[Discuss-gnuradio] Another question about GPS

[Jesus Cea]

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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