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Re: [Discuss-gnuradio] Another question about GPS
From: |
Matt Ettus |
Subject: |
Re: [Discuss-gnuradio] Another question about GPS |
Date: |
Mon, 28 Mar 2005 09:10:58 -0800 |
User-agent: |
Internet Messaging Program (IMP) 4.0-cvs |
> But if you don't know anything, in my understanding, you must:
>
> 1. Try every (32) PRN code, since you don't know what satellites are
> over your local horizon.
Yes
> 2. Try multiple frequencies in order to compensate doppler deviation of
> unknown satellites respect to your unknown position.
Yes
> Since PRN codes are repeated every millisecond and they are 1023 bits
> long and 32 PRN codes in total, suppossing a non parallel correlator you
> would need about 1-2 seconds per PRN code, 32-64 seconds in total. But
> you must do that searching in several frequencies, because the doppler
> thing.
Yes.
> So, my questions are:
>
> a) Are my appreciations basically right?.
Yes. it is a very big search space.
> b) What is the maximum doppler offset you could possibly expect?.
+/- 5kHz from satellite motion. You usually search +/- 10kHz if you don't know
how bad your own clock accuracy is.
> c) What could be the frequency "step" in that interval?.
If you search for 1ms at a time, you can do 500 Hz steps. If you search for 10
ms at a time to find weaker satellites, you need to do finer steps.
> From b and c, and the 32-64 seconds figure for full PRN code search, we
> could get the initialization time for a GPS "full dead cold start". An
> intuitive first aproximation, brute force search, could be "hours" :-p
It's not that bad. But if you have a limited number of correlators and you
don't use fft methods it can be as long as 15 minutes if you are unlucky.
Matt