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In article <greg.714049408@coombs:, greg@coombs.anu.edu.au (Greg Price) writes:
: Hi, I have a couple of questions about how GPS works, but no-one know the
: answer too around here. See if you can help me here....
: 1) How does the receiver separate out the different signals from a number
:    of satellites if they are all transmitting on the same frequency (using
:    180 degree phase modulation) and using one non-directional antenna?

The short answer is that they aren't.  The long answer is that they are using
spread spectrum technology to module the signal.  Each satellite transmits
with a different pattern of frequencies, called Gray codes.  The challenge
is for the reciever to lock onto and track each of the satellites that are
within range.

: 2) The satellite has an accurate internal clock, and the receiver has
:    another internal clock which it syncs with the satellites, and uses
:    the propagation delay to determine this distance. How do you initially
:    sync the receiver clock with the satellites from scratch?

You don't.  The time in the receiver is generated via techniques that I
currently do not understand, but appear to generate a kind of 'virtual time'.
Each satellite transmits a kind of "I am here at this time".  The receiver
generates a 'virtual time' to figure out where and when it is.  As a spinoff
GPS allows you to get a time very close to UTC.

-- 
Kenneth Ng
Please reply to kdn5669@hertz.njit.edu for now.
Apple and AT&T lawsuits: Just say NO!