[time-nuts] Re: NRCan PPP observations

[Bob kb8tq]

Hi

There are some pretty well known issues that these processing services 
run into. Often they show up as a jump at midnight GPS time. Many folks just
check that the jump is at the “expected time” and adjust it out of the data.

There are normally three data sets you can get at. One is a “couple of hours
later” set. The next is a “couple of days later”. The best of the best is a couple
of weeks later. Since the data comes out on the same sort of “this or that GPS time "
basis, exactly when that meshes with your data set makes the “when available”
time a bit ambiguous.

One simple answer to this is to align your data to whatever the time / date
setup is for the data set you are after. Midnight GPS time is always a good bet. Which
day to start on is the next thing to dig into. The data gets reduced to 30 second
chunks by most outfits. That’s 30 second ( wait for it … GPS time) ticks. It’s amazingly
easy to set things up for UTC and have the leap second nonsense get in the
way of that …. 

Fun !!

Bob

> On Jul 22, 2022, at 10:06 AM, Skip Withrow via time-nuts <time-nuts at lists.febo.com> 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
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com