[time-nuts] Common sky pps errors for any GPSDOs?
SAIDJACK at aol.com
SAIDJACK at aol.com
Tue Jan 6 06:59:18 UTC 2009
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
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