Comment by sandworm101
1 year ago
Tracking is an issue, but doppler can also be a thing. At orbital speed (actually up to 2x orbital speeds) the doppler effect between two satellites can change the frequency enough to cause interference. Moving a scope to track a moving target is one problem, allowing the algorithms to adapt at the frequency shifts on the fly another.
Indeed Iridium had to deal with the same thing (or I guess, didn’t):
“ Cross-seam inter-satellite link hand-offs would have to happen very rapidly and cope with large Doppler shifts; therefore, Iridium supports inter-satellite links only between satellites orbiting in the same direction.”
https://en.m.wikipedia.org/wiki/Iridium_satellite_constellat...
There were some experiments with communicating over Iridium to small cube-like sats back in the day, but we couldn't make the system on a chip beefy enough to do the Doppler shift calculations on the fly and survive a launch; it was close though. I think its possible to do now.
Don't all satellites orbit in the same direction? Launching retrograde is just throwing away free velocity.
In the context of the full article (https://en.wikipedia.org/wiki/Iridium_satellite_constellatio...), it's clear they're talking about the polar orbits used by the Iridium constellation, which have "seams" around the Atlantic and the Pacific as the "first" set of satellites passing north-to-south overlap with the "last" set of satellites coming back south-to-north on the other side of their orbits. So of the 6 orbital planes used by the Iridium satellites, each plane covers 1/12th of the globe for each "half" of its over-the-poles orbit. So there are two "seams" where handoff is not supported, one off the eastern seaboard and one roughly over Japan.
There's an animation on linked article that explains this pretty well: https://upload.wikimedia.org/wikipedia/commons/thumb/9/90/Ir...
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The Iridium satellites are in what you might call "parallel" orbits, if you stretch the meaning of the word a little bit.
The wikipedia link above explains it well:
""" Orbital velocity of the satellites is approximately 27,000 km/h (17,000 mph). Satellites communicate with neighboring satellites via Ka band inter-satellite links. Each satellite can have four inter-satellite links: one each to neighbors fore and aft in the same orbital plane, and one each to satellites in neighboring planes to either side. The satellites orbit from pole to same pole with an orbital period of roughly 100 minutes.[8] This design means that there is excellent satellite visibility and service coverage especially at the North and South poles. The over-the-pole orbital design produces "seams" where satellites in counter-rotating planes next to one another are traveling in opposite directions. Cross-seam inter-satellite link hand-offs would have to happen very rapidly and cope with large Doppler shifts; therefore, Iridium supports inter-satellite links only between satellites orbiting in the same direction. """
The 'seams' have interesting implications for latency when I was working on Global Data Broadcast.
Doppler is not a big problem with lasers because the carrier frequency is so much higher than RF that it doesn't matter; it's bang-bang AM modulated.
I'm assuming two things: That something like Manchester coding is being used so that some clock skew is tolerable, and that the laser carrier is not in fact being frequency or phase modulated. Last I checked FM and PM of optical frequencies was not yet practical outside of laboratories, but I'm happy to be corrected.
Links between satellites closing range near 2x orbital speed have two problems: - bigger doppler - the lifetime of the link is much shorter