[Starlink] Starship's 4th flight test was magnificent

Mon Jun 10 06:51:42 EDT 2024

On 10/06/2024 2:22 pm, Michael Richardson via Starlink wrote:
> Dave Taht via Starlink <starlink at lists.bufferbloat.net> wrote:
>      > When they kicked into the Blue Danube, during Starship's coast phase, I
>      > started dancing around the boat. The landing phase was a real
>      > nailbiter, but seeing starlink maintain connectivity through nearly it
>      > all of that plasma was amazing, too. To be massively cheered up about
>
> Silly legal/technical question:
>
> When starship is in a suborbital, ~140km apogee (coast phase) over Atlantic
> Ocean or Africa, whose jurisdiction are the power limits set by?

The power limits for transmission are ultimately set by the ITU, to be 
enforced by the authority of whichever state has approved the spacecraft 
(which would be the FCC in this case I guess). But there's a catch or 
five here.

The main power limit is for power hitting the ground (downlink to end 
user), because that might get into the receive antenna of some sort of 
ground based system and wreak havoc there.

In this context it's helpful to understand that interfering signals 
don't have to be on the same frequency that you are trying to receive. A 
transmitter's output might have >99% of its power within the band you 
want it to be in, but the rest distributes across the remainder of the 
spectrum - as a rule of thumb, the closer you are to the "intended" 
frequency with your receiver, the more of those spurious emissions you 
see. These emissions may be a few orders of magnitude below the intended 
signal in power, but because RF communication deals with intended 
signals that can differ by many orders of magnitude, your receiver will 
still hear them - you need the help of distance, antenna directionality 
and in some cases even filters in order to suppress that unwanted signal.

Similarly, receivers aren't just listening on the frequency they're 
tuned to - they're similarly listening on all other frequencies, too, 
except that any signal there needs to be a bit stronger to break 
through. Your cheap car radio will demo that effect to you when driving 
past an FM broadcasting transmitter.

So in the case of the power limits in non-GSO satcom, you have a whole 
raft of microwave receivers on the ground - some pointing skywards - and 
too many to dodge them individually (you'll notice Starlink not being 
offered around radio telescopes - these are super-sensitive and even the 
usual ITU limit is too high there). Moreover, they are approximately the 
same distance as your intended receiver - so whatever you throw at your 
intended receiver, they can snap up as interference.

The opposite case occurs (in principle) when transmitting from the 
ground (or a re-entering spacecraft) to a non-GSO sat above you, with a 
geostationary sat lurking behind your satellite in the distance. In this 
case, the bit of signal beam (think light beam as in car headlights) 
that shines past the non-GSO sat - you could jam that satellite's 
receiver and spoil someone's Superbowl (or worse, Superbowl ads!). This 
is somewhat less critical than the opposite direction, though, mainly 
because you're trying to hit a target that's maybe 1000 km away whereas 
the part of the geostationary orbit that cops the beam leftovers is 
40,000 km or so away. This means that your signal (which suffers 
spherical spreading) is around 40^2=1600 times lower at the 
geostationary sat than at your intended receiver. That's about 33 dB in 
difference, which helps, but then again, the geostationary satellite 
user's uplink suffers the same spread - and may not be aiming at 
producing the same signal level at the satellite antenna as you are 
aiming for on your non-GSO satellite. So there is also a risk here of 
some interference. But in most cases, transmitting up is OK and power is 
more of a matter of what you can afford down on the ground (OK, it's 
more complex than that, but that's for another post).

Now in the case of a video link from a re-entering spacecraft, you get 
the "break" in reception due to signal being absorbed in plasma on 
spacecraft re-entry. That plasma isn't an atmospheric layer, it's simply 
air molecules getting disturbed in their daily business by a spacecraft 
passing by at entirely unreasonable speeds, knocking their electrons 
off. Now that plasma forms wherever the spacecraft grates with the 
atmosphere - read mostly below and around the sides. So what you then 
get is a comet-like "tail" of plasma - a bit like a cone, and there's 
comparatively little plasma inside the cone.

Conventionally, if you had to communicate your re-entry video or audio 
feed to a ground station, you had to communicate *through* that cone's 
wall. Similarly, if you wanted to go "up", you had to go to a TDRSS 
satellite, of which there were only a small number in orbit - and the 
one visible to you would have been on the other side of the plasma cone 
wall with high probability. With Starlink, you have potentially a few 
dozen satellites within field of view, and the chances of having one 
within view out the back of the cone are relatively good (but not 
guaranteed). The other day, they got lucky that the star(link)s lined up ;-)

Quite how that lucky satellite that got to handle the re-entry video 
feed got to downlink it is another question. Splashdown was NW of 
Australia - I don't have an accurate location at this point, but if it 
was close enough to WA, it could have been handled by one of the 
gateways there. Otherwise, the laser ISLs do also a good job at 100 
Gb/s, and it could have come down at a gateway near you...

> If at 140km altitude, that's 20% to 30% closer to the satellites than on
> land.  OTH, it's moving really fast, and does the hand-off really work?
> Are they just using TCP (or maybe QUIC) for data, or something else?
That'll ultimately be their choice, but I suspect it'd be something 
UDP-based off the shelf. Zoom maybe?
>
> Also, during the entire Tonga situation, Ulrich provided lots of really good
> explanation of how hard it was to do a downlink.  That was before the space
> lasers.  I would guess that this data link required inter-satellite lasers,
> since downlink over oceans (and probably Africa) would be difficult.
>
>      > Lastly, I couldn't help but imagine small repair robots deploying once
>      > in orbit to get a full view of every tile on starship, and perhaps
>      > effect repairs. Call 'em Hewey, Duey and Louie....
>
> Naw. R2-xx are Astro *MECHS*. https://www.youtube.com/watch?v=gfMoXBOjWBM
>
> --
> ]               Never tell me the odds!                 | ipv6 mesh networks [
> ]   Michael Richardson, Sandelman Software Works        |    IoT architect   [
> ]     mcr at sandelman.ca  http://www.sandelman.ca/        |   ruby on rails    [
>
>
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Dr. Ulrich Speidel

School of Computer Science

Room 303S.594 (City Campus)
Ph: (+64-9)-373-7599 ext. 85282

The University of Auckland
ulrich at cs.auckland.ac.nz
http://www.cs.auckland.ac.nz/~ulrich/
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