[time-nuts] Re: Measuring oscillators against GPS

[Bob kb8tq]

Hi

As everybody runs off to scoop up the NetRS gizmos on eBay:

Like a lot of gear, they have a flash memory card in them. That card is 
(fortunately) removable. Some of the cards are well past their likely end
of life point. Just as with other devices, working out which card makes
them happy can involve some trial and error. 

No, not a big deal. 1 GB CF cards aren’t crazy large anymore. In fact
they are getting more rare simply because they are obsolete. As old
as the NetRS is, that stack of unused cards you stockpiled back in 
2006 probably will do just fine ….

They “expect” to run with a 12V antenna. The Trimble versions from
that era had ~50 db gain amps in them. A “modern” 3.3V 24 db gain 
antenna running into 200’ of RG-58 may not be ideal for use with them. 

Bob

> On Mar 7, 2022, at 4:44 PM, John Ackermann N8UR <jra at febo.com> wrote:
> 
> 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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