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, [email protected] 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
