Starlink routing

Mon Jan 23 22:04:44 UTC 2023

My present understanding is that starlink satellites with lasers are not
designed to communicate inter-plane. Each launch of starlink satellites is
put into exactly the same orbital inclination (53.2 degrees or the more
rare near polar orbits now launched from Vandenberg).

In the weeks and months following their launch they spread out into an
extended line all following each other in the same plane. Plane change
maneuvers are extremely expensive in delta-v for any satellite and are
generally avoided unless absolutely necessary. Best conjecture is that
starlink satellites' on board propellant for hall effect or ion thrusters
(or whatever they're using that has an ISP above 3000) is used almost
exclusively for thrusting prograde to maintain altitude.

If you view a launch of 45 or 50 starlink satellites in a live animated
satellite tracking application, based on their TLE orbital data, they all
follow each other in a line. Satellites in the same line may be using
inter-satellite lasers to speak to the unit immediately in front of it, and
immediately behind it, forming a conga-line like network of linked
satellites until they get to one that is generally above a starlink earth
station/terrestrial network facility. At which point the traffic is
transferred.

Starlink has recently made service available for purchase in Nunavut and
all of the other high-latitude areas of northern Canada, which means that
they clearly think they have sufficient (82 degree plus) inclination sets
of satellites *and* inter-satellite links working to provide service in an
area that definitely has no terrestrial fiber or starlink earth stations.

On Mon, Jan 23, 2023 at 11:29 AM Thomas Bellman <bellman at nsc.liu.se> wrote:

> On 2023-01-23 19:08, I wrote:
>
> > I get that for 1310 nm light, the doppler shift would be just under
> > 0.07 nm, or 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).
> > [...]
> > 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.
>
> And as I was bicycling home, I of course thought of another aspect
> of the doppler shift: the timing between the symbols in the signal,
> or in other words the baud rate.  There will be something like a
> phase-locked loop (PLL) in the receiver in order to know when one
> symbol ends and the next one starts, and that PLL can only deal
> with a certain amount of baud rate shift.
>
> But we can use the same formula.  And in general, the doppler shift
> for 16 km/s is about 53 parts per million.  So e.g. a 112 Gbaud signal
> would be received as 6 Mbaud faster or slower than it was sent at.
> And here I have to confess that I don't know how generous typical
> receiver PLL:s in network equipment are.
>
>
> Another potential aspect might be the decoding of phase-shift keying,
> i.e. when phase modulation is used for the signal.  My *very*vague*
> understanding is that the typical way to decode a phase-modulated
> signal, is to mix the incoming signal with a reference carrier wave,
> generated locally by the receiver, and the interference between the
> two gives you the actual signal.  But to do that, the reference must
> have the same frequency as the received wave, and, I guess, must
> match very closely.  Can they adapt to an incoming wave that is 53 ppm
> offset from what it should be?
>
> Or have I misunderstood this?  Analogue signals is very much *NOT*
> my forte...
>
>
>         /Bellman
>
>
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