beecher at beecher.cc
Mon Jan 23 18:54:49 UTC 2023
Appreciate that. Definitely becoming clear to me that a lot of my knowledge
here was rusty. Lots of papers on this specifically (Doppler effects on
optical ISL) that I need to call in some favors to get access to.
On Mon, Jan 23, 2023 at 1:08 PM Thomas Bellman <bellman at nsc.liu.se> wrote:
> On 2023-01-23 17:27, Tom Beecher wrote:
> > What I didn't think was adequately solved was what Starlink shows in
> > marketing snippets, that is birds in completely different orbital
> > inclinations (sometimes close to 90 degrees off) shooting messages to
> > other. Last I had read the dopplar effects there were so much larger due
> > relative speed deltas it just couldn't currently be done. If there is
> > out there on that solution, be glad to read up on what info anyone may
> > on that if they can share.
> Worst case would be if the satellites are moving directly towards or
> directly away from each other. Each satellite will be moving at a
> speed of slighly under 8 km/s, and they will thus approach or depart
> from each other with a relative speed of somewhat less than 16 km/s.
> I get that for 1310 nm light, the doppler shift would be just under
> 0.07 nm, or 12.2 GHz:
> l0 = 1310 nm
> f0 = c / l0
> f = f0 / sqrt((1 + 16 km/s / c) / (1 - 16 km/s / c))
> l = c / f ≈ 1310.0699 nm
> f0 - f ≈ 12.2 GHz
> In the ITU C band, I get the doppler shift to be about 10.5 GHz (at
> channel 72, 197200 GHz or 1520.25 nm).
> (Formula from https://en.wikipedia.org/wiki/Relativistic_Doppler_effect
> first entry in the table under "Summary of major results".)
> These shifts are noticably less than typical grid widths used for
> DWDM (±50 GHz for the standard spacing), so it seems unlikely to me
> that the doppler shift would be a problem.
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