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GPS satellites send precisely timed signals containing their position information.
Receivers calculate distances by measuring how long satellite signals travel.
Trilateration combines multiple satellite measurements to determine precise device locations.
GPS, or the Global Positioning System, is a satellite-based technology that allows devices to determine their location, altitude, speed, and precise time. From smartphones and cars to navigation systems, GPS has become part of everyday life. But behind a simple location pin lies a network of satellites, ground stations, precise clocks, and complex mathematical calculations.
For GPS to operate, several satellites need to be in orbit at an altitude of about 20,200 kilometers above Earth's surface. These satellites keep emitting radio waves that contain the satellite's exact location and very precise time.
Every GPS satellite contains an atomic clock. Every GPS satellite must contain an atomic clock because it ensures precise timing for calculating the GPS receiver's location.
Accurate timing is essential, as GPS uses timing information to determine distance.
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Upon detecting the signals from a particular satellite, a GPS receiver can compute when the signal left the satellite and when it reached the receiver. With such information, the receiver can compute the distance between itselfa considerable gap in measuring and the satellite.
The speed at which GPS signals are received is close to the speed of light, at 299,792,458 meters per second. Hence, even the slightest discrepancy in time can be considered a considerable gap in distance measurement.
With knowledge only of the GPS receiver's distance from a satellite, the exact location cannot be determined.
GPS determines position through a process known as trilateration. If a receiver knows its distance from one satellite, its possible location could lie anywhere on a large sphere surrounding that satellite. A second satellite narrows the possibilities to where two spheres intersect. A third satellite narrows the position further.
In practical GPS positioning, a fourth satellite is normally required. It helps the receiver calculate not only its three-dimensional position but also the error in its own clock.
This is also why GPS is more accurately described as using trilateration rather than triangulation. The system primarily calculates position using distances, rather than angles.
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The GPS satellites have very precise atomic clocks; however, the receivers, such as smartphones, don’t have any.
A small discrepancy in the receiver’s own clock will affect the distance calculation. To measure this discrepancy, the fourth satellite sends out a signal to the receiver.
Using four satellites, the location (latitude and longitude), altitude, and time are determined.
Satellites are just one element of GPS. The control segment consists of a series of ground stations that monitor and help maintain the system's precision.
This control segment tracks satellite orbits and performance, updates navigation information, and helps maintain the precision of satellite clocks and positions.
The information managed by the control segment is vital since the GPS receiver relies on precise satellite position and time information for location calculation.
GPS does not always provide the same level of accuracy. Signals can be affected by atmospheric conditions, buildings, reflected signals, and the arrangement of satellites in the sky. These factors can reduce positioning accuracy.
Under basic GPS service, the FAA says positioning accuracy is approximately 7 meters 95% of the time. NIST notes that modern GPS can locate a position to within about 4.9 meters, while advanced receivers can achieve substantially better precision.
The process can be summed up in a few steps: a satellite sends a precisely timed signal; a GPS receiver receives it; the receiver calculates the time it took for the signal to travel; the distances from multiple satellites are determined; and those measurements are combined to determine the device’s location.
That combination of satellite signals, precise timing, and mathematical calculations is what allows a smartphone or navigation system to determine its location without a physical connection to the satellites.
What does GPS stand for?
GPS stands for Global Positioning System, a satellite-based technology used to determine location, altitude, speed and precise time.
How does GPS determine your location?
GPS receivers calculate distances from multiple satellites and use trilateration to determine the device’s precise three-dimensional position.
Why does GPS need four satellites?
Four satellites help calculate latitude, longitude, altitude and receiver clock errors, improving the accuracy of the positioning process.
How far are GPS satellites from Earth?
GPS satellites orbit roughly 20,200 kilometres above Earth, continuously transmitting signals containing their position and precise timing information.
Why can GPS accuracy vary?
Buildings, atmospheric conditions, reflected signals and satellite arrangements can interfere with GPS signals and reduce positioning accuracy in certain locations.
