[time-nuts] U-blox teaser

[Magnus Danielson]

Hi,

On 2021-02-27 17:18, 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.

The uBlox F9 and Septentrio Mosaic does this. They output observables
such that good realtime and postprocessing can be achieved. By doing
double frequency, you can do trivial ionspheric delay observation and
compensation of that observation, such that both L1 and L2 can be
compensated. This way you can compensate with actual observed ionspheric
delay rather than using the Klobuchar model which on average only
removes half the ionspheric error. Also, that you get both code and
phase observations helps. All contributes significantly to push things
down. At the same time, doing this for as many satellites as a L1 only
receiver do provide more observables and more precision. Antenna and
reflections end up being more of a challenge in addition to calibration,
but it is worth the next level of improvement to push the errors towards
1 ns or sub 1 ns that you find in metrology type of comparisons. With
the F9 and Mosaic, the price of such receivers just dropped significantly.

TMAS is built with older type of receiver, but I suspect they will be
interested in upgrading for the next generation, as things push the
limit for what is possible on reasonable budget.

Speaking of which, I need to do some work on that. :)

Cheers,
Magnus