How Does GPS Work?

What are the three segments of GPS?

The Global Positioning System (GPS) is a utility owned by the United States that gives users positioning, navigation and timing services.^[1] It is made up of three parts: the space segment, the control segment and the user segment.^[1]

The space segment is the constellation of satellites, which send out one-way signals giving each satellite's current position and the time.^[1] The control segment is a worldwide set of monitor and control stations that tracks the satellites, keeps them in their proper orbits, adjusts their clocks and uploads updated navigation data.^[1] The user segment is the receiver equipment, such as the GPS receiver in a mobile phone, which takes in the signals and uses them to calculate a three-dimensional position and the time.^[1][4]

How many GPS satellites are there, and how high do they orbit?

The United States is committed to keeping at least 24 operational GPS satellites available 95% of the time.^[2] To meet that commitment, the U.S. Space Force has flown 31 operational satellites for well over a decade.^[2] GPS.gov counted 31 as of July 3, 2023, excluding decommissioned spares still in orbit, and points readers to the NAVCEN website for more up-to-date status.^[2] NIST describes the fleet differently, saying twenty-seven of the 31 satellites are in operation and the others are backups.^[4]

GPS satellites fly in medium Earth orbit at an altitude of approximately 20,200 km (12,550 miles), and each one circles the Earth twice a day.^[2] NOAA's geodesy tutorial gives a lower height, approximately 11,000 miles.^[6] The satellites are spread across six equally spaced orbital planes, each with four baseline "slots," so that users can see at least four satellites from virtually any point on the planet.^[2] In an expansion completed in June 2011, three of the 24 slots were expanded, and GPS now effectively operates as a 27-slot constellation.^[2]

• Satellites committed (available 95% of the time)24
• Baseline slots after the 2011 expansion27
• Operational satellites on July 3, 202331

Source: GPS.gov (National Coordination Office for Space-Based Positioning, Navigation, and Timing) [2]

How does a receiver turn signal travel time into a position?

GPS satellites constantly broadcast signals carrying their time and position.^[4] The time is placed in the codes each satellite broadcasts, so a receiver can always tell when a signal was sent.^[3] The receiver takes the difference between the broadcast time and the time the signal arrived and uses it to compute its distance, or range, from that satellite.^[3]

The calculation is distance = rate × time, where the rate is the speed of the radio signal: the speed of light, 299,792,458 meters per second.^[5]

Working out a position from the distances to at least three known points is called trilateration, and in GPS the known points are the satellites.^[5] NOAA explains it with imaginary spheres: one distance measurement puts the receiver somewhere on a sphere whose radius is the distance to that satellite.^[6] A second satellite limits the receiver to the circle where two spheres intersect, and a third narrows it down to just two possible points.^[6] A fourth satellite is used to correct for the error in the receiver's own clock and find the precise position.^[6]

The FAA puts it in terms of clocks: three ranges would be enough only if the receiver had an atomic clock synchronized to GPS, and a fourth satellite's measurement removes that need.^[3] With four satellites, the receiver computes latitude, longitude, altitude and time.^[3]

Why do atomic clocks matter for GPS?

Distance is measured by timing a signal that moves at the speed of light, so small clock errors become large distance errors.^[5][7] NASA, writing about spacecraft navigation, notes that a clock that is off by one millisecond, a thousandth of a second, produces a distance error of 185 miles (300 kilometers).^[7]

Accurate time is the basis of GPS positioning, so each GPS satellite carries several atomic clocks.^[4] NIST says these clocks keep time to within three billionths of a second, while NOAA's tutorial puts the figure at 40 billionths of a second.^[4][6] An atomic clock combines a quartz crystal oscillator with a group of atoms to make its timekeeping more stable.^[7] Even so, the clocks on GPS satellites drift and must be sent updates two times per day, drawn from more stable atomic clocks on the ground.^[7]

How accurate is GPS on a phone?

GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky.^[8] NIST gives the same 4.9 meters (16 feet) and adds that modern phones using additional signals and post-processing can be more precise.^[4] The FAA describes the basic GPS service as accurate to approximately 7.0 meters, 95% of the time.^[3]

Accuracy worsens near buildings, bridges and trees.^[8] Common causes of error are blocked satellite signals, indoor or underground use, and signals reflected off buildings or walls, called multipath.^[8] The ionosphere and troposphere also slow the signal, and the receiver must account for that delay.^[3] High-end users improve accuracy with dual-frequency receivers, augmentation systems or both, which can give real-time positions within a few centimeters.^[8]

The U.S. government's accuracy commitments apply to the signals transmitted in space, not to GPS devices: it commits to keeping a signal measure called user range error at a global average of 2.0 m (6.6 ft.) or less, with 95% probability.^[8] On April 20, 2021, the actual figure was 0.643 m (2.1 ft.) or less, but GPS.gov stresses that range error is not user accuracy.^[8]

Who runs GPS, and what other systems exist?

The U.S. Space Force develops, maintains and operates the GPS satellites and the control segment.^[1] The civilian service is freely available to all users on a continuous, worldwide basis, while a military service is available to U.S. and allied armed forces and approved government agencies.^[1]

GPS is not the only system of its kind: the FAA lists three other constellations that provide similar services, which are GLONASS, developed and operated by the Russian Federation; Galileo, by the European Union; and BeiDou, by China.^[3] Together with their augmentations, these constellations are called Global Navigation Satellite Systems, or GNSS.^[3] All of the providers have offered free use of their systems to the international community.^[3]