On 3/30/17 11:06 AM, Peter Monta wrote:
I am curious if the first local oscillator on a GPS receiver must actually
be locked or coherent to the reference oscillator in the GPS receiver
typically running at some 10 MHz approximately. Or as long as the first LO
is quite stable it doesn't matter because the receiver can track the code.
It doesn't matter, so long as the first LO is in the ballpark so that the
Doppler search is not needlessly large. I'm not so familiar with the early
receivers, but I imagine a single reference oscillator serves for
everything---there would seem to be no reason to have more than one unless
the antenna/downconverter were physically separate from the rest of the
receiver. If an older receiver used a physical source at 10.23 MHz, it
would still need to be offset slightly for each satellite because of "code
doppler", but this choice of frequency might slightly simplify the
circuitry. Current receivers would use any convenient physical rate, then
synthesize the code rates.
BTW a lot of GPS receivers don't have a "first LO".. they are more like
a Tuned RF receiver - an input BPF for L1, L2, or L5, then direct
sampling at around 30-40 MHz - something that makes the GPS signals
alias down somewhere convenient (and always have positive frequency
offset from zero, even at max negative Doppler)
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