[time-nuts] GPS Survey

[Dan Kemppainen]

Hi All,

Had to set up a GPSDO at a different location for a test. It allowed me 
to record some data for a neat little plot that some of you may be 
interested in.

The attached plot shows the TIC phase between the GPS PPS and OCXO, and 
the DAC value used to steer (yank around!) the OCXO. The graph starts 
during the survey and continues after the survey is done. The green 
trace indicates survey process running (hi) and done (low). The Blue 
trace is GPSDO saw tooth corrected phase compared to the OCXO. The 
Yellow is DAC value in counts used to steer the OCXO.



A few things to note:

The antenna location is NOT optimal. It's on a file cabinet inside a 
mostly wooden single story building, with one metal wall approximately 
20 feet east of the antenna. A few feet of snow on the shingle roof. No 
windows east, south, or west.

The GPS is a Ublox LEA-6T.

GPSDO TIC resolution is around 40pS (Big round numbers).

The blue phase plot is saw tooth corrected.

The DAC is a 20bit DAC. Each count represents approximately 4uHz worth 
of steering applied to the EFC (big round numbers here!).

P-P phase difference before and after the survey is approximately 185nS 
and 18.5nS respectively.

The oscillator is drifting at around -1.35E-10 per day.

Not much here that could be used quantitatively, however it gives a 
qualitative feel for the difference between running a survey or not.



Happy Holidays!

-Dan



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