[time-nuts] U-blox teaser

[Lux, Jim]

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.