[time-nuts] Some results of PRS10 and Trimble Resolution

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Jun 29 22:34:26 EDT 2006

Hello Tom,
I had a conversation with Sam S. from TSC the other day, and he said that  
it's probably not possible to get <10ns GPS accuracy anyways due to the  
multipath issues, Ionospheric issues, antenna survey issues, thermal issues  etc.
I tend to agree with Sam; to get this kind of accuracy to UTC ([or better:  
GPS time]  a documented 2ns within 300 hours as tested on the M12+ by  
Synergy/USNO) one needs to average over several hours, or even days as done by  USNO.
This is documented in many long term plots of GPS versus Cesium.  
Instantaneous drift in all of these plots seems to be >>+-5ns due to  diurnal effects etc.
Our units typically average the GPS 1PPS over 30 minutes, so having less  
than 3.33ns error on the 1PPS capture may not improve things much because  while 
the error stays always at +-3.33ns it get's averaged over the  measurement 
Of course this would be different if we use a Cesium 1PPS output to  lock to. 
Also, one must ask the question: what is the correct time? Is  the relative 
time between two GPS disciplined stations close to each other such  as are used 
for CDMA base stations (some of the errors cancel out between the  two 
stations' frequency differences) good enough? Certainly diurnal and  ionospheric 
effects between the two stations will cancel if they see the same  sat's.
Sam had a point in that to get 1ns accuracy from your GPS, you must have a  
fantastic antenna not affected by environmental effects, and also know it's  
position to within 1 foot in all three dimensions!

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