bellman at nsc.liu.se
Mon Jan 23 18:08:34 UTC 2023
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 each
> other. Last I had read the dopplar effects there were so much larger due to
> relative speed deltas it just couldn't currently be done. If there is more
> out there on that solution, be glad to read up on what info anyone may have
> 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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