GPS is an abbreviation for global positioning system, a satellite-based navigation system that allows anyone with a GPS receiver to determine their three-dimensional position on, or above, the surface of the earth. The GPS consists of a constellation of 24 satellites, six ground stations located around the world and GPS receivers, which can be handheld or mounted in vehicles, vessels or aircraft.
The physical principles of GPS include quantum theory, which led to the invention of the atomic clock, and Einstein’s general theory of relativity. The basic principle behind GPS, however, is the measurement of distance, or range, between the satellites and the receiver. The signal from each satellite, moving at the speed of light, takes a finite amount of time to get to the surface of the Earth. A GPS receiver calculates the distance to a satellite by multiplying the difference between the time a signal is received and the time it was sent by the speed of light.
A GPS receiver calculates its precise latitude, longitude and altitude by measuring the distance to four separate GPS satellites. If the GPS receiver measures the distance to a single GPS satellite, it can determine that it is somewhere on an imaginary sphere with radius equal to the distance to the satellite. If it measures the distance to two satellites, it can determine that it is somewhere on the line where the two spheres intersect. If it measures the distance to a third and a fourth satellite, it can calculate its precise geographical position.
Signals from GPS satellites travel to earth at the speed of light, so accurate time measurement is paramount to the operation of the GPS. GPS satellites carry the most accurate clocks available, known as atomic clocks, which are accurate to one billionth of a second per day. These clocks allow GPS satellites to broadcast signals that are synchronized with those from other satellites.
Einstein’s general theory of relativity correctly predicts and explains that gravity appears to slow down, or dilate, the flow of time. In other words, the difference between the strength of the gravitational field on the surface of the Earth and the 12,000 miles or more above it means that the clock on a GPS receiver runs a fraction of a second slower than the clock on a GPS satellite. Satellite-based clocks are deliberately manufactured to run slightly slower, when on the Earth, so that they keep the correct time once in space.