[time-nuts] GPSDO - probably a stupid question.

Jay Grizzard elfchief-timenuts at lupine.org
Wed Aug 17 16:01:10 EDT 2016

Wouldn't you also not be able to actually sync to the individual chips, 
since you can't really see the start of any given chip so much as you 
just see the correlation over larger sections of the stream? Plus you'd 
have to track only one SV at a time (right? Since I doubt the edges of 
every chip are perfectly aligned across all SVs even under the best 
conditions), so things like brief multipath excursions or even 
atmospheric/ionospheric fluctuations would push you off by a bit as well...

(which is why, of course, you have the long control loop that GPSDOs use)


On 2016-08-17 11:41 , Didier Juges wrote:
> In fact, you do not want to "update the crystal one million times/second".
> The whole point of a GPSDO is to combine the excellent short term stability
> of the crystal with the excellent long term stability of the GPS signal. If
> you update the crystal in real time from the GPS data, you do not need the
> crystal...
> The control loop of GPSDOs usually have an effective bandwidth measured in
> minutes or even hours in the case of rubidium oscillators.
> On Wed, Aug 17, 2016 at 11:57 AM, Peter Reilley <preilley_454 at comcast.net>
> wrote:
>> You can get crystal oscillators that have a frequency control signal and
>> are more
>> stable than the run of the mill oscillators.   Changing the GPS oscillator
>> would
>> require modifying a very tightly populated circuit board.   Perhaps not
>> possible.
>> What about some of the SDR (software defined radio) projects that aim to
>> implement GPS functionality?   If you used the GPS chipping rate (1.023
>> MHz)
>> to dicipline the 10 MHz oscillator then you are less sensitive to crystal
>> instabilities.
>> You are updating the crystal one million times a second rather than once
>> per second.
>> This is assuming that the chipping rate of the transmitter is just as good
>> as the
>> 1 PPS signal.   This info from here;
>> https://www.e-education.psu.edu/geog862/node/1753
>> and here;
>> https://en.wikipedia.org/wiki/GPS_signals
>> Even using the 50 bits/sec data rate of the GPS signal would allow
>> updating the
>> GPSDO faster than the 1 PPS signal.
>> Pete.
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