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

[John Ackermann N8UR]

Hi Tim --

See below.

On 10/4/22 18:31, Tim Lister via time-nuts 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

I think that's a good assumption.  Even a single frequency GPS can 
easily be accurate and stable to <100 ns.  If you're using standard 
computer hardware the latency and jitter of IRQ handling and CPU 
overhead is far more than that.  The improvement from a dual-frequency 
receiver isn't going to be noticeable in that application.

> 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 ?

Most GPS receivers these days have extremely sensitive front ends and 
there's no magic required for a splitter -- many people have used cable 
or satellite TV splitters with good results.  You do need to worry about 
the DC path so that the LNA at the antenna gets power, and none of the 
receivers sees a DC short on its antenna input, but that's pretty 
straightforward.

With dual frequency systems you do need to pay some attention to the 
frequency response of the system since the signals are separated by ~300 
MHz.  And you need to make sure the splitter doesn't have any filtering 
that will pass L1 but not L2 -- most "real" GPS splitters are L1 only.

> 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 ?

Historically L1/L2 antennas have in fact been insanely expensive, but 
that's starting to change.  u-blox sells a mini-hockey-puck mag mount 
L1/L2 antenna for ~$100 IIRC, and there is at least one Chinese eBay 
source for L1/L2/L5 "base" antennas for ~$200.  Both of those will work 
just fine for timing, though they don't have the precise phase center 
needed for geodetic measurements.

Hope this helps!

John