Re: Starlink routing

[Dorn Hetzel <dorn () hetzel org>]

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