[time-nuts] Re: Use of dual frequency GPS for NTP server and other purposes

[Bob kb8tq]

Hi

The main issue with using the output of a GPS receiver directly is 
dropouts. They can be caused by a variety of things. When the GPS
“goes away” you loose all timing information.  Do these various things
( jamming, birds, lightning, …..) impact you? If they do, depending on 
just how the loss of timing impacts you, you do or don’t have a problem. 

Into an NTP server, the impact will still be there. How great an impact
it is depends a *lot* on what the next level source going into the server
is. The commercial NTP gizmos tend to have things like OCXO’s or Rb
standards in them. That gives them a bit better performance than a 
typical DIY computer based NTP server. 

If you *do* go with a dual freq GPS, you can do a very good job of working
out the location of your antenna. With some effort that can improve the
net timing accuracy. If post processing timing correction is ok in this 
case, that also is a lot easier / more accurate with a full set of dual freq
GPS data to correct against. 

So, when it works, you might be down into the < 10 ns range. When that
bird sits on that antenna …. microseconds …. takes a nap … milliseconds ….

Lots of tradeoffs.

Bob

> On Oct 4, 2022, at 6:31 PM, Tim Lister via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi everyone, wanting to pick the collective brains of the group
> regarding the use of dual frequency GPS antenna/receivers for NTP time
> servers. I work for an observatory that runs telescopes at various
> locations around the world and we had previously used Symmetricomm
> S250 and S300 NTP time servers to allow remote management and to
> provide NTP and IRIG-B signals but these are now affected by the Week
> Number Roll Over problem. With the lowering cost of dual frequency
> L1/L2 GPS receivers and antennas, I was thinking it would be nice to
> have a (likely separate) dual frequency receiver to allow estimation
> of the precipitable water vapor above the observatory. Timing
> precision needed for the NTP signals is the ~2-3ms expected over local
> Ethernet and ~20-50us for a 1PPS output for the future; commercial
> solutions rather than homebrew strongly preferred for the time server.
> 
> So assuming that there aren't dual frequency time servers or a great
> deal of benefit of dual frequency receivers in a time server for this
> needed time precision:
> 1) Is putting a 2 way splitter in front on the time server to split
> the GPS signal for the time server and the experimental dual frequency
> receiver going to cause issues ? Are there recommended 2 way splitters
> that pass L1 and L2 well ?
> 2) Any recommendations for good quality but not insanely expensive
> L1/L2 antenna that will survive being on a pole on a remote high
> mountaintop for ~decade ?
> 
> Thanks for any input,
> Tim
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