or twelve and a half minutes, to broadcast the full message. That message is made up of five subframes, each 300 bits long and lasting six seconds. Subframes four and five are repeated 25 times, so it takes 25 complete frames to finish the data set. Each frame has five subframes, and each subframe contains ten words that are 30 bits each. Altogether, the message is 37,500 bits long. Because of the transmission speed, it takes exactly 750 seconds to send the whole thing. Every 30-second frame starts right on the minute or half-minute, as set by the atomic clocks aboard each satellite.
The GPS message is divided into frames, and each frame has five subframes. The first subframe gives the week number, the time within that week, and information about the satellite's health. The second and third subframes carry the ephemeris, which are precise orbital details for that satellite. The fourth and fifth subframes hold the almanac, which includes rough orbit and status data for up to 32 satellites in the system, as well as error correction information. To figure out where a satellite really is, a GPS receiver has to read the message from each satellite it uses in its calculations for about 18 to 30 seconds. To gather all the almanac data, the receiver must process messages for 732 to 750 seconds—over 12 and a half minutes.
Every satellite in the system broadcasts on the same frequency, but they each send unique signals that let your receiver tell them apart. They do this using a method called code-division multiple access, or CDMA. There are two kinds of CDMA codes: one is the coarse/acquisition, or C/A, code, which anyone can use. The other is the precise P(Y) code, which is encrypted and only available to the U.S. military and certain NATO allies who have been granted access to the key.
The GPS system relies on data files called ephemeris and almanac to guide users to their location. The ephemeris is refreshed every two hours and remains accurate for about four hours, though it can be updated as infrequently as every six hours if needed. Meanwhile, the almanac usually gets a new update once every twenty-four hours. In case of delays in uploading information, the system also stores data for a few weeks ahead.
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Every GPS satellite transmits signals on two main frequencies: 1.57542 gigahertz for the L1 signal and 1.2276 gigahertz for the L2 signal. They use CDMA spread-spectrum, mixing actual data with high-speed pseudo-random codes unique to each satellite. Receivers need these codes to decode messages. The civilian C/A code moves at 1.023 million chips per second, while the military P code runs at 10.23 million chips per second. Each satellite's internal clock is set to exactly 10.22999999543 megahertz to account for relativity time differences. The L1 carrier carries both C/A and P codes, but L2 only carries the P code. The P code can be encrypted as P(Y) code, restricted to military users with proper key. Both C/A and P(Y) codes include precise timing information for users.
The L3 signal, operating at 1.38105 gigahertz, carries information from GPS satellites down to Earth, and it’s used by a system called the United States Nuclear Detonation Detection System, or USNDS. That system helps detect nuclear explosions in the atmosphere and near space, and one of its jobs is to monitor compliance with nuclear test ban treaties. The data sent through this signal plays a role in ensuring these international agreements are being followed.
The L5 frequency band at 1.17645 GHz was added during GPS modernization, placed in a protected range for aeronautical navigation to ensure no interference. The first Block IIF satellite carrying this signal launched in May 2010, with the final one, the 12th, going up on February 5, 2016. This frequency consists of two in-phase quadrature carrier components, each modulated by a separate bit train through bi-phase shift keying. "L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy."
In 2011, the FCC granted LightSquared a conditional waiver to operate a terrestrial broadband service near the L1 band, despite the company applying for a license in the 1525 to 1559 band as early as 2003. The agency asked LightSquared to work with the GPS community to test receivers and identify potential problems from stronger signals. The GPS community hadn't opposed earlier applications until November 2010, when LightSquared requested a modification to its Ancillary Terrestrial Component (ATC) authorization. That filing, SAT-MOD-20101118-00239, asked for much higher power in the same frequency band, effectively turning what was meant to be a quiet space for satellite signals into something like a cellular network. Testing in early 2011 showed minimal interference in the lower 10 MHz, but concerns remain about the upper 10 MHz, which may seriously affect GPS devices, especially for consumer use. Aviation Week magazine reported in June 2011 that the latest tests confirmed "significant jamming" of GPS by LightSquared's system.
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Because all the satellite signals are sent on the same L1 frequency, they have to be separated after being received. This is done by giving each satellite its own special binary code called a Gold code. Once the signals come in, the receiver uses these Gold codes to pull out the information from each satellite. The process involves adding in the Gold codes that match the satellites being tracked. That way, even though they're all on the same frequency, the system can tell them apart and figure out where you are.
When a GPS receiver already has almanac data, it uses the unique PRN numbers—between 1 and 32—to know which satellites to track. If the almanac isn’t stored, the receiver goes into search mode, waiting until it locks onto a satellite’s signal. That requires an unobstructed view of the sky. Once locked, it can download the almanac and figure out which satellites to follow. Each satellite sends its own distinct C/A code, so the receiver can tell them apart. But even after detecting a signal, there’s still a delay of up to 30 seconds before the device can give a position estimate because it has to read the ephemeris data.
The navigation message is processed in order to figure out when the signal was sent and where the satellite was at that moment. This step is part of a larger process called demodulation and decoding, which is described in more detail in a section titled "Demodulation and Decoding, Advanced." The information from this processing allows users to determine their location based on timing and position data from the satellite.
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The receiver in a GPS system relies on signals from satellites to figure out where they are and what time it is. Each satellite's position in three-dimensional space—designated by x, y, and z coordinates—and the time the signal was sent are represented as [xi, yi, zi, si]. The subscript i refers to each individual satellite, numbered from 1 up to n, with n being at least four. The system uses these data points to calculate the receiver’s location. The time indicated by the on-board receiver clock marks when the message was received.
The receiver’s clock is always slightly off from the highly accurate clocks aboard the GPS satellites, and this difference is called the receiver clock bias. This bias affects all signals received at once, as long as the satellite clocks are perfectly synchronized. The time it takes for a signal to travel from a satellite to the receiver is what we call the message's transit time.
In order to figure out where you are, a GPS receiver needs to know how far it is from you to each satellite in the sky. That distance, called the geometric range, is represented by the letter *d*, with an *i* to show which satellite it's measuring from. The receiver doesn’t just need that one number—it also needs to know the exact position of each satellite and your own location in three-dimensional space, using x, y, and z coordinates. So for every satellite, it calculates a distance based on where they are and where you are, and then uses all those distances together to triangulate your position. That’s how GPS works: by comparing these geometric ranges from multiple satellites.
To figure out where you are, a GPS receiver needs to solve equations with four unknowns: the three coordinates of its position and something called clock bias. That means signals from at least four satellites are required to try and solve them. These equations can be worked out using algebra or numerical methods. The question of whether a solution exists and is unique was looked at by Abell and Chaffee. If there are more than four satellites involved—meaning more equations than unknowns—the system becomes overdetermined, and then you have to use a fitting method to find the best answer.
The accuracy of GPS results depends on where the satellites appear in the sky. When satellites cluster together, it leads to bigger errors. To deal with this, GPS receivers estimate position error by multiplying their basic resolution by numbers called geometric dilution of position, or GDOP factors. These are calculated based on how the satellites are positioned relative to the receiver.
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When a GPS receiver measures distances to satellites, those measurements include errors from the clocks on board. In an idealized model where the clocks are perfectly synchronized, the true distances form what we call "pseudoranges." Each pseudorange represents the radius of a sphere centered on a satellite. With signals from at least three satellites, the receiver’s location lies at the intersection of those spheres' surfaces. This method is known as trilateration or true-range multilateration. Typically, the three spheres intersect at two points—only one of them is the actual position of the receiver, while the other moves quickly and doesn’t appear on Earth's surface.
When calculating position using signals from satellites, several sources of error come into play beyond just clock differences. Random errors can throw off results, and if the satellites are positioned too close together, subtracting their distances can lead to loss of precision. That’s why relying on just three satellites often isn’t enough for accurate location. Adding more satellites helps because random errors tend to balance out, and having a wider spread between satellite positions improves accuracy. Still, even with more signals, the spheres don’t usually meet at one exact point. Instead, a close approximation is found—usually through a method called least squares—and the more data available, the closer the estimate comes to the true location.
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When more than four satellites are available, the GPS calculation can choose to use just the four best ones, or it can incorporate more than four at once—up to every satellite that's visible. The decision depends on how many receiver channels the device has, how much processing power it can handle, and something called geometric dilution of precision, or GDOP. This factor measures how well the satellites are spread out in the sky, which affects how accurate the position fix will be. Using more satellites generally improves accuracy, but only if the system is capable of handling the extra data.
When you have more measurements than you need—say, more than four—to figure out where you are, you end up with an over-determined system. That means there’s no single answer that fits all the data perfectly. Instead, you have to find the best possible solution by using a method called least squares. This technique helps you balance out the differences in your measurements so you can get as close as possible to the truth. If some of your data is more reliable than others, you might use weighted least squares to give more importance to the better readings. It’s a way of making sense of messy data and finding the most accurate result from a set of equations that don’t quite line up.
The solution to finding the best estimates for position and clock bias involves minimizing a specific function. This function calculates the sum of squared differences between observed and computed pseudorange measurements. The variables being estimated are the x, y, z coordinates of the receiver's position, and b, which represents the clock bias. The method uses a mathematical operation called arg min to determine the values that make this sum as small as possible. The result is a set of estimates for the receiver’s location and its clock offset, based on data from multiple satellites. This process is fundamental to how GPS determines where you are.
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S. Bancroft developed a closed-form solution for the equations at hand, and its characteristics are widely recognized. Among its claimed advantages, it's said to perform better in low-GDOP scenarios when contrasted with iterative least squares techniques.
Bancroft's method is an algebraic approach to determining position using signals from satellites, and it can work with four or more satellites. When four are used, the process involves inverting a 4x4 matrix and solving a quadratic equation with one variable. This method delivers either one or two possible solutions for where you are. Usually, there are two solutions, but only one will be sensible—close to Earth and physically meaningful.
When more than four satellites are involved, a receiver can apply Bancroft’s approach, which uses what's called the pseudoinverse to determine position. But another path exists through iterative techniques such as the Gauss–Newton algorithm, which is often preferred for tackling over-determined, non-linear least squares problems. These methods tend to produce more precise outcomes compared to relying solely on the pseudoinverse method.
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In the United States, GPS receivers fall under the Federal Communications Commission's Part 15 rules. As stated in the manuals of GPS-enabled devices sold there, the device "must accept any interference received, including interference that may cause undesired operation." Specifically for GPS devices, the FCC requires that manufacturers "must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum." For the past thirty years, GPS receivers have operated near the Mobile Satellite Service band and have successfully distinguished between GPS signals and those from services like Inmarsat, without any problems.
The FCC set aside the GPS L1 band from 1559 to 1610 MHz, but since 1996 it had also allowed LightSquared, then called Motient Services, to use frequencies from 1525 to 1559 MHz. On March 1, 2001, LightSquared’s predecessor applied to use those frequencies for a satellite-terrestrial service. In 2002, the U.S. GPS Industry Council reached an out-of-band-emissions agreement with LightSquared to keep its ground stations from interfering with GPS. The FCC adopted that agreement in 2004 as part of its authorization for LightSquared to deploy ancillary tower components, making clear that the terrestrial network would remain secondary to their satellite service. The approval was reviewed and supported by the U.S. Interdepartment Radio Advisory Committee.
In January 2011, the FCC conditionally approved LightSquared's wholesale partners—like Best Buy, Sharp, and C Spire—to only buy and resell a combined satellite-ground service using LightSquared’s 1525 to 1559 MHz frequencies, but only on devices that use the ground-based signal. Before that, in December 2010, GPS receiver makers told the FCC they were worried about interference from LightSquared's signal. The FCC’s decision didn’t address how many ground stations could be built or how powerful they could operate. Final approval depended on a working group led by LightSquared, with help from GPS experts and federal agencies, to study the interference. Then, on February 14, 2012, the FCC started proceedings to cancel that conditional order after the NTIA concluded there was no practical way to stop potential GPS interference.
GPS receiver makers sometimes design their devices to use spectrum that goes beyond the band set aside for GPS. In certain areas, these receivers can tap into up to 400 MHz of space on either side of the L1 frequency at 1575.42 MHz. That’s because mobile satellite services are broadcasting from space to Earth in those regions, using power levels similar to mobile satellite operations. Under FCC Part 15 rules, GPS receivers don’t get protection from signals outside their allocated spectrum. This is why GPS and the Mobile Satellite Service bands sit right next to each other—both can work together without interference, thanks to how their spectrum is shared.
In February 2003, the FCC allowed MSS licensees like LightSquared to build a few ground-based towers in their spectrum to use terrestrial wireless more efficiently. The FCC said at the time that MSS ATC and CMRS would operate in different market segments and wouldn’t directly compete. In 2004, they clarified the towers had to stay ancillary, not stand-alone. Then in July 2010, the FCC expected LightSquared to offer an integrated satellite-terrestrial service similar to what terrestrial providers offered. But GPS receiver makers argue that the 1525 to 1559 MHz spectrum wasn’t meant for high-speed wireless broadband, since the 2003 and 2004 rulings made clear the ATC was supposed to support, not replace, the satellite service.
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