How does a GPS unit work?

Question

How does a GPS unit work?

Answer 1

The satellite transmit their orbit\positions and timestamps.

Each satellite tracks its own longitude, latitude and elevation and has an onboard clock.

The satellite signal consists of the satellite's position and the exact time that the transmitted it.

The speed of the transmission is known and we have the time, so we can get the distance.

Now, distance is no good alone...because distance from what?

Each satellite also sends its ID and its position.

We have the transmission speed (this is a constant) and the height. From that we get the time it takes to reach the ground in a straight line. This can be used to find elevation of the reciever..

Anyways, since the tramission is unidirection, we can see how for a reciever is from a satellite. Unfortunately, this will only produce the target somewhere in the satellite's transmission radius.

Data from 2 satellites will give you 2 possible locations that you can be located. Still not good enough and you have no clock-sync, so if your device's clock is off, you could be way off route.

With 3 satellite, your location can only be a single point were all 3 distances intersect. From the clock average of all 3 satellites, you can find the current local clock time and use it in reference for computation. With elevation of the satellite can be used in conjunction with the speed of the transmission and thus the time it takes for a transmission to reach the ground in a straight line and comparing this with the time computed from the transmission distance average against all 3 sats, you can find the height...

Its simple trig, really...

Satellites may transmit extra data, such as their true elevation and true transmission times and speed for a more accurate computation, but in the US and Euro sats, this data is encrypted because it would give extremely accurate data that can be used as targeting data.

Russia's system doesn't have this encryption and this data is transmitted freely...