[time-nuts] Tom Van Baak: a conversation about GPS timing

Estanislao Aguayo eaguayo at thinksrs.com
Tue Jan 26 17:58:58 UTC 2016

Hello Time-nuts mailing list,

Let me introduce myself, my name is Tani Aguayo, I am a physicist 
working on precision clocks for Stanford Research Systems (SRS). We had 
the pleasure of having Tom Van Baak visit our lab yesterday, and I want 
to write down some of the topics we talked about, and I thought this 
list would be a good place to keep it, and share it. And also have Tom 
go over it, and correct me if I got something wrong.

We talked GPS, and here is a couple of thought inducing comments from Tom:

Q: Why is there an timing offset of 20ns average between identical 
commercial off-the-shelf GPS modules?
TVB: A low-cost (~$25-$50) GPS receiver gathers timing information via a 
data lock to the local oscillator in the receiver, meaning the the 
timing information is based on data passed to the receiver and then the 
receiver uses that to produce a signal based on its own oscillator. This 
is one part of what can explain the differences between units, the 
stability of the receiver local clock will mask the precision  timing of 
the GPS 1 pps. The other part is that this timing data is based on the 
GPS antenna parameters, and so any disturbing effects (i.e. cable 
tempcos, impedance mismatches, active filtering in the antenna that 
could introduce group delays... ) on the transmission path from antenna 
to receiver, will affect the timing precision of the GPS 1pps at the 
receiver with respect to UTC.  This effect could change from unit to 
unit, as different antennas would have different effects on the signal.

Q: When it comes to using a GPSDO to measure timing performance (Sigma 
-Tau) any hiccups on GPS could make this calculation fail and you would 
not know until after the fact, is there a way around this dependence?
TVB: It is hard to get a beautiful 10MHz and see how in a matter of 
days, the timing quality is lost due to aging, tempcos, GPS hiccups... 
you name it. A better approach is to keep a log of your clock against 
GPS (without locking to it) and use an offline approach to compute your 
timing performance (Sigma-Tau) and this way you can look at your clock 
performance without mixing the GPS locking loop parameters.

Thank Tom for sharing this insight with us!

- Tani Aguayo

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