[time-nuts] Strange GPS behaviour
mkperrett at gmail.com
Sat Dec 29 05:14:16 UTC 2012
That is simply not accurate - if the solution rate is 1/second, then all
parameters are solved in that time frame. There are 4 indpendent variables
and minimal processing power is required to solve all four equations.
Although I am not very familiar with commercial receivers, that is what
happens in the Rockwell, Trimble and IEC military units. If the output is
more than once per second it is *usually* an output of the Kalman Filter,
not a "true" measurement.
Michael / K7HIL
On Fri, Dec 28, 2012 at 6:20 PM, Bob Camp <lists at rtty.us> wrote:
> …. 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>
> > 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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