The surprising part: GPS is a clock system
People often imagine GPS as satellites that track your phone. It is the other way around. The satellites do not know where you are. They continuously broadcast radio signals saying, in effect, “this is my position and this is the exact time.” Your receiver listens, measures how long each signal took to arrive, and works out its own location. A receiver only listens, which is why many devices can use the same satellites at once and why the satellites cannot see who is using them.
GPS is the name of the system operated by the United States. The general term is global navigation satellite system, and other countries and regions run their own, including Galileo, GLONASS and BeiDou. Many modern receivers can use several of these at the same time. This article uses GPS as the common example, and the ideas apply to all of them.
From travel time to distance
Radio signals travel at the speed of light, about 300,000 kilometers per second. If you know how long a signal took to arrive and when it was sent, distance is simple: distance equals speed multiplied by time.
The satellites orbit about 20,200 kilometers up, so a signal from one directly overhead takes about 0.067 seconds to reach you (20,200 divided by 300,000). Because light travels so far in so little time, the clocks must be extremely precise. An error of one microsecond, which is one millionth of a second, equals about 300 meters of distance. A billionth of a second equals about 30 centimeters.
That is why each satellite carries very accurate atomic clocks, and why the ground control network keeps checking and correcting them.
Finding a position from several distances
One distance measurement tells you only that you are somewhere on a sphere around that satellite. A second satellite narrows it to a circle where two spheres meet. A third narrows it to two points, and one of them is usually far out in space or otherwise impossible, so the receiver can discard it.
That would be enough if the receiver had a perfect atomic clock. It does not. A phone or car navigation unit has a cheap clock that drifts. The receiver therefore treats its own clock error as one more unknown. With three position unknowns (in effect, latitude, longitude and height) plus the clock error, it needs signals from at least four satellites. Having more than four helps accuracy and reliability.
This is also why the receiver gets very accurate time as a side benefit. Many networks and systems use satellite time for synchronization.
What is in the signal
Each satellite signal carries a repeating code that identifies the satellite and lets the receiver measure timing precisely. It also carries a navigation message with the satellite’s orbital information and clock corrections. A receiver compares the arrival times of codes from different satellites to determine each delay.
The signals are weak by the time they arrive, which is why GPS works poorly indoors, in tunnels and in deep urban canyons. Buildings can also reflect the signal, so it arrives by a longer path, a problem called multipath that can make a position appear several meters off.
Why a constellation is needed
Medium orbit satellites circle the Earth about twice a day. A GPS-style constellation is arranged in several orbital planes so that a receiver almost anywhere on the surface can normally see at least four satellites above the horizon. The nominal design calls for at least 24 satellites, and operators typically keep more than that working.
Sources of error
A typical phone in open sky is often accurate to within several meters, though results vary. Common error sources include:
- Atmosphere. The ionosphere and troposphere slow and bend signals slightly. Receivers use models, and some use signals on more than one frequency, to correct for this.
- Satellite geometry. Satellites clustered in one part of the sky give a poorer fix than ones spread widely.
- Reflections and blocked sky, as noted above.
- Clock and orbit errors, kept small by monitoring and corrections.
- Relativity. The satellites move fast and sit higher in Earth’s gravity, so their clocks run at a slightly different rate than clocks on the ground. The system is designed to compensate for this, and without compensation, errors would build up quickly.
How accuracy is improved
Phones also use assistance from mobile networks and Wi-Fi to find a first position quickly and to keep working where satellite signals are weak. Surveyors and farm machinery use correction services and reference stations to reach centimeter-level accuracy; this is a specialized method that needs extra equipment. Motion sensors in a phone can fill brief gaps, for example when passing through a tunnel. Cell towers, which operate as part of 5G networks and earlier systems, can provide rougher positioning when satellites are not visible.
Vulnerabilities
Because the satellite signals are faint, they can be disrupted by interference, known as jamming, or faked, known as spoofing. This is a reason that critical systems often use backup timing sources and multiple navigation methods rather than relying on one signal alone.
Where it is used
Navigation apps are the obvious use. Beyond them, satellite positioning supports aviation, shipping, surveying, precision farming, emergency services and the timing used by networks and power systems. It also supports self-driving cars and autonomous drones, which typically combine satellite positioning with cameras, radar and other sensors instead of trusting any single source.