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pmonta / GNSS-DSP-tools

GNSS code generators, acquisition, and tracking
MIT License
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Offset to L1 carrier frequency #3

Open RazaJaved opened 6 years ago

RazaJaved commented 6 years ago

I am trying to implement Galileo E1 acquisition and tracking. but I have some doubts:

  1. What is this offset to L1 carrier frequency? (Line no: 96 in Track_galileo_E1b.py)
  2. While parsing command line arguments, why the sampling frequency is different from the initialised value at the top of acquisition and tracking code?

Thank you Raza Javed

pmonta commented 4 years ago

Sorry for the very late response on this.

  1. This offset represents the frequency difference between the L1 carrier and the center frequency of the input stream, which consists of complex samples. For my hardware this sample rate is 69.984 MHz, and so the input stream spans frequencies from -35 MHz to +35 MHz roughly. This passband has been frequency-translated from the original L-band signal; for my hardware, the center frequency of the passband is near 1584 MHz so that the L1 channel can cover both GPS and GLONASS. Thus, the offset of -9.33 MHz is the difference between 1584.xxx and 1575.42. If your sample stream is different, you'll have to adjust the sample rate and offset. By the way, the offset need not be overly exact---any change in the offset just gets forwarded to the same change in the doppler estimates. You do need to get it within the doppler search range, though.

  2. Some of the files contain Python comments with example command lines showing sample rate, offset, etc. The ones that contain sample rates of "68873142.857" are obsolete; they were for a previous firmware revision of my receiver hardware. They should all be changed to "69984000" for consistency. Thanks; I'll see about revising (or eliminating) these example command lines.

Cheers, Peter