[time-nuts] Strange GPS behaviour

Bob Camp lists at rtty.us
Sat Dec 29 01:20:15 UTC 2012


…. except… A navigation GPS doesn't care much about the time solution. Updating the location is a much higher priority than updating the time. The typical "solution" is to let the time estimate coast for a while and update it much less often than the location.


On Dec 28, 2012, at 7:18 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> 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.
> Cheers,
> Magnus
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