|Quoting SlamClick (Reply 2):
As to exactly how this works, I'm sure someone on here will be acquainted enough with that side to take you as far down the rabbit hole as you'd like to go.
Heres a quick rundown of what I learnt last time I looked into this:
Receivers get a signal from a satellite which contains a unique Satellite ID
, time signal sent and some other technical data called a psuedo random code. The receiving unit has a set of data in it which tells it where all the GPS satellites are at any given time on any day ever. Minor positioning error corrections are sent with the GPS signal.
Receiver measures time difference between sending and receiving (travel time), calculates the distance from the satellite to itself, while keeping the receiver and sender clocks in sync is a function of the psuedo random code signal. The psuedo random code does a lot more, like allow for amplification of the signal with very small antenna, and ensure that each satellites signal is unique and doesnt block each other. The satellites have atomic clocks on board, and the receivers can reset their own clocks based on each satellites time reading, ensureing that the receiver is in sync.
This gives it a possible number of locations on the earth it can be based on the distance from that satellite, which works out to a sphere the radius of that distance. Do this with more satellites to cut down on the possible locations by intersecting the spheres calculated for each satellite, you are whereever those spheres intersect. The more satellites you can receive a signal from, the smaller that area becomes, and also the longer you spend receiving signals from the same satellite, the smaller the area becomes, which is why if you leave a GPS receiver for a period of time, it keeps increasing in accuracy over that time.
In general, it takes 3 satellites to give you two possible locations for your receiver. Why two? Because thats where the third satellites sphere intersects the other two satellites spheres based on how the measurements are done. You can wait for a fourth satellite to come into view to be able to determine 100% which of those two points you are actually at, but in reality one of those two points nearly always is 'impossible', ie its not on Earth (IE its in orbit or further out), or that points velocity based on updates from the satellite is far too high for it to be you. A fourth satellite is also needed when your GPS receivers clock is out of sync with the satellites clocks - 3 satellites give you a rough area and the fourth satellites measurement gives you enough information to reset the receivers clocks and recalculate its position with the new accurate time reading.
So in points:
1. You need distance measurements from the satellite to you
2. You need 4 satellites to guarantee a 100% accurate location
3. In general only 3 satellites are required because most of the possibilities are ridiculous or impossible
4. A 4th satellite can give you speed and altitude measurements, allowing for aircraft navigation, as well as correcting timing issues
Hope that helps!
[Edited 2007-08-15 17:44:39]