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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