[time-nuts] Re: Measuring oscillators against GPS

[Stewart Cobb]

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