[time-nuts] NRCan PPP observations

[Skip Withrow]

Hello time-nuts,

I have been trying to characterize a very good oscillator (MHM-A1 hydrogen
maser) and have resorted to post-processing dual-band GPS observations with
Natural Resources Canada (NRCan) PPP submissions. I have some observations
that might help others trying to do the same, or if others have discovered
better techniques, I'm all ears.

My setup is to run the DUT (maser) into the external clock input of a
Trimble NetRS GPS receiver that is operated in fixed position mode with
clock steering off.  Daily data files are collected that can be converted
to RINEX format and submitted to NRCan.  I like NRCan because the output
gives you a graph of the clock offset and a data file with the same
information.

1. There may be jumps in the output data/graphs.  At first I thought this
might be due to jumps in the maser oscillator, but have come to the
conclusion that they are primarily software processing artifacts.  I seem
to see them in the 'Rapid' solutions, and the 'Final' data seems smooth
(though I haven't processed a lot of data with the final solutions yet).

2. In an effort to get a longer time series for the clock offset I looked
at taking several daily files (Rapid data solution) and gluing them
together.  It doesn't work because there are significant offsets between
each graph.

3. I have also taken several days of raw data and processed them into one
RINEX file.  This works 'mostly', but is subject to the same jumps that are
seen in the daily 'Rapid' processed files.

4. Taking the same combined data and processing it with the NRCan 'Final'
data seems to get rid of the jumps and gives a smooth graph.  HOWEVER, I
have found that the magnitude (slope) of the data may be significantly
different over the data collection period.  This is true for daily files as
well as week long files.

What I have learned is that with very good oscillators patience is a must.
You really can't make adjustments based on NRCan 'Rapid' data (it will at
least give you a clue).  Waiting for the 'Final' data (2-3 weeks) is more
accurate, but is very non-real-time (though if your oscillator is very good
things should be about the same several weeks later).

I still have lots of things to still try.  One is adding an a priori
position to the RINEX header and seeing if that makes any difference (it is
currently listed as 0 0 0).  I also have another dual-band receiver that
will record clock offset directly, so I want to make a comparison of the
noise on that data compared to post-processed data.

Do know where my GPS antenna is now.  Daily files give 2mm x 3mm x 9mm
error volume, week files give 1mm x 1mm x 4mm error volume.

Regards,
Skip Withrow