[time-nuts] Re: NRCan PPP observations

[John Ackermann N8UR]

Hi Skip --

I've been doing something similar, but though I've been collecting data 
for several months I haven't gotten as far along in analyzing it as I 
wanted to.

But I thought I'd pass on a couple of things I learned in private email 
with the NRCan staff:

1.  Many of the phase jumps are an artifact of the processing and they 
say is completely appropriate to address them by adjusting the offset 
values at the skip point to add or subtract the appropriate correction 
to all offset records going forward.  I wrote a Python script that goes 
through the clock offset file and makes a correction whenever there's a 
jump of a specified value or greater.  500 ps seems to work pretty well.

2.  It's much better to concatenate multiple days data into one RINEX 
file, than to combine phase records after processing.  There are 
discontinuities introduced as each file is processed, and that can lead 
to phase jumps when you combine the clock data.  Putting as much data as 
possible in each RINEX file is the best approach.

3.  The NRCan website says you can submit up to 7 days of data in a 
single file, but after you read the fine print it turns out that the 
limit is actually 6 days.

John
----

On 7/22/22 14:06, Skip Withrow via time-nuts wrote:
> 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
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