Speaking of Doppler shifts, back when we were doing the GPS class we were catching satellites at >10kHz Doppler using the recorded data we found, and that really shouldn't be possible for a receiver at rest (relative to the Earth surface).
I never looked into it further but i am suspicious of the doppler numbers we get from the soft correlator. -Nathan On Thu, Jun 4, 2015 at 10:59 AM, Jamey Sharp <ja...@psas.pdx.edu> wrote: > Awesome! And nice job on the pretty pictures! > > I think this is strong evidence that we should get access to a real > satellite simulator, soon. > > Though I don't understand why it saw a -14.8kHz Doppler shift. The original > file is a zero-IF sample. Did you set the HackRF at a center frequency > different than 1575.42MHz, or multiply the file sample by a sine wave, > perhaps? That seems outside the range of reasonable clock errors to me. > > Hey, does your copy of soft-correlator produce the same output mine does for > the looped original prn1? It looks like we didn't push the change that sets > the IF center frequency to 0 instead of 1.95MHz. > > Notice the bogus satellites have very low signal/noise ratios (although you > should be skeptical of my math for computing those, FYI). Apparently my > heuristic is wrong for guessing that a satellite is visible; I based it on > my clock error estimate, and all of that is something I made up, not > something I've seen documented as the usual way to build a receiver. > > Jamey > > On Jun 4, 2015 10:27 AM, "Kenny" <ke...@romhat.net> wrote: >> >> For all you folks following along at home who want to see some pretty >> pictures, here you go! >> >> >> https://drive.google.com/open?id=1pnFV5MWkx5YR9ZlutetgDrejNG9SZx6Zp6xbdKl48Ps&authuser=0 >> >> I sometimes have blissful moments of forgetting that Google wants all of >> my data, always. In this particular instance, we may as well let them >> help out. So there is a Google Drive folder linked at the bottom of the >> doc which contains "gpslog_big-outside.raw" along with various other >> files we have been talking about. >> >> Transmitting the prn1 converted to complex floats through the HackRF and >> grabbing a sample with jGPSv3 correlates! Check out that >> signal-to-noise ratio for SV 1!. >> >> $ ./soft-correlator < data/gpslog_prn1_rx.raw >> # frequency of 1-bits: i-sign 51.7%, i-mag 33.6%, q-sign 48.8%, q-mag >> 30.0% >> # SV, S/N ratio, doppler shift (Hz), code phase (chips), sample clock >> error (chips/s) >> * 1 601.457508 -14801.532243 253.000000 9.611385 >> * 2 14.721936 -799.801893 330.000000 0.519352 >> * 4 15.425549 -6801.062185 669.000000 4.416274 >> * 11 14.114000 -10801.489275 302.000000 7.013954 >> * 17 15.250208 2199.254809 77.000000 -1.428088 >> * 24 14.148221 -7794.920364 335.000000 5.061637 >> * 28 13.145979 14199.180477 545.000000 -9.220247 >> * 30 13.199176 5798.007429 466.750000 -28.764940 >> # 8 satellites in view; average clock error -1.598834 chips/s >> >> I'm surprised to see that the added noise was enough to correlate the >> pseudo-noise sequence from other satellite vehicles. This was done in >> the basement, so I know it wasn't accidentally picking up any real >> satellites. >> >> -- >> Kenny >> >> -+---+++-++-++++--+------+-+-++--++--+-+-++--+++-++----+-++-+++---+----+--+----+ >> >> >> >> > > _______________________________________________ > psas-avionics mailing list > email@example.com > http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics > _______________________________________________ psas-avionics mailing list firstname.lastname@example.org http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics