SpaceX just got FCC approval to launch 7,518 satellites

Re:SpaceX just got FCC approval to launch 7,518 satellites by Tuna-Fish on Thursday November 15, 2018 @02:38PM (#18462522)

Traditional satellite internet uses satellites in geostationary orbits, or 35,786km high above the equator. This is great because satellites in that orbit are stationary relative to the ground, so you just need a parabolic antenna pointed at a specific point in the sky to get great reception. But it's also terrible for latency because the signal takes 120ms just to get up, and another 120ms to get down. Assuming you live on the equator, directly underneath the satellite. For someone living in the temperate regions, typical real rtts are in the 700ms area.

Starlink and OneWeb are different in that they intend to use a lot of satellites in low orbits to maintain constant coverage. This is technically much harder, not the least because you need thousands of satellites to get reasonably good coverage, but also because the ground station and the satellite transceiver both need to track each other. This was not technically feasible before, but modern AESA antennas can steer their signal without having to move the antenna, and can both transmit and receive multiple simultaneous beams and very rapidly move the beams around when doing time-sharing.

The minimum round-trip time to something very close to you and also close to it's own ground station will be on the order of 10ms. However, where the system really shines is long distance communication. The satellites will pass the signal between each other using lasers, and will get the signal to the other side of the earth much faster than terrestrial fiber, both because laser in vacuum travels substantially faster than one in fibre, and also because the fibres don't get to follow ideal great circle paths.

Independent researches have evaluated the likely latencies of the system, and the results are frankly shocking. For example, today on the existing fiber network, rtt between London and Singapore is ~160ms. On Starlink, the rtt will be ~90ms.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by walrus01 on Thursday November 15, 2018 @02:49PM (#18462634)

Not all satellite internet, currently, is dependent on geostationary. Read up on o3b's network and architecture, which has been operational for several years now. The cost, however, is basically "if you have to ask, you can't afford it". O3b is a market option for ISPs and telecoms in very remote or island areas that were previously dependent upon leasing Ku or C band transponder megahertz.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by shaklee3 on Thursday November 15, 2018 @08:09PM (#18465262)

Or cruise ships:

https://www.ses.com/blog/behind-scenes-royal-caribbean-cruis...

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Re:SpaceX just got FCC approval to launch 7,518 satellites by Filligree on Thursday November 15, 2018 @02:44PM (#18462584)

> Traditional satellite internet uses satellites in geostationary orbits, or 35,786km high above the equator. This is great because satellites in that orbit are stationary relative to the ground, so you just need a parabolic antenna pointed at a specific point in the sky to get great reception. But it's also terrible for latency because the signal takes 120ms just to get up, and another 120ms to get down. Assuming you live on the equator, directly underneath the satellite. For someone living in the temperate regions, typical real rtts are in the 700ms area.

This doesn't check out.

The radius of Earth is 6,300km. Assuming a receiver on the north pole, the total distance should only be sqrt((35786 + 6300)^2 + 6300^2) = 42555km, which isn't notably larger than the 36000km at the equator.

I'm sure you're still right about the latency, but it can't be just distance doing it.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by walrus01 on Thursday November 15, 2018 @02:50PM (#18462656)

in practical real world use the absolute minimum ping time you'll see from a 1:1 dedicated capaacity (SCPC) link via geostationary, from end terminal to larger earth station, is 489 to 491 ms. This will vary up to 495ms depending on modem modulation/framing and FEC type, and actual line of sight distance to satellite (low inclination aim angle for the antenna, if you're not directly under the satellite, vs two sites that are directly under it as viewed from geostationary).

Ignoring TDMA oversubscribed VSAT networks for the moment, there are a number of different possible modulation schemes and FEC types, and FEC code rate (payload vs FEC percentages), and things that are unique to different SCPC modems, which will vary the modulation by a few ms beyond pure speed of light.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by dr_orpheus on Thursday November 15, 2018 @02:54PM (#18462715)

The latency from the distance light travels does not vary that much. The rest of the ~600 ms latency is caused by the same thing as latency on ground systems (routers, etc.). And the processors on spacecraft are not particularly fast.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by ryanmarsh on Thursday November 15, 2018 @03:07PM (#18462817)

Who builds routers that go in space? Cisco? That's got to be some crazy hardware.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by dr_orpheus on Thursday November 15, 2018 @03:21PM (#18462912)

Cisco has built routers for space multiple times before. The first time was way back in 2003: https://en.wikipedia.org/wiki/CLEO_(router)

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Re:SpaceX just got FCC approval to launch 7,518 satellites by ehead on Thursday November 15, 2018 @06:59PM (#18464744)

I believe companies like Boeing or Space Systems Lorale do that kind of work in their satellite design groups. Same with Orbital ATK.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by dzhiurgis on Thursday November 15, 2018 @04:20PM (#18463463)

Apparently Elon is gonna try use consumer hardware or build their own, thats kinda part of it's cost advantage.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by kartickv on Thursday November 15, 2018 @10:35PM (#18466160)

Is lower latency the main benefit? Because there are other ways to combat that, like locating servers closer to where users are. Yes, there are some applications like HFT or gaming or whatever that need lower latency, but I think it would be more useful to the world at large if Starlink provides people all over the world with affordable broadband Internet access with a minimum speed of say 20mbps, say. People far from cities, people in developing countries with bad infrastructure (till a few years back, I had a 4mbps connection), people on islands with bad connectivity, trains, ships, planes, etc. Does it do the latter?

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Re:SpaceX just got FCC approval to launch 7,518 satellites by intrasight on Thursday November 15, 2018 @09:47PM (#18465911)

>The satellites will pass the signal between each other using lasers

I thought there were regulatory barriers to doing that

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Re:SpaceX just got FCC approval to launch 7,518 satellites by reaperducer on Thursday November 15, 2018 @02:52PM (#18462690)

SpaceX released a video about a week ago estimating that SF-London times were be around 60 ms, IIRC.

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Re:SpaceX just got FCC approval to launch 7,518 satellites by pjscott on Thursday November 15, 2018 @07:11PM (#18464847)

That sounds like a realistic number, considering the measured latency for the test satellites they launched earlier this year:

https://www.reddit.com/r/spacex/comments/8mccic/musk_tintin_...

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