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LEO vs. GEO networks are like plutonium bombs vs. uranium bombs. One is simple to engineer, but resources are limited and expensive. The other is extremely com
by umvi 8y ago
LEO vs. GEO networks are like plutonium bombs vs. uranium bombs.
One is simple to engineer, but resources are limited and expensive. The other is extremely complicated to engineer, but resources are cheaper and more available.
In this case GEO systems are simple: satellite is stationary (ignoring slight drift which causes microsecond changes in latency), beams are stationary, antennas on people's roofs have no moving parts. However, the GEO satellite is extremely expensive, the spectrum it is using is expensive, and the GEO orbital slot is expensive. Also, it's so far away that it's high latency making online gaming nonviable.
LEO systems are extremely complex by comparison: satellites are moving, thus the beams are moving, antennas are moving (motor driven, unless using electronic phased array which are currently super expensive). With everything moving you have to do constant beam hand offs, doppler correction, etc. But, the satellites are much cheaper and the latency is much smaller.
Kudos to whomever can pull it off - there are several contenders other than SpaceX like OneWeb.
- gpm 8y agoI believe the plan is for phased arrays, at both ends, at substantially lower prices than are currently commercially available. Since SpaceX has already built the "first batch" of satellites (delivered to launch site as of a few days ago), one has to imagine they've largely solved the manufacturing problem and reduced the cost from 250k per antenna.
- umvi 8y agoIIRC, OneWeb's initial plan was to use phased arrays as well, but it proved too expensive and they switched to mechanically steered antennas (at least for first gen). Phased array is definitely the way to go if you can get cost down. No moving parts means fewer hardware/mechanical failures and less noise
- nielsole 8y agoWhat is the difference between phased array and MIMO? Isn't MIMO a solved problem and shouldn't the technology be transferable to this use case?
- andbberger 8y agoA difference is that phased arrays need sub-wavelength tolerances in phase between emitters to work
- dreamcompiler 8y agoI can easily imagine economies of scale in MMICs (Monolithic Microwave Integrated Circuits) bringing the price down enough to make phased arrays affordable. I sincerely hope it happens.
- mises 8y agoWhy are electronic phased arrays so expensive? I am no radio expert, but it seems that the pricey bit might be the engineering required rather than cost of production. From what I remember of my polar calculus, it sounds like a lot of painful calculus written once and fed henceforth into a computer.
- bsder 8y agoPresumably you need a high-end FPGA or a custom VLSI chip to process the signals. Both of these are very expensive in low-volume.
- loeg 8y agoIt also needs to be rad-hard for space. Terrestrial FPGAs are subject to occasional "SEU" (single event upsets)[1] which can arbitrarily affect behavior because gates can flip. Robust terrestrial FPGA systems have to handle SEUs; you have to imagine that these events are more frequent in space and need to at least be handled. (This may something you already had in mind, summarized as "high-end FPGAs;" I just wanted to add some interesting color some folks might not be aware of.) [1]: https://en.wikipedia.org/wiki/Single_event_upset https://en.wikipedia.org/wiki/Single_event_upset
- mises 8y agoGood to know, thanks. I suppose at the scale of SpaceX, ASICs may be an option.
- loeg 8y agoI'm not sure 4000 is enough to break even on ASICs, but I'll admit I don't know much about the costs of either high end FPGAs nor ASIC production at quantity 4000.
- mises 8y agoFrom what I am reading here, a large part of the cost is up-front with chip design and setup. I suppose it depends on pricing. They also may wish to sell more after those four thousand units. http://blog.zorinaq.com/asic-development-costs-are-lower-than-you-think/ http://blog.zorinaq.com/asic-development-costs-are-lower-tha...
- 01100011 8y agoAre hand-offs really all that complicated? Cell networks have to handle it. I don't think you need to move the antenna, but you need a wide footprint and probably overlapping coverage between satellites. How does Iridium work? I'd think this has already been done(quite a while back even).
- dreamcompiler 8y agoLEO satellite handoff is tough because you effectively need two antennas: One points at the "old" satellite while one points at the "new" one. You can achieve this with a single phased array antenna divided in half and controlled by software, but each half must still be large enough to give the needed S/N ratio. Iridium uses an omnidirectional antenna which is fine if all you want is 1200bps. But if you want 5Mbps, you have to point the antenna. So Iridium is not applicable here.
- ncallaway 8y agoCell hand-offs don't happen nearly as frequently as they would for an LEO constellation. The orbital period for a satellite at 550 km is ~01h 36m. That means any given satellite is going to be visible for a very short period of time, which means hand-offs will have to be extremely frequent.
- anticensor 8y agoApproximately every 2.5 minutes in fact.
- umvi 8y agoThat's per satellite; each satellite is going to have multiple beams, so you'll have to do a beam handover like every 15 seconds and a satellite handover every couple of minutes
- childintime 8y ago> Are hand-offs really all that complicated? The fundamental problem is this: https://en.wikipedia.org/wiki/Two_Generals%27_Problem Don't know how cellular handles this.