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
>>
>> -+---+++-++-++++--+------+-+-++--++--+-+-++--+++-++----+-++-+++---+----+--+----+
>>
>>
>>
>>
>
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