[time-nuts] Strange GPS behaviour

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 29 00:18:42 UTC 2012

On 28/12/12 23:35, Bob Camp wrote:
> Hi
> The GPS does an estimate against the local crystal frequency. It generates the PPS off of it's estimate. The less often it updates the estimate the more odd things you see as the crystal drifts.

A typical GPS off the shelf solves the position solution every second, 
having a 1 Hz report rate. This includes clock corrections. Some GPSes 
is capable of higher report-rates.

> Of course, the crystal can have trouble all it's own. If the crystal has a rapid rate of frequency change over a narrow temperature range, the GPS simply can't keep up with the crystal.

Most GPS receivers only have TCXOs, and even if tossing in an OCXO, 
excessive heat can throw the frequency and hence the GPS solution way of 
the mark. For many GPS reference stations, rubidiums is used to steer 
the internal clock, and the quality of that lock can affect how well it 
tracks it and have secondary frequency issues.

So, it comes as no surprise that the GPS module is temperature 
sensitive. The metrology labs measure and compare the temperature 
stability of various GPS-receivers,

There are also filters that can provide temperature effects, but the 
TCXO is where it usually hurts most.


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