[time-nuts] GPS first LO need to be locked?

Peter Monta pmonta at gmail.com
Thu Mar 30 14:06:00 EDT 2017

> 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.


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