Re: [Starlink] Starlink for Tonga?

[Ulrich Speidel <ulrich@cs.auckland.ac.nz>]

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I've been watching (and commenting on) this ever since one of our MPs 
tweeted Elon Musk with a plea for help for Tonga (and as the PhD 
supervisor of Tonga's first PhD student in Computer Science and someone 
who has had Pacific connectivity at the core of his work, I've got a 
fairly good understanding of what "Tonga offline" means in practice). 
That said...

- Practical range from gateway ground station to user ground station is 
more like 300-400 km. Fiji-Tongatapu is twice that distance.
- Countless images of gateways (and fencing around it) suggest that 
satellite elevation from gateways needs to be a bit more than 25deg.
- Satellite density in the tropics is lowest for Starlink's currently 
operational 53 degree inclination network.
- It's my understanding that dishys don't actually track using their 
motors, which they use only to position themselves initially in the 
direction of the highest satellite density (=wherever your closest 53rd 
parallel is, basically). That's a strategy that doesn't really work in 
the tropics because you can't see the sky above the 53rd parallel. So 
here, your dishys would need to know to point west-north-west.

While it's encouraging to see Starlink send folk to Fiji to scout things 
out (there are plenty of Fijian outlying islands where Starlink service 
would be welcome, too, at least by the locals, not necessarily by the 
regulators), I don't hold my breath on them making serious inroads when 
it comes to alleviating the current connectivity crisis.

The cable ship (CS SubCom Reliance) tasked with repair of the cable is 
on site and, judging by its AIS track, has located the end of the cable 
coming from Tongatapu. It appears to have shifted a significant distance 
south (around 1-2 km), possibly as a result of a turbidite flow 
(underwater landslide) from a seamount in the area that was triggered by 
the eruption at Hunga further north (my going theory anyway - there's a 
seamount right where it needs to be for that sort of damage and there's 
no other event that explains the timing of the cable outage). Either 
way, if the area is suitable for re-laying the cable, Tonga should be 
back online within the next week. Otherwise add a week or two.

On 8/02/2022 8:06 am, Nathan Owens wrote:
> You can simulate it on starlink.sx 
> <http://starlink.sx>, 
> maybe Mike will chime in with what he found in doing that.
>
> On Mon, Feb 7, 2022 at 11:05 AM Christian von der Ropp <cvdr@vdr.net> 
> wrote:
>
>     But in practice the satellites won't be sitting and waiting at the
>     edge of this 940km radius. They are moving in and out the radius
>     and the question is if satellite density is high enough so that
>     once the serving satellite loses its gateway link there's another
>     satellite in the 940km radius which also covers Tonga. And then
>     this new satellite cannot be within certain elevation angles
>     (~60-80° at 0° azimuth) where the geostationary arc crosses Fijian
>     skies and the gateway antennas have to seize emission. My gut
>     feeling is that availability in Tonga would be <90% simply because
>     it's too far out at the edge of a Fijian gateway's range where
>     there will be frequent service interruptions.
>
>
>     Am 07.02.2022 um 19:51 schrieb Nathan Owens:
>>     The current coverage radius of a gateway/ground station with a 25
>>     degree minimum elevation is ~940km, so nothing in theory.
>>
>>     On Mon, Feb 7, 2022 at 10:50 AM Daniel AJ Sokolov
>>     <daniel@sokolov.eu.org> wrote:
>>
>>         Hello,
>>
>>         I hope this is not offtopic: Starlink wants to build a ground
>>         station on
>>         Fiji to supply Tonga with internet.
>>
>>         The distance between Tonga and Fiji is about 750 km minimum.
>>         That's
>>         quite the distance.
>>
>>         What does Starlink have to do to make this work?
>>
>>         Cheers
>>         Daniel
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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@cs.auckland.ac.nz  
http://www.cs.auckland.ac.nz/~ulrich/
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