[time-nuts] Impact of GPS antenna height measurments
michaeljwouters at gmail.com
Mon Jun 27 08:31:14 EDT 2016
Poor coordinates for the antenna couple into the apparent distance to
a satellite in such a way that the error in the distance tracks a
parabola-shape as the satellite rises and descends, with a
corresponding error in the apparent satellite time. At any instant,
the combination of N satellites then leads to an apparent GPS time
with a mean offset roughly equal to position error, and peak to peak
scatter in the satellite times equal to the position error as well.
The offset isn't a problem, unless you care about time of day. How the
noise feeds into your GPSDO depends on the time constant of the servo.
I tried a simple numerical experiment, where I post-processed raw code
measurements from a timing receiver for different antenna heights,
comparing the results with those for its true position, known with an
accuracy of a few cm. The output of this calculation is a CGGTTS
time-transfer file, where the satellite time is averaged over 780 s.
For a 15 m error in height, at my latitude (30 deg) the offset is
about 28 ns and the peak to peak scatter is about 50 ns. If you then
try to do what the GPS receiver does, ie take an average of the
satellites, you get a peak to peak noise of about 10 ns.
I plotted the TOTDEV of this noise.
As you can see, at about 1000 s, the instability is about 10^-12,
averaging down to about 10^-13 at one day.
This does not compromise a GPSDO's frequency stability at these
averaging times. At shorter averaging times there may be a problem
though, depending on the time constant of the GPSDO, and the stability
of its oscillator.
On Sun, Jun 26, 2016 at 11:02 PM, Mark Barettella via time-nuts
<time-nuts at febo.com> wrote:
> Using auto survey mode my gps antenna height is significantly off. I estimate my antenna’s actual height at about +5 m high and the gps indicates -17 m.
> My question is will this adversely influence the accuracy of the gpsdo output? Would there be any benefit to adjust this manually via the scpi commands?
> Windows 7 Pro SP1 64 bit
> GPScon-JLT v 2.007
> Fury firmware rev. 1.22 w/ M12M upgrade
> PCTEL 8171D-HR-DH-W antenna
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