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Re: [questions] Re: GPS+PPS vs NTP server, why a huge offset ?

I like the adafruit gps hat

Chip Blach

On Fri, Jun 17, 2022, 1:16 PM Terje Mathisen <terje.mathisen@xxxxxxx> wrote:
David Woolley wrote:
> On 17/06/2022 16:34, chris wrote:
>> As for compatibility, while a mismatched connection may work, it's bad
>> practice to do that, where you are dealing with microsecond timing
>> and want to avoid jitter. Use the correct interfaces and do the job
>> right, then you can fit and forget:-)...
> RS232 isn't optimal for PPS as it is slew rate limited to 30V/µs, which
> means it will take at least 0.5µs from a resting level until it has
> fully cleared the transition region, if implemented to standard.
> TTL is the more natural logic family for high accuracy PPS.
> GPS should be able of achieving time transfer accuracies of better than
> .03µs.

GPS, even at the $80 Shure evaluation board level, have been directly
measured at the ~25 ns level, which is pretty much what you claim here. :-)

The key idea is of course that in order to know where a GPS is located
with better than 3 m precision, the unit by implication also knows what
time it is, to within 10 ns of UTC(USNO). The only problem is to be able
to convey that info to a connected NTP server.


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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