Duplex comms can be disconnected due to suboptimal routing that only minimizes latency

[connorjak]

This is going to be a hard one to explain.

I have a transatlantic GEO and a Europe/Moscow GEO, both geostationary, both with directional L-band and S-band.

The transatlantic comms work fine with duplex Pleumeur-Bodou <> GEO Atlantic <> Andover. The transatlantic GEO also services the USNS Vanguard.

The euro GEO is working fine with duplex Pleumeur-Bodou <> GEO Euro <> Moscow.

For Cuba/Moscow telecom, the seemingly obvious route Moscow <> GEO Euro <> Pleumeur-Bodou <> GEO Atlantic <> Tapaste is not being taken. I can't tell why. Here are a few pics:

Pic 1: Connection takes a detour through USNS Kingsport, even though we don't have enough power for that. Also through USNS Vanguard for some reason, even though Pleumeur-Bodou would be less power.

Pic 2: More of the same, except with euro GEO tilted up a lot so it only tries to access through Pleumeur-Bodou, not Kingsport.

Pic 3: Working link, where a passing extra satellite COR-3 Comm 1 takes over from the euro GEO.

Got ideas of what this could be? Could you explain how the "limited by capacity: available in simplex" reporting works, or maybe the antenna route selection?

CKAN Modlist:
ckan_modlist.txt

---

After a tweak to the orbit and the pointing of the L-band antenna, I think I've fixed it. The Tapaste -> Moscow completely skips the GEO Atlantic.

Still worth taking a look at why the link didn't work as I expected though. Maybe GEO Atlantic was being saturated on bitrate/bandwidth?

[connorjak]

Could this also be due to accepting the Our World contract?

Also, after the recent update I no longer can see ground station labels. Is that an issue with this mod or with RealAntennas?

[periodically-makes-puns]

Route selection is currently very naive. Each connection takes the shortest latency route available that is capable of serving the connection, and we route each connection in sequence.

I suspect what is happening is that the Our World contract is doing a bunch of power-inefficient relaying through Kingsport/Vanguard because those links offer slightly lower latency than relaying through Pleumeur-Bodou. This manifests as power issues since the maximum data rate to Kingsport/Vanguard is lower, which increases proportionate power draw.

Ground stations disappearing is a known issue and fixed in the RA/Skopos repositories currently, it’s just not been deployed to CKAN yet.

[connorjak]

Alright thanks that makes sense. I can compensate for that limitation by either greatly increasing the transmit power of the antennae or doing Tundra/Molniya/MEO satellites instead.

I'm a programmer; could you point me to the link path selection code? I might eventually be able to contribute an improvement.

[periodically-makes-puns]

Skopos/Telecom/routing.cs

Line 258 in fbfbc7e

private PointToMultipointAvailability FindChannels(

FindChannels is the primary routing algorithm, every contract connection calls this at least once (duplex twice, everything else once). Right now it's just running Dijkstra's. Note that this is the primary performance bottleneck in Skopos at the moment, since running Dijkstra's on the RA graph once for every contracted connection every FixedUpdate is somewhat slow.

[periodically-makes-puns]

Looking at this more closely, this also doesn't work because of latency requirements. Two GEO satellites relaying a single duplex connection both ways has a sum round-trip latency of about 120ms * 8 = 960ms, higher than the 800ms the contract requires.