[time-nuts] Re: Measuring oscillators against GPS

[John Ackermann N8UR]

I've been working on frequency/stability measurements using the Trimble 
NetRS and newer Mosaic-T dual-frequency receivers.  Very early data 
indicates that the NetRS does just about as well as the Mosaic, assuming 
you are only interested in GPS and not GLONASS or the other 
constellations the Mosaic-T covers.

I have been sending data to the Natural Resources Canada PPP processing 
system (https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php) and 
am just finishing a set of python scripts to completely automate the 
process from pulling the files from the GPS via FTP to building an 
ever-growing file of phase values that can be fed into TimeLab.

Attached is an ADEV plot for 18 days of post-processed data.  I don't 
know whether the noise floor at around 2e-15 is the measurement system, 
or the reference.

In a few weeks, I hope to have some more complete results to show off, 
and the python scripts uploaded to Github for anyone else who might want 
to try this.

If NetRS's are becoming available as Stewart noted, that's a Very Good 
Thing as they really do work well and are easy to manage.  The only 
downsides are (a) GPS only, and (b) the data files are in a proprietary 
format that needs to be converted to RINEX.  (My script handles the 
conversion process, using a couple of third-party programs.)  And 
remember they require a dual-frequency antenna.  Those have been scarce 
but there seem to be some hockey-puck antennas available now for <$100 
that might do the job.

John
----

On 3/7/22 4:02 PM, Stewart Cobb wrote:
> Skip Withrow's writeup is very interesting. As Skip mentioned, the Trimble
> NetRS is a survey-grade dual frequency GPS receiver which accepts a 10 MHz
> clock input.
> 
> It appears that the US Coast Guard has recently replaced a large number of
> these NetRS units, and sent the old ones to surplus. They are now appearing
> on eBay for approximately the price of a Thunderbolt (they used to be
> several times higher). This situation probably will not last.
> 
> It seems possible to use a NetRS, plus an OCXO and an external computer
> such as a Raspberry Pi, to build a dual-frequency GPSDO which would be
> immune to ionosphere effects.
> 
> It seems possible to extend such a system by running RTKLIB on the external
> computer, fed with global GPS correction data from ITRS via NTRIP, to build
> a GPSDO with PPS precision of a few nanoseconds.
> 
> Implementation is left as an exercise for the reader (but see Ole's hints
> at https://www.efos3.com).
> 
> Cheers!
> --Stu
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