[time-nuts] Positional accuracy of the M12+T
Tom Van Baak
tvb at leapsecond.com
Thu Jan 4 14:39:44 EST 2007
> Tom,
>
> Good points. I think that a lot of people are unaware of the diurnal
> shifts that occur due to atmospherics. These can be many 10's of
> noseconds compared to UTC. This is true for every receiver I have ever
> worked with. The ionospheric correction algorithms are good, but they
> are not perfect. Can't wait for a civilian L2.....................
>
>
> Randy
Right. What gets in the way of accurate UTC time from
an M12+, or just about any other GPS receiver is:
(not in any particular order):
-- antenna cable feed delays
-- antenna delays
-- antenna phase center errors
-- internal receiver hardware delays
-- external receiver connector or other cabling delays
-- trigger level or zero-crossing errors
-- antenna preamp, RF filter, or splitter delays
-- humidity
-- tempco in all of the above
-- voltco in some of the above
-- receiver firmware delays
-- sawtooth errors
-- 1PPS quantization errors
-- imprecise zero-D position measurement
-- PPF (pigeon poop factor ;-)
-- multi-path errors (large)
-- GPS SV clock errors
-- GPS SV ephemeris errors
-- ionospheric errors (large)
-- tropospheric errors (small)
-- UTC(USNO) errors
Some of these vary with ~12 or ~24 hour periods;
some of these vary with 1 year periods; some of
these show sudden jumps; some of these show
gradual drift; some of these just wander around
over time.
You get the idea. Some are ps, ps/K, some are
ns, some are tens of ns. I wish I could give you
a nice list with hard numbers but I don't know.
Perhaps Tom Clark does?
I also don't have any data to back up this bold
claim, but: I would be surprised if any of us, me
included, has UTC at home closer than maybe
20 to 50 ns. -- with the exception of DougHo
(with his USNO calibrated, real-time JPL corrected,
frequency-steered, 5071A-driven, post-processed,
dual frequency Z12T) and the one or two of you
on the list that work at TSC.
What most of us time-nuts use GPS receivers for
is quartz, rubidium, or eBay-cesium *frequency*
measurements, and so all these fixed, or slowly
varying *time* offsets, have little or no effect on our
measurements.
/tvb
http://www.LeapSecond.com
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