Hi Bob,
The actual receiver logic is that you have some sampling point in time,
the tracking phase of a channel is being sampled. As you do for multiple
channels, the relative phase of each channel is sampled.
In order to extend this phase into a pseudo-range, one needs to guess
how many integer multiples of the code there is from each GPS to the
receiver. A bunch of multiples is assumed from the orbit, as there is at
least the delay of the shortest distance, and then you can make a rough
estimate by the sub-code phase of the birds, as they hook up like a set
of clock-work gears. That gives you a first approximate guess, which
might be wrong, but as we try to make it fit, we can solve this equation
and out pops a first rough estimate, from that we can then maintain a
correct guess from then on.
For your rocket, you have a known stable situation at the launch-pad.
That cuts out the guess-work, as at that point, you can assume that
there is no multiple as your measurement nodes are within range.
Cheers,
Magnus
On 04/04/2015 05:51 AM, Robert Watzlavick wrote:
To head off a bunch of replies - I think I stumbled upon what is being
suggested. To extract the pseudorange, you have to figure out the
offset of the locally generated PN code against the one that is
received. In this reverse GPS case, I assume each ground station would
have to start their local PN codes at the same time? Then you would be
able to get the pseudoranges at each ground station and use those values
for the multilateration equations. You still would have an uncertainty
of one clock cycle since the phases of the local clocks at the stations
wouldn't be aligned but several folks have suggested ways around that.
-Bob
On 04/03/2015 10:12 PM, Robert Watzlavick wrote:
I have an amateur radio license (mostly CW/HF and some VHF/UHF
experience) and I've written some driver software for an IQ
demodulation board but I have to admit, I would have no idea how to
begin setting up that system as initially described by Attila and
expanded by you and others. I have a rudimentary understanding of the
modulation schemes involved but I don't fully understand how the
various codes mentioned fit in. I've poked around a bit at some
articles on PN codes and I can see how data would be transmitted but I
think I'm missing something key that allows you to extract positions,
velocities, etc. out of the various links. I think I have some more
reading to do :)
Thanks,
-Bob
On 04/03/2015 06:08 AM, Magnus Danielson wrote:
I think this is a good idea, and it is relatively straight-forward to
do.
You can observe both code and carrier phase this way, given that the
transmitting radio is coherent with the code generation clock.
Doppler also pops out of the tracking station.
A good coding-gain reduces the need for a strong transmitter.
The issue might be the allowed width of the signal being transmitted,
forcing the chipping rate down.
Cheers,
Magnus
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