Space-based radio stations?

Factual Questions
1

It seems so simple, and yet I can’t think of an instance of it being tried:

Has anyone ever created and launched a geosynchronous satellite that broadcast a normal, analog radio signal (AM or FM, your choice) that can be picked up by anyone with a Walkman within the targeted region? Not a digital signal, and not in any sense directional one (other than that it’s pointed at the Earth, as opposed to out in space somewhere). Like your average ClearChannel affiliate station, just stuck on an orbital platform and moved some 22,300 miles up.

What kind of wattage would a station like that need to be heard reliably on the ground? Would the frequencies normally used in AM or FM work even penetrate the atmosphere?

If nothing else, I’ll learn more about RF radiation from this thread. :slight_smile:

2

Arthur C Clarke wrote a story about geosynchronous satelites being used to broadcast outlaw televsion stations. IIRC It was titled “I Remember Babylon”

3

Such a satellite (if it were technically feasible, although I don’t see why it wouldn’t be) would interfere with the licensed broadcasts of dozens, if not hundreds of radio stations in dozens of countries. So you’d be breaking the laws of a number of countries and angering the owners of all those stations. This would probably result in heated calls and letters from lawyers and law enforcement types.

So to do it, you’d have to find a frequency with as few conflicts as possible, persuade the owners of those stations to abandon their frequencies, and get the legislatures of every country in the hemisphere to change their laws to permit such a system. I leave it as an exercise for the reader to determine which of these tasks would be harder/more expensive.

That said, I’m laying even money that Rupert Murdock starts working on it within ten years. (Thanks for giving him the idea, Derleth!)

4

There absolutely are such satellites, though most of them are LEO satellites, and none broadcast commercially. I’m speaking, of course, of Amateur Radio satellites, a number of which are currently in orbit. I know of several which transmit and/or receive in the 30 Mhz (10-meter), 50 MHz (6-meter), 144 MHz (2-meter), 220 MHz and 440 (70-cm) MHz bands. They operate in almost all modes, including CW, voice, RTTY, packet, and SSTV. The SST, Mir, and the IST all carry amateur-band equipment, and sometimes (though rarely) the astro(cosmo)nauts will talk to Earth-bound Hams! Now, doesn’t that make yu want to run otu and study up for that Tech class ticket?

73 de KB2YYR

5

But amateur radio satellites require directional antennas to use. The OP was talking about a signal strong enough to receive on a regular radio.

It says here that a low-power FM station (10-100 W) has a range of 3.5 miles. I’ll be optimistic and take that to mean most radios can pick up a 10-W transmittion 3.5 miles away. That means if the power is distributed optimally, you need 0.06 W per square mile (10W / (4 * pi * 3.5[sup]2[/sup]) ). If you use an orbital antenna with a circular beam pattern barely large enough to cover the continental US, you need 200 kW of power to achieve 0.06 W/mile[sup]2[/sup]. That’s 10 times more power than is available on a Boeing 702 communications satellite (one of the largest available, at 5 tons launch weight).

6

No, they do not, necessarily. I worked the Mir packet station, R0MIR on 145.800 Mhz down and 145.200 Mhz up, using a Collins Ringo 2m colinear (essentially a stacked pair of vertical dipoles fed in phase). I could have received the signal with a bit less antenna, but to transmit up required a bit of gain, but not overly much.

7

What about high atmosphere balloons? I’ve read a few articles that seriously discussed them as an alternative to some satelites. They would be cheaper, need less power, allow you to focus on a smaller area. Of course, launching an illegal transmitter balloon would draw the wrath of the FAA as well as the FCC.

8

“This is Radio Clash on a pirate satellite!”

(Sorry. ;))

So, the sat would need to be purpose-built. Would it be possible to equip it with large enough solar panels to give it the desired wattage, or would we need something like small fission plants (as in the Cassini probe)?

And, assuming it does get through most of the time, would the signal be strongest in the day or at night? (That is, would a signal from outside Earth bounce off the ionosphere?)

Q.E.D., I was aware of the amateur radio birds. I also knew that they were strictly LEO and, therefore, not quite what I was looking for.

DocCathode, I didn’t think of those when I was forming my OP, but they also aren’t quite what I’m looking for. In this thread, at least, I’m interested in satellites.

9

I don’t think a 200-kW solar panel is practical. The Boeing 702 satellite is friggin’ huge. Its solar panels are as big as the wings on a Boeing 737 airliner, as illustrated here.

The Cassini probe uses RTGs (Radioisotope Thermoelectric Generators). These are just lumps of radioactive material that undergo natural decay, producing heat. The heat is converted into electricity by thermoelectric generators. These aren’t very powerful; the three RTGs on Cassini only produce 870 W each.

I think the only way to get 200 kW is to use a nuclear reactor. Both the US and Soviet Union have flown reactors in the 30 kW range, IIRC, and 200 kW is not out of line compared to some missions proposed recently.

In any case, all that is needed just to broadcast one channel. It doesn’t strike me as a very cost-effective method of broadcast, considering digital satellite radio is far more efficient. XM Satellite Radio uses two Boeing 702 satellites to broadcast 120 channels across the entire country.

I believe AM would be stronger during the day, but FM would not be greatly affected by the ionosphere.

10

Which do you want, small area or nationwide coverage? For broadcast, the power requirement is strictly a function of the service area you want to target. It doesn’t matter if you send a very tight beam from far away (e.g. geostationary orbit), or a broad signal from closer (e.g. balloon).

But you are right, a balloon it might be a reasonable choice for a moderately large (e.g. Texas size) coverage. But since the wind is rather strong up there you need a maneuverable balloon, i.e. a blimp. I’ve seen some design studies along these lines but nothing practical yet. I think these platforms would be especially useful for cell phones. Distance does matter if you want to keep the transmitting antennas (i.e. the one on the phone handset) compact.

11

scr4: Fascinating. I didn’t think we’d ever launched real reactors, only thermocouples with radioisotopes inside. How were those 30 kW reactors designed? (And what missions now under development would require 200 kW?)

And, yes, my goal with this is to see if a satellite broadcast with continent-wide coverage (that is, all of the Lower 48 in the USA, or a big chunk of Europe, or something equivalent) on normal, bog-standard radio would be feasible.

Apparently, it would depend on how much money the backers would be willing to spend per bird and how much R&D they could get their hands on, as there are no satellites yet made which could manage this task.

12

There’s a vast difference between AM and FM. I’m not sure I buy the 200kW number, but being late on a Friday night, I’ll have to get back to you.
As a rule of thumb, Fm works by ‘line of sight’. There can be a very powerful transmitter (100 kW) that’s completely blocked out, if you live in a valley. I seem to remember a transmitter in the Florida Keys that was 100 kW+, which was needed because of the distance between mainland and the furthest point of Key West. Of course, it covered a lot of empty water, as well.

From being a PD for radio stations for more years than I care to remember, I can tell you that what we always seek is altitude. The higher the transmitter, the lower the power needs to be to penetrate.

If you think of that line of sight again, and realize that concrete, wood, glass, almost any solid material, will interfere with a signal, you realise that going through one wall hampers the signal. going through several buildings to reach your radio will hamper it exponentially more.

Or put it another way: A transmitter on the top of the Sears Tower will not only beam horizontally, but vertically as well. That means a person at ground level, 3 miles away will get a clear strong signal. Same person, with same receiver, might have trouble picking up a signal coming from only a mile away, if the transmitter (of the same wattage) is at ground level, what with there being so many buildings and other stuff between the source and the receiver.

I’d say it’s a question of logistics. As someone noted above, should I put a transmitter on the ISS and start broadcasting at 104.1 MHz, I’m gonna have a lot of pissed off radio stations on my back. The FM dial is limited. However, if there is a chance of finding a single frequence which is not occupied in N. America, I think a 10 kW transmitter on a satellite would do.

However, if we’re talking about AM, then it’s a totally different ballgame. When I was an intern at WGN720, it was considered a ‘clearchannel’ (not to be confused with the company CC) station, that reached 30+ states at night. The transmitter was 50kW. Putting the same transmitter in space - well, it’d reach half the globe. Easily.

13

Sorry, turns out I misread the figures - the SNAP 10A satellite (the only American satellite with a reactor) had a 45kWt (thermal output) reactor but the electrical output was only 650 W (cite). The Russian RORSAT had 100kWt reactors producing 3kW of electricity.

As for more recent programs, this document says the Prometheus project is aiming for 100 kW. This would be mostly for ion propulsion.

It’s based on the “3.5 miles at 10 W” number which I pulled out of Google, as I explained. If you can come up with a better estimate i’d appreciate it. I don’t have a very good feel for radio signal power.

14

Actually, not. The reason AM reaches 30 states is that radio waves at those wavelengths bounce off the ionosphere. Satellites are above the ionosphere, so AM radio waves from there are going to bounce away from the Earth.

15

Yes, that’s what I thought. So, would the power requirements of an AM station be significantly different from an FM station in this circumstance?

16

Not so. When I still lived at my father’s house, and had access to all of his Ham equipment (yes, I’m licensed, but too poor to buy equipment of my own) I used to use the RS10/11 satellites all the time, up on 2 meters, down on 10, with only omnidirectional antennas. 45W output on 2meters (144mhz-148mhz), and we listened on the 10 meter band.

</nitpick>

-Butler
N1OQF

17

IANAPhysicist or radio engineer, but are you sure about the last part of this? It seems to me that the bouncing of a terrestrial signal is like skipping a stone on a pond, and that a signal coming in from outside the ionospher would be like dropping a rock in a pond. I.e., the ionosphere might attenutate the signal a bit, but wouldn’t be an impenetrable barrier. Just a hunch I have about the situation.

18

Arthur C. Clarke not only predicted the idea of communication satellites in geosynchonous orbit, but way back in 1959 he wrote an essay titled “Voices from the Sky” in which he speculated about what we would now call direct satellite broadcast. One section of the essay was a Cold War speculation that he titled “How to Conquer the World without Anyone Noticing”:

What of course happened is that the West got going on communication satellites (although not quite the direct broadcast system Clarke envisioned), and today fifty years later English is the main language of mankind :stuck_out_tongue:

The main limitation of such a scheme is broadcast power, as noted in previous posts. Not only the size of the solar arrays, but also the fact that geosynch orbit is in the middle of the outer Van Allen radiation belt, which degrades solar cell performance and limits the useful lifetime of satellites (along with the need for small amounts of propellent for attitude and station control). This limits how large, complex and expensive is practical for a com sat in geosynch orbit.

I wonder if anyone has seriously considered a solar-thermal power supply for a large communication satellite? If the satellite had a turbine powered generator, and used an ultra-efficient ion engine for station keeping, it could have a very long service lifetime.

19

There’s a group at NASA developing Stirling engines for space use, and one of their stated uses is a solar power system. I don’t think anyone has flown such a system yet. Solar cells are a proven technology, and space agencies always stick to proven technologies when they can. Also don’t forget any generator powered by heat requires a powerful cooling system. Dumping heat is a very difficult task in space - there’s no air or water so you need to rely on radiative cooling, which isn’t very efficient.

Ion engines are already used on some geosynchronous satellites for stationkeeping, and on interplanetary probes for propulsion.

20

I’m not a physicist either, but as I understand it, the ionosphere and the Earth act as a waveguide for AM frequencies at night. If this waveguide works like others I’m familiar with (e.g. optical fibers) then where you can hear such a broadcast depends on the index of refraction of the ionosphere. That is, the signal will only make it through the ionosphere directly below the satellite and in a circle around that point. The size of the circle depends on the index as well as the height of the satellite.

Since geostationary satellites are directly above the equator, I would not expect the US, Canada or Europe to be within the circle. However, I don’t know what the index of refraction is for the ionosphere, so I can’t say for sure.