> 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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
