[time-nuts] Designing an embedded precision GPS time
kb8tq at n1k.org
Wed Nov 1 10:22:58 EDT 2017
Ok, local RF interference sounds like a significant part of the problem. I would
suggest that swapping antennas might make sense. Not all “super interference
rejecting” antennas are created equal.
> On Nov 1, 2017, at 9:55 AM, MLewis <mlewis000 at rogers.com> wrote:
> I wish.
> It's using GLO and GPS now, yet gets reception dropouts.
> That's why I'm hoping to eventually get the firmware update that will add GAL to the mix.
> I had anticipated reception issues, which is why I went with the M8T for its sensitivity, multi-constellation and it's a timing module so a good PPS on a single sat - only to get surprised that my version didn't have GAL enabled. But I didn't envision reception would be so bad that not having GAL would be material.
> I'm also too close to that tall building that is reflecting the sats over the Bering Strait at me. It's a military computer site, which I thought would be pretty tight on stray RF, but it has antennas. I asked a friend who works there about my GPS issues and if RF from the site may be influencing things. He hesitated, then said "'Yes'. That's all I can say."
> For first power up I had obtained an active antenna for multi-constellation and a pre-filter that "provides protection from near frequency or strong harmonic interfering signals."
> As it's just for NTP accuracy, I may be better off letting all the multipath through than getting dropouts. I'm starting to think that part of my problem is that I know the GPS is capable of getting better than I need, so instead of working to get what I need, I want what it should be capable of.
> But I feel a better solution is a reliable holdover capability, which I should have anyway for failsafe. So perhaps the reception dropouts are a way to make me address holdover properly rather than limp in and get surprised later.
> Hence adding a 'precision' RTC to the Pi.
> On 01/11/2017 8:45 AM, Bob kb8tq wrote:
>> For NTP levels of accuracy Glonas is quite fine. Combining that with GPS should
>> get you a pretty good “time source” even under your extreme conditions.
>>> On Oct 31, 2017, at 11:14 PM, MLewis <mlewis000 at rogers.com> wrote:
>>> 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:
>>>> 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.
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