How long will satellite repeaters last?

July 24, 2009 11:16 AM

Ham Operators: In the event of the apocalypse, how long will satellite-based repeaters last?

Are some of the satellite repeaters in completely stable orbits, thus requiring no fuel to maintain orbit? Are the electronics expected to die after a certain period of time? It seems as if the oldest fully-functioning amateur satellite repeater was launched in 1974.

On a side-note, in the event of societal collapse, is the complication of using satellites to bounce signals around the planet worth the effort? Is there a much easier method of communicating (ie. short-wave, higher-power, etc.)?

posted by jsonic to technology (10 comments total)

I don't think any of them absolutely require stationkeeping— unlike geosynch satellites, it's not important for them to stay in a very specific place in the sky. Some ham satellites have been in low enough orbits that they've deorbited due to drag, but many are high enough up that I don't think that's significant. I'm not sure what limits their life in practice. Gradual radiation damage to their semiconductors? Battery charge-discharge cycles? Infrequent but deadly solar flares? In any of those cases I wouldn't expect them to last more than a few decades.

Post-apocalypse, and post-failure of preëxisting space infrastructure, it seems unlikely space-based comms would be at all practical, because rocket launches are so expensive. Shortwave and such can get very long ranges without a satellite (via skip or ducting), but there's a limit on the total worldwide data capacity (I'd say "bandwidth", but this is a context in which bandwidth actually has a real meaning still). There are also techniques like meteor-bounce and aurora-bounce, which are still in some use today. If information technology survives the apocalypse, I'd guess that ground-based store-and-forward networks (a la packet radio or the internet) would be the easiest route. If IT doesn't but radio does, then ground-based store-and-forward networks (operated by humans).
posted by hattifattener at 12:36 PM on July 24

it seems unlikely space-based comms would be at all practical, because rocket launches are so expensive

Totally agree. I was talking about using the already-existing satellites to communicate over long distances. Especially since I wouldn't need a massive transmitter to use them.

Picture a small group of people with some batteries, solar power, and some radio equipment. How long could they expect to use satellites to communicate long distances? Assuming they had limited energy resources, would they be better off using some other approach, such as short wave?
posted by jsonic at 12:44 PM on July 24

Most OSCAR satellites are powered by NiCd batteries recharged via solar panels; presumably the batteries will eventually lose their charge, though some satellites have been positioned to receive nearly continuous sunlight, so they could theoretically operate straight off the juice from the panels. I don't know how stable the orbits are over time.
posted by Civil_Disobedient at 12:47 PM on July 24

According to this, LEO satellites are limited to 5-7.5 yrs, MEO and GEO are 10-15 years. YMMV.
posted by blue_beetle at 12:49 PM on July 24

Well, note that OSCAR-7 is still operational after 35 years, though its battery has died so it only works when it's in sunlight. The ham satellites I've seen look like they have much lower drag:mass than commercial satellites, probably because they don't have as much power-hungry stuff on them requiring big solar panel area.

jsonic, I'd say yeah, shortwave. If you're really interested, take a look at the amateur radio sub-hobbies of DXing (long-distance) and QRP (communicating with minimal power) and see what kinds of techniques and bands they use.
posted by hattifattener at 12:56 PM on July 24

I'm not currently a licensed HAM, but I am a commercially licensed radio engineer, with a number of years of experience. If that will do, I would offer, first, that the lifetime of all artificial Earth satellites at the apocalypse, may not be materially altered, if that event is non-violent. If Armageddon happens, existing artificial Earth satellites of all types will probably slowly fail due to energetic solar events and collisions with debris, see their orbits decay under gravitational force and high atmosphere drag, in the 1000 years Satan will be cast down in the abyss, thereafter.

For an ELE, a lot depends on the nature and proximate cause of the event. If a big enough object (or a gravitationally dense enough one) intersects the Earth, the resulting collision could effectively destroy all functioning artificial satellites in minutes. But, if you are just talking about the hypothetical situation where all humans go missing instantaneously and permanently, thus ceasing all tracking, orbit stabilization and control, then there are still a number of factors that can result in early, unpredictable demise of artificial satellites.

• Solar flares pump out enough energetic mass to materially affect our satellites. The approximately 11 year "solar cycle" observed since the 1958 IGY may not be clockwork, but it is a predictor that even the luckiest, best performing amateur repeater satellites are vulnerable in a 22 year period, to failure from high energy solar radiation. Indeed, few of the OSCAR satellites launched in the late 80's and early 90's survive, as operational birds. Some of the newer FM transponder birds (like AO-51) have better shielding and are likely to better handle energetic solar events, but at an MEO altitude of about 720 miles, require occasional orbit correction, and will exhaust their fuel in a time span not much greater than loss to energetic solar event would otherwise dictate.
• More stuff goes into earth orbit all the time, and so more space junk is constantly being created. This is beginning to be significant for low to medium earth orbit vehicles like AMSAT OSCARS, which need to be close enough to Earth for low power signals to be used to contact them. The increasingly dense cloud of space junk isn't well modeled, but it is increasingly apparent that radar tracking of debris down to a couple of centimeters diameter vastly underestimates the potential damage from such junk.
• AMSATs use, wherever possible, mechanical systems like torquer rods and passive paint/tape designs to provide inertial attitude control from sunlight heat and photon pressure. But there is a limit to what such systems can correct, and for small satellites in MEO, significant gravitational events on the Earth could swamp their attitude control systems easily.

So, for a sci-fi story, (and you are writing a sci-fi story, aren't you?), a figure of 20 years after mankind goes dark, would probably be a good horizon for average AMSAT bird life.
posted by paulsc at 1:14 PM on July 24 [3 favorites]

You don't really need a lot of power (or a satellite) for traditional DX work if you have a good high gain antenna pointed in the right direction. At night. Along with good propagation conditions.

The big problem for them is going to be their radio batteries failing, in say 10 years.
posted by okbye at 1:54 PM on July 24

Don't forget that jumbo-sized satellite of Earth, aka "The Moon". Ham radio operators can moonbounce signals. Granted, that depends on waiting until the moon is in the spot you need, so you can't use it all the time.
posted by fings at 5:23 PM on July 24

. Ham radio operators can moonbounce signals.

Moonbounce, however, is tough and slow -- you need decent amps, big gain antennas, and you need them at both ends. You can do voice, if you have 10-20m diameter dish antennas at both ends (giving you on the order of 40dBi gain) Most EME work is done with very weak signals and lots of computer processing to dig that out of the noise -- I'm going to suggest that if you destroy the sat infrastructure of the world, the semiconductor industry is going with it, and those computers will fail far before.

Far, far easier to drop to SW and count on skywave propagation to get across the ocean -- esp. since you don't need that high a tech to get the power levels and antenna gain needed.
posted by eriko at 7:16 PM on July 24

I think ground-based communications on the HF bands would be the way to go.

Actually, a large percentage of the human population disappearing would make for some great DX, since it would make the bands extremely quiet. Once the power grid failed, you wouldn't have any transformer noise (one of the main noise sources for HF, in urban/suburban areas) unless you were creating it in your own station somehow, nor anything else. You'd still have lightning and ionospheric/aurora noise, but no man-made interference.

The combination of that low noise floor (which hasn't been seen on Earth since the dawn of radio), plus a modern transceiver (which are designed to be very good, since they frequently operate in a very noisy environment), and a decent antenna would get you transcontinental communications when the ionospheric conditions were right, probably on a pretty regular basis.

For whole-planet coverage though, you'd be talking about some network of linked stations. It would probably not be real-time, either. Due to the way HF propagation works, you typically don't get good long-distance communication on one side of the planet at the same time that you get it on the other. So you'd probably want a very low-bandwidth store-and-forward network. That could be as low-tech as stations taking messages for each other and meeting at predetermined times to pass them off, or some sort of asynchronous, email-like digital system.

It's worth pointing out though that Ham satellites don't let you talk to the other side of the planet either; they greatly increase your range, but (at least that I'm aware of) they're not networked like the Iridum constellation is. It's basically up and back down, not up, over (to another satellite), and down. It's possible my information is out of date here, however.
posted by Kadin2048 at 7:42 PM on July 24