[time-nuts] Common sky pps errors for any GPSDOs?

[SAIDJACK at aol.com]

Hi Matt,
 
I must admit I don't fully understand your requirements. Are you looking  for 
correlation between errors, or absolute UTC accuracy, or short term  
jitter/wander?
 
If you have two systems with self-surveyed antenna positions, you will  
likely have 1 - 10 feet of antenna height error in the self survey on the  Motorola 
timing receivers (typically).
 
This position-hold error in itself will give you much more than 140ps  error 
(offset, drift, wander) between the units as satellites fade in and  out of 
solution, even if the units are sitting right next to each other and are  seeing 
the same systemic GPS errors.
 
For example, let's say both units share the same antenna, and after  
auto-survey one reports it's height as 10 feet MSL, the other unit as 15  feet MSL 
(M12M's have easily more than 5 feet height error after  self-survey).
 
So if you now compare the outputs of the units, satellites  directly overhead 
could cause a 5 feet, or ~5ns error, while sats at  the horizon will not be 
affected by the height error, but rather the long/lat  errors (which are much 
smaller).
 
So unless you have a perfectly surveyed antenna position stored in the two  
receivers (to within < 1 foot) you will get GPS systemic errors as well  as 
timing errors due to position error - especially due to antenna height  errors.
 
When we say units typically have 25ns unit-to-unit variation on the 1PPS on  
un-calibrated units, then I believe most of this is caused by the auto-survey  
position errors of the GPS receiver. One could get much better performance by 
 manually entering the exact position-hold position of the antenna, and then  
calibrating for antenna cable delay (in 1ns steps).
 
This seems to yield down to 2ns performance as reported by  
Motorola/Synergy/NIST with careful calibration, and using a "proper" GPS timing  antenna with 
multipath choke-ring etc.
 
But again, this requires a perfectly surveyed antenna position, as well as  
offset correction due to antenna cable length delay.
 
There are also antenna cable length variations due to ambient temperature  
changes :)
 
Bruce and others had discussed these errors not too long ago. 140ps error  
(or 70ps per GPSDO unit) may be possible on a long antenna cable just  due to 
temperature changes on the cable..
 
Lastly, our units seem to have a residual PLL tracking noise floor  of down 
to 1.9ns rms when using a good double oven OCXO as can be seen  on the unit 
running in Mexico using a properly surveyed antenna position:
 
   _http://resco.ucol.mx/Fury/gpsstat.htm_ 
(http://resco.ucol.mx/Fury/gpsstat.htm) 
 
Getting 140ps matching offset error between two different units'  1PPS 
outputs may be tough to achieve.
 
bye,
Said 
 
 
 
 
In a message dated 1/5/2009 22:04:10 Pacific Standard Time,  
boyscout at gmail.com writes:

That  makes it sound a lot more difficult than it really is.  The  vast
majority of the error in GPS is systematic, such that two GPS  systems
with antennas near each other should have highly correlated  error.
This is the basis of differential GPS.  It doesn't matter if  the
absolute error is hundreds of feet, as long as  both devices have  the
same error.

I spent a couple of years nearly a decade ago doing  differential GPS
for steering heavy equipment.  You can get  sub-centimeter errors over
baselines in the tens of km.  Again, this  is relative  error.

Matt