[time-nuts] Designing an embedded precision GPS time

MLewis mlewis000 at rogers.com
Tue Oct 31 23:14:14 EDT 2017


I'm stuck with a near ground level antenna site (~16" above grade?), 
with half a sky view (thankfully to the SSE), less some low blocking 
buildings with regular mutlipath, plus multipath bouncing off a taller 
building to the SE that bounces sats from the NW at me from low over the 
Bering Strait. The building I'm in is concrete with flat steel under 
each floor from the construction method. As I write this I'm down to two 
green sats in LH.

A number of times a day, it will drop to one sat, and there's a few 
dropouts a day where it goes to none of sufficient signal. How many 
times and for how long varies by the day. It's worse when it's wet out, 
which it is right now. If I lower the signal strength threshold, then I 
end up with tons of multipath signals.

If I can ever get a bios update to my NEO-M8T, then I'll have GAL in the 
mix and should experience fewer dropouts, potentially none.

An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one 
to 20 minutes.

While I wrote this, LH was typically showing two or three green sats, 
once up to five and once down to one. And I just hit a dropout... for a 
minute and a half; the one remaining green sat went behind the corner of 
the building's entrance canopy, then back out.


On 31/10/2017 10:30 PM, Bob kb8tq wrote:
> Hi
>
> Under what conditions would you expect to loose GPS? I seem to be able to
> do just fine sitting in an armchair here in the family room. That’s hardly a
> fancy setup.
>
> Bob
>
>> On Oct 31, 2017, at 10:27 PM, MLewis <mlewis000 at rogers.com> wrote:
>>
>> I'm intending to add a "precision" (well, precision to the Pi world) RTC to my Pi 3 to use for a holdover source when it hasn't got PPS from the GPS module.
>>
>> On 31/10/2017 10:04 PM, Chris Caudle wrote:
>>> On Tue, October 31, 2017 7:19 pm, MLewis wrote:
>>>> ...the "better" quality RTCs seem to be DS3231 based
>>>> How does one translate that into an expected 24 hour holdover?
>>> For the RTC, or for an NTP server?  If the NTP server is running it will
>>> not make a difference, modern operating systems do not use the RTC for the
>>> system clock, only to get close to the correct time at startup.
>>>
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