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. Thanks! On Mon, Jan 23, 2023 at 1:08 PM Thomas Bellman <bellman@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 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.
/Bellman