View Full Version : Global positioning systems

15th Dec 2001, 05:01
I'm sure there is an obvious answer to this, but I've been thinking about GPS and how it works. It takes a minimum of three satellites in orbit each sending out a signal that your receiver picks up and then calculates the distance to each satellite and hence your position in relation to them. Now that's fine.
How do the satellites know where they are? If all of the satellites were one arc second out of position i.e. the net is as expected just slightly rotated out of position, what system is built in that tells the satellites where they are?


Self Loading Freight
15th Dec 2001, 06:02
The satellites don't 'know' where they are. They're basically just very, very accurate clocks hooked up to transmitters. A network of ground stations -- which do know where they, the ground stations, are -- monitors the signals and from the relative timings/dopplers work out where the satellites are. The ground stations then upload that information back to the satellites, and this is transmitted to GPS receivers along with the timing information. The satellites drift around in space quite a lot; the ground stations stay put (well they don't, but their movements due to tectonics, etc, are known compared to the space segment drifting around up there).

The basic references in the GPS system are the positions of the ground stations and the clocks on the satellites. From those, the rest of the system can find out where and when it is.


15th Dec 2001, 07:10
Thanks, I'd wondered the same.

15th Dec 2001, 07:59
I have read in the last few days that during North hemisphere winter, the north pole sinks and the northern hemisphere shrinks, pulling the equator north a bit, so expanding the southern hemisphere. Total earth shape and size remain the same. This recent scientific discovery was reported in the lay press, but without mentioning the amount of movement.

1. How do they know - did GPS tell them?

2. Could GPS still give accurate positions when these movements are happening?

I suspect the answers to 1. and 2. are mutually exclusive.

15th Dec 2001, 21:56
Thanks SLF,
I had assumed that it had something to do with reference to a ground station (or more)but knew no more.

Thanks again. :)

Self Loading Freight
17th Dec 2001, 04:47
I haven't heard about the Northern hemisphere's movements! Have you got a reference?

GPS may well have shown this movement (or it could have come from radar or lidar surface mapping through other satellites, which is more likely). Part of GPS operation includes monitoring errors, and a systematic error where something is regularly and unexpectedly changing will stand out a mile. Or a millimetre, which is just as good.

Nothing is fixed in GPS; all it does is coat the planet in a mesh of signals that can be decoded to provide an arbitrary code that everyone understands relates to positions on the ground and to the compass. But the ground is moving and the poles are moving, all in three dimensions. Where there's a discrepancy between the mesh and the real world, it'll be spotted and, in time, coped with.


17th Dec 2001, 14:57
it takes four satellites - you need an extra one to get the timing right.


18th Dec 2001, 11:31

it takes 3 for a 2D fix, 4 for a 3D fix.

One might wonder whether the movement caused by summer/winter is large enough to measure without the use of DGPS (as they do in geology). GPS uses a model of the earth's shape anyway (WGS84) which in itself causes small position errors in reference to the earth. As all users in aviation use this same model, there is not really an issue.

Or should we start using different summer and winter charts? :D :D