[time-nuts] nuts about position

Bob kb8tq kb8tq at n1k.org
Wed May 30 17:02:39 EDT 2018


It’s not real clear what the magic L1 / L2 / L5 gizmo does for you right now today. As far as I can tell, 
the free processing services all only do two at a time. You can do L1 / L2 or L1 /L5. Possibly at some
point in the future that will change. 

Even with “only two” a fancy device will get you down to numbers that simply are not believable. Any
device that thinks it’s getting into 0.000x meter precision needs to back up and look at some of the 
reference points being used.  

Getting back to timing, if you guess that 0.3 m is about a nanosecond, the M8T is off by about a nanosecond
in lat and lon. It’s off by about 2 ns in elevation.  The time solution will be some sort of RMS of this. Unless
the one and only sat you can see passes directly over you, the full 2 ns isn’t going to come in. 

Location will always be one of a list of errors in getting time data. The bigger question will be how well the 
other stuff on the list is controlled / known. Coming up with an RF delay number that is good to < 1 ns on
any of these gizmos may be a bit tough…..

If frequency rather than time is the goal, the position error will indeed add to the “swing” over a day. Your 
antenna location / sky view will be a big part of the “how much” part of the question. You might see a 
nanosecond of swing on an hour to hour basis …. you probably would see less. Simply put, it’s in the
3x10^-12 or less range frequency wise. Most GPSDO’s struggle below a few parts in 10^-11 for a variety
of reasons ….


> On May 30, 2018, at 3:46 PM, Mark Sims <holrum at hotmail.com> wrote:
> Ok, I did the experiment.  I took an 8 hour M8T RINEX file with GPS,SBAS,GLONASS,GALILEO data in it.  I then used to TEQC to extract GPS only and GLONASS only data (CSRS-PPP ignores SBAS and GALILEO data).   The 95% confidence error ellipse estimates ("rapid" orbits) were:
> GPS+GLONASS:    lat 0.245m    lon 0.219m    alt 0.582m
> GPS ONLY:             lat 0.269m    lon 0.239m    alt 0.616m
> GLONASS ONLY:   lat 0.610m    lon 0.755m    alt 1.835m
> Conclusion... if you are just interested in an accurate antenna position,  a $25 LEA-5T should do just fine.  GLONASS brings very little benefit (call it an inch) to the party.  24 hours of observations can reduce those numbers by around 50%   I used one of those $70 Chinese L1/L2/GLONASS/BEIDOU antennas from Ebay.   Note that my antenna location/environment is HORRIBLE... 
> Things might change if you get into expensive L1+L2(or L5) receivers.
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