Every one of us had a moment in life when roads find us strange, the darkness of night is falling all over us and you don’t know the way back to home. Ah! GPS helped us! All thanks to it. But have you ever tried to go behind the scenes and find out how it works. If you are a TGC reader, you are never going to miss the shot.
Global positioning system is powered by 27 earth orbiting satellites, 24 in power and 3 extra if any one fails. The orbits of these satellites are arranged in such a way that there are always 4 satellites visible in the sky.
In order to calculate your position, the GPS receiver has to perform two chores-
- Investigate the location of at least 3 satellites above you.
- The distance between you and each of those satellites.
The receivers iron out this by scrutinizing low-power radio signals from the GPS satellites. Radio waves are electromagnetic energy, which means they travel at the speed of light. The receiver can figure out how far the signal has traveled by the time it took to arrive.
At a specific time the satellite starts transmitting a long, digital pattern called a pseudo-random code. The receiver also starts running the same pattern at the same time. When the satellites signals reaches the receiver, the transmission of pattern will lag a bit behind the receiver’s playing of the pattern. The length of the delay is equal to signal travel time. The receiver multiplies this time with the speed of the light to know how far the signal travelled.
In order to make this measurement, the time of both the satellite and the receiver need to be synced to nanosecond which is only possible with the help of atomic clocks. But atomic clocks being expensive are only used in satellites whereas quartz clock substitute atomic clock in receivers. The quartz clock constantly resets itself to maintain the accuracy with that of atomic clocks in space. In other words, there is only one value for the “current time” that the receiver can use. That time value is held by all the satellites in space. This means that GPS receiver gets atomic time in discount.
When we measure the distance to four of those satellites, we lure four spheres intersecting at one point. The all four spheres won’t intersect if the measurements are incorrect. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionately improper.
The receiver can automatically find out the adjustments required to make the entire four spheres to intersect at a point. It will reset its clock to be In sync with atomic clocks, which means its calculations are near to accurate as expensive atomic clocks in the space.
To make the distance information useful, the receiver has to trace the location of satellites. This isn’t very much difficult as the satellites revolve in very high and predictable orbits. The GPS receiver maintains an almanac to tell the location of the satellite at a given point of time. Things like gravity from moon and sun etc do change the orbits slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites’ signals.