[time-nuts] (no subject)

[Lux, Jim]

On 1/18/21 7:53 AM, Skip Withrow wrote:
> Hello time-nuts,
>
> Came across this poster of the CHOMPTT cubesat mission.
> https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3494&context=smallsat
>
> Obviously, they are doing measurements on a per orbit basis.  But if
> you had an optical ground station and the clock difference information
> (correction) you could calibrate your home clock.  Basically, this is
> what we already do with GPS with CORS correction information to get
> much better than 20ns accuracy.  One question that I have is the
> uncertainty with optical paths lower than for GPS (RF)?  I suspect
> maybe.

Almost certainly better - at L-band, the ionization of the ionosphere is 
a big uncertainty, both because the propagation velocity changes as the 
ion concentration, but because that effect can cause refraction, so the 
"ray path" from satellite to you isn't straight.  Dual frequency 
receivers can solve for the velocity difference (since it's inversely 
proportional to frequency squared) but they can't solve for the 
non-straight-path. 
(https://apps.dtic.mil/dtic/tr/fulltext/u2/a142725.pdf, Klobuchar 1983)

For optical paths, there are some refractive effects close to the 
horizon but at elevation angles that are higher, they are much reduced.


>
> A system such as this might also provide an independent source of time
> to GPS.  But don't we already have that with GLONASS, Beidou, and
> Galileo?

Yes.
However, there's always interest in time transfer that doesn't use GNSS, 
particularly in space applications.


more on CHOMPTT
https://digitalcommons.usu.edu/smallsat/2017/all2017/49/ is from 2017 
(pre launch)

https://digitalcommons.usu.edu/smallsat/2019/all2019/108/  is from 2019 
(post launch)

They used the same "narrow temperature range" CSAC that I did on my 
satellites (one of which was on the same launch)



>
> Regards,
> Skip Withrow
>
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