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charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable Subject: [Starlink] Why ISLs are difficult... X-BeenThere: starlink@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: "Starlink has bufferbloat. Bad." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 01 Sep 2022 12:19:22 -0000 As this seems to have branched out... There are a whole bag of issues=20 with ISL's and routing, really, and again we know diddly squat about=20 what Starlink actually intend to do. My 5 cents worth: - Linking two satellites that follow each other on the same orbit is the=20 easiest exercise. I gather that Starlink have ticked that one off. It's=20 probably not too useful on its own for most real scenarios though:=20 ground stations move through orbital planes. Also, two arbitrary ground=20 stations between one would want to forward will probably not be=20 connectable by a chain of satellites all in the same orbital plane. - Linking two satellites that are in different but adjacent orbital=20 planes is one notch up but probably not a lot harder if you master=20 gimbal / mirror control. You have some relative movement, but most of=20 the time it's slow. Low hanging fruit if it's not already been picked. - Linking two satellites in range of each other that satisfy some=20 arbitrary criterion (minimum distance, desired direction): A bit harder. - Turning this into a global network in the shell: Even harder. Let me elaborate a bit on this. Let's assume we have one or more gimbals that allow us to point our=20 space laser(s) at other satellites in range. Or a mirror arrangement -=20 doesn't matter. One unknown that we have is what the receiver side of these links will=20 look like. As we'll see in a moment, this is actually quite important. There are in principle two options for the receiver: 1) A receiver with a wide angle lens that can receive laser signals from=20 multiple other satellites at once. This is a pretty simple arrangement=20 and may not even need moving parts. 2) A receiver that gets pointed back at the transmitting satellite,=20 perhaps with a telescopic zoom lens. This adds a little weight and could=20 be on the same gimbal as a laser, so we could communicate both ways=20 between the satellites. Moreover, the zoom lens would be like antenna=20 gain in a link budget, so would allow a higher data rate between the=20 satellites and / or less power. Now 2) seem clearly superior, right, if we can handle a few extra grams?=20 Then we could give each satellite n TX/RX gimbals and could, say, get=20 each of our satellites to connect to its n nearest neighbours. And=20 bingo, we'd have a network that spans the globe, right? Not so simple. Two problems, and they're serious ones as it turns out: A) What happens if one of our n nearest neighbours doesn't have us among=20 its n nearest neighbours? Then they won't point their gimbal back at us.=20 How do we resolve this? B) If n=3D3 and I have Dave, Mike, and Brandon as my nearest neigbours,=20 Dave's 3 nearest neighbours are Mike, Brandon and me, Mike's nearest=20 neighbours are Dave, Brandon and me, and Brandon has Dave, Mike and me=20 as his nearest neighbours, then David, Dick and Sebastian who may be=20 orbiting a bit further away from us don't get to link to our elitist=20 cluster and our dream of a global network turns to dust. Now, Problem B (which also occurs for outward links from clusters with=20 receiver type 1) can be mitigated by requiring a minimum distance to a=20 neighbour, but in combination with a), we seem to have a nasty little=20 overlay graph problem to solve. Oh, and we'd want to do that in a=20 distributed fashion if possible, and every few seconds from scratch, please= . --=20 **************************************************************** Dr. Ulrich Speidel School of Computer Science Room 303S.594 (City Campus) The University of Auckland u.speidel@auckland.ac.nz http://www.cs.auckland.ac.nz/~ulrich/ ****************************************************************