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[Discuss-gnuradio] Another question about GPS

From: Jesus Cea
Subject: [Discuss-gnuradio] Another question about GPS
Date: Mon, 28 Mar 2005 18:31:15 +0200
User-agent: Mozilla Thunderbird 1.0 (X11/20041206)

I've just read the GPS specs.  Good material. Thanks for the reference.

I have now a more relevant question for this list: How can you easily lock the GPS sats when a) you don't have an almanac, b) you don't have a clock aproximate reference and c) you don't know your approximate location.

As far as I understand GPS, when you have all three said datasets, you know more or less what satellites you could "see" in your sky, its PRN codes and aproximate doppler deviation. So, "cold" adquisition is about 45 seconds, in order to download current ephemeris. Nice.

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.

2. Try multiple frequencies in order to compensate doppler deviation of unknown satellites respect to your unknown position.

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.

So, my questions are:

a) Are my appreciations basically right?.

b) What is the maximum doppler offset you could possibly expect?.

c) What could be the frequency "step" in that interval?.

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

Thanks for your time and attention.

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