[ntp:questions] Sure Electronics GPS board: Amazing performance. :-)
"terje.mathisen at tmsw.no" at ntp.org
Fri Apr 1 20:05:50 UTC 2011
Chris Albertson wrote:
> On Thu, Mar 31, 2011 at 11:03 PM, Terje Mathisen<"terje.mathisen at
> tmsw.no"@ntp.org> wrote:
>> I don't think any GPS comes even close to 2 ns, given that the entire GPS
>> constellation is only required to be within 14 ns or so of UTC.
> First off this is a "timing gps". That means the receiver "knows" the
> antenna is not moving and is fixed to a surveyed location..
> I think if the receiver does not meet their specs you could ask for a
> refund. They do place that 2ns spec near the top of their data sheet.
> I think they do two things
> 1) The error is predictable, one of the outputs from the device is an
> error prediction that looks like a sawtooth function if you plot it.
> The 2ns only applies if yo use the error prediction.
> 2) One can get much better than even 2ns if you average many seconds of signal.
That is a totally different problem!
Getting a GPS to be UTC-aligned on average is far simpler than what
we're usually discussing here, which is getting the jitter value (really
rms timing error) as low as possible.
> OK, I think I may have confused two email lists. This one is for NTP
> where most people don't need even microsecond accuracy. But other do
> use GPS for calibrating lab instruments and are needing oscillators
> with error at the 10E-10 or even better level.
Chris, I soldered together my first UT+ many years ago, based on the
TAPR board. I do know the difference between timing and position GPSs.
My corporate setup has 6 distributed servers (spread over 3 locations),
they use the 12-channel Oncore receivers. (With the Oncore driver the
sawtooth corrections are of course included.)
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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