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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