On Mon, Jan 5, 2009 at 10:59 PM, <saidj...@aol.com> wrote: > 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. >
I see what you are saying here. I guess that it will be necessary to have some way to calibrate out the long term phase variations from the received signals just like in VLBI. I think it would be interesting to see the raw data from a time interval counter on the PPS of two identical GPSDOs sharing an antenna. If anyone has that sort of data, maybe over a day or two, I'd love to get a copy. Thanks, Matt _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.