I think it's also likely that only modest, if any, WDM is required on those links, because the goal in most cases will only be to go far enough to get down to a ground station (excepting some low latency transatlantic use cases I have read might be in the offing), and because the satellite RF uplink/downlink capacities shouldn't seriously challenge the bandwidth available on the optical links.

At least in the current case of general purpose internet access with dynamic IP addresses, I suspect the IP of a user-terminal is related to the ground station serving it, and there is just a parade of satellite intermediaries, but the terminal and ground station remain fixed, so the routing can be more of a connection oriented type.


On Mon, Jan 23, 2023 at 1:10 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