Chris:
So you are not tracking the signal in the traditional sense, only
looking at the frequency and delay estimate of your FFT acquisition. How
far spaced are your Doppler bins in the FFT acquisition? If you are
using a 1 ms integration time, and hence a 500 Hz bin spacing, if the
signal's true Doppler is towards the middle of a bin it is ambiguous
which bin the peak power will reside in due to noise. I have observed
that the USRP's clock is jittery (PLL tracking is only possible by
opening up the bandwidth beyond the recommend 18 Hz for a 3rd order
PLL), however I have never observed a 500 Hz instantaneous jump in
frequency (which would cause all of the channels to dump). Do you have
tracking data with multiple SVs which also point to a 500 Hz frequency jump?
--
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