[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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