[time-nuts] "GPS once a day issues" ?

[Bob Camp]

Hi

Gee, now after a few cups of coffee … yes that does appear to be the sun.

————

The GPS system does it’s best to model the ionosphere and transmit that data. Unfortunately the model / model resolution is not as good as it could be. That lets the ionosphere creep into the solution more than it might with a perfect model. My *guess* (as in I have no data) is that constellations with a significant number of low(er) angle sats *and* a sun rise / sun set over one end of the constellation are the worst ones. That could easily be pure bunk.

Bob

> On Oct 20, 2014, at 7:31 AM, John C. Westmoreland, P.E. <john at westmorelandengineering.com> wrote:
> 
> Bob,
> 
> You mean the Sun, correct?
> 
> Regards,
> John
> On Oct 20, 2014 4:16 AM, "Bob Camp" <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> Yes, but there’s this large object in the sky that modifies the ionosphere
>> as it travels in a “about one a day” track. It appears to be coming up just
>> about now, but I do need more coffee to be sure …
>> 
>> The combination of the constellation and the ionosphere are what I believe
>> give you the once a day (rather than once per 12 hours) bump.
>> 
>> Bob
>> 
>>> On Oct 20, 2014, at 3:43 AM, Magnus Danielson <
>> magnus at rubidium.dyndns.org> wrote:
>>> 
>>> Bob,
>>> 
>>> Since the satellite orbit the earth with a period of 11 hours and 58
>> minutes, it is actually twice a day.
>>> 
>>> Cheers,
>>> Magnus
>>> 
>>> On 10/20/2014 03:50 AM, Bob Camp wrote:
>>>> Hi
>>>> 
>>>> The GPS constellation repeats roughly once a day. It is not at all
>> uncommon to have a “worst case” sattelite  geometry for a given antenna
>> location. If you have one, it will repeat once a day and show up as a bump
>> in the timing out of your GPS module. If you track long term data, it will
>> / may / can keep you from getting to the sort of stability you would expect
>> in the 100,000 second range. It’s one of the main reasons that things like
>> GPSD-Rb’s lock up with time constants much longer than 100K seconds. Yes
>> having a Cs or something similar helps a lot looking for this sort of thing.
>>>> 
>>>> Bob
>>>> 
>>>>> On Oct 19, 2014, at 9:26 PM, Bob Stewart <bob at evoria.net> wrote:
>>>>> 
>>>>> Hi Bob Camp,
>>>>> 
>>>>> 
>>>>> In your response to Chris, you said: "Once you have it “right” you
>> really need to check it over a month or two to watch for GPS “once a day”
>> issues. "
>>>>> 
>>>>> Could I ask you what you meant by these "once a day issues"?  Was this
>> a general comment, or was it about something specific?  As you know I'm
>> working on a GPSDO and am doing a lot of testing, so if there's something
>> else I should be looking for, please let me know.
>>>>> 
>>>>> 
>>>>> Bob - AE6RV
>>>>> 
>>>>> 
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