On 2/27/21 8:18 AM, Dana Whitlow wrote:
Thanks, Bob.
It seems to me that, depending on the positions of sats visible to one's GPS
antenna and the spatial distribution of free electron density in the
ionosphere,
the ionospheric contribution to position errors could sometimes largely
cancel.
But that observation may (or may not) reflect strongly on one's ability to
get
accurate absolute time from GPS on "average" days.
During my Arecibo Observatory days we used NIST's TMAS service to keep
our H-maser-based station clock synced with UTC. Our user community
(mainly VLBI and pulsar timing people) seemed pretty satisfied with +/-
100ns
accuracy, so I tried to do better by keeping things well within +/- 50 ns
during
my reign. IIRC, NIST was claiming that TMAS could produce results mostly
within about +/- 20 ns.
To be honest I'm baffled by how time transfer much better than that could
be achieved in practice.
Regarding Q3, yes I'm aware that *some* GPS receivers do the estimation of
ionospheric delay. What I was asking was: Do any of the relatively
inexpensive
receivers to which we time-nuts have access do this? Here I'm speaking of
those being sold for no more than a few hundred USD.
Dana
If it does dual frequency, then it probably compensates for the
ionosphere. The algorithm isn't complex, and really, there's no reason
to do dual or multiple frequency otherwise - You can get plenty of
satellites with a L1 only, so the increased number of observables,
alone, isn't a good reason for dual frequency.
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