In article ,
wrote:
What frequency are GPS units on?
And would that be AM or what?
I dont really understand satellites.
Just curious.

They're on a couple of frequencies in the 1-2 GHz band.

dig, dig, 1575.42 MHz and 1227.6 MHz.

And then it gets complicated. The buzzword is Direct Sequence Spread
Spectrum.

There's a digital circuit called a pseudo-random number generator that
generates what looks like a random sequence of bits. But if you have
two such circuits, you can transmit that signal with one, and the other
(in a receiver), will be able to match (correlate) the data it receives
so that you can know where in the counting sequence of the random
generator at the transmitter is operating at. In other words, the time
at the transmitter. To an resolution of the bit rate that's transmitted.
For GPS it's around 1 MHz and 10 MHz. Since time is distance, that's
300 or 30 meters resolution.

Next, there's a data stream that gives the satellite's particulars. (What
bird, what time it is to some offical standard, where it is, etc...).
This is transmitted at some slow rate, 50 bits per second, as I remember.
Phase Shift keyed, I think. This gets combined with the spreading signal
to a fuzzy blob of RF at the microwave frequency given above.

The receiver then looks the frequencies with a set of fancy digital
filters that match the spread spectrum signals. (One trick with spread
spectrum is that you can have more than one transmitter on the same
frequency at the same time. So the one receiver can be looking at a
half a dozen or more satellites at the same time). Each one of these
receiver channels, when matched up, will give the time, and also the
data stream for a satellite. Then the receiver can take this data,
and calculate the time difference for pairs of satellites and with their
known position from the data stream, calculate the position on the earth
where this receiver's antenna is.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)