posted
I have a bit of a sci-fi specific that's been driving me nuts for a little bit - I'm ignorant as to the magnitudes of the quantities involved.
Suppose there was a space ship, equipped with decently sensitive signal receivers, located in a solar system similar (but not identical) to our own. In particular, suppose that this ship is located at a distance from a target planet equal to the distance between Mars and Earth.
I know that radio waves (em waves in general) have a geometric dispersion over distances, and that the intensity can also be lowered by intervening medium (like a planet's atmosphere).
1. If the target planet has a satellite system similar to or perhaps far more powerful than Earth's own, would it be reasonable for a ship to be able to receive any transmissions at a distance of Mars, even if the signals were not directly targeted at the ship? Would a ship be able to "listen" and reasonably expect to find transmission signals from so far away? (of course acknowledging that such signals would be n minutes old when they reached the ship)
2. If the target planet had civilization that used radio technology, but did not put out satellites - and they broadcast signals in a fashion identical to the way radio stations on Earth broadcast, at what distance from the planet would the signals be unintelligible? Would a population on such a planet be able to justifiably assume that no ships passing by would notice terrestrial radio signals?
posted
Receiving a radio signal depends largely on the receiving antenna's configuration and how the signal transmits data, analog or digital, frequency modulation or amplitude modulation, amount of interference noise on the wavelength, and signal blocking medium in the line of sight. Low power to a signal is not an impediment to a clear transimission along a line of sight in a vacuum. Range is not a major issue in the vacuum of space.
I see nothing implausible in the parameters set forth, except for the assumption that local radio transmissions wouldn't propagate out into space nearly infinitely. More detailed information is available; Wikipedia: Antenna, Yagi Antenna, Parabolic Antenna. etc.
[This message has been edited by extrinsic (edited May 20, 2009).]
posted
I understand that signals propagate infinitely -- but are you saying that, despite atmospheric interference, a receiver that could fit on a ship the size of a yacht could hear your local NPR station from Mars orbit? The signal will get there, but would it really be distinguishable from noise? I know my car antenna isn't that powerful, but given that I lose a station a good hour or so out of the city, I'd think that even with a killer parabolic antenna, you'd lose one outside the atmosphere.
In particular, said ship doesn't have an enormous satellite (of the size you see driving in the southwestern US deserts). I'm assuming technology that really isn't that much better than what you can buy today.
posted
For what it's worth, ham radio operators can bounce their signals off the moon. And that's just with home grown equipment, nothing fancy.
I would think part of your answer would come by asking what kinds of signals you decide your people are using. There's all sorts of FCC broadcasting rules (a ham radio manuel would probably be able to give you the specifics) that basically state certain types of audio and video signals must be sent out in such a manner so as not to cause interference with other types of signals. They "yield" so to speak.
I don't remember the technical term, so sorry if this is all a little fuzzy, but an example of what I mean is as follows: before the digital conversion, trucker radio frequencies would often interrupt the signal my rabbit ears picked up. I'd be listening to my TV program and all of a sudden there would be voices overriding my audio track. So the TV signal was "most passive" and it would be least likely to actually make it to Mars. The truckers' (rather entertaining comments) about other drivers, on the other hand, stood a better chance of it.
posted
Transmitter antennas come in a variety of styles as well as receiving antennas. A directional transmitter antenna focuses a signal on a narrow footprint. Like an optical lens, a directional antenna has a focal point where the transmission is most focused then spreads out to cover a target region with the intended signal amplitude. Modern day satellite TV signals broadcast from space using directional antennas. Nonetheless, there's still leakage. Radio, like light, has a wave behavior that proportionately expands over distance from a source, but the waves carry the same information in the same sequence regardless of strength or direction.
However, a planetary mass absorbs all but the lowest, ultralong-wave frequencies. Ground-based transmitters for ground based reception are usually omnidirectional with most of the signal lost to outer space or to the ground around an antenna tower. In some atmospheric conditions in some frequencies, signals bounce off the sky and reach distant receivers. Short-wave frequencies are more prone to absorbtion than long-wave. Submarine radio communication is in ultralow frequencies. I live near a WW II era installation of a submarine transmitter that broadcast signals which could be received anywhere underwater in the North Atlantic. The antennas are long gone, replaced by more advanced technology.
Radio astronomers track thunderstorms on Jupiter and Saturn with radio telescopes. The erie sounds of storms in Jupiter's atmosphere closely resemble ghostly sounds popular in B-grade horror movies of the '50s.
Edit: Yeah, a Mars orbiter could, and many do, as well as ground rovers, receive Earth transmissions and vice versa. As long as transmitter and receiver are in clear line of sight, with correction for time delay, an FM NPR broadcast would be received legibly at Mars, even groundside, or on Quaoar for that matter. AM would probably be very static and warbling though.
[This message has been edited by extrinsic (edited May 20, 2009).]
posted
Radio waves won't go forever in practice, because of noise. There's background noise from radio sources all around, some artifcial, some natural and unavoidable, all interfering with the signal you're listening for: the further away you are, the more the signal you want will get lost in the noise of other signals. Furthermore, the receiver itself makes noise which will drown weaker signals; pump up the volume and the louder hiss still obscures the signal you're straining for.
Because of noise, reception quality will depend on the strength of the signal transmitted, and whether the sending antenna is directional or not. We tend to put only as much power into a transmission as is needed to get it to its destination. (I suspect that the relationship between power and transmission distance is an inconvenient logarithmic one: double the power and the transmission distance is rather less than double.)
To save power (which is expensive to generate) we use directional transmission antennae, to focus available power on the intended receiver. E.g. satellites use directional antenna to focus transmission on the Earth below and avoid wasting it on the black sky above.
Thus, it's not necessarily true that a signal will be easy to receive way beyond the distance between transmitter and intended audience, especially if the transission antenna is directional and the eavesdropper is not in the intended footprint. (These are problems the SETI people recognise and strive to overcome with highly sensitive, low noise, directional receivers and the hope that someone, somwhere, is pumping out enough power to traverse interstellar distances.)
If the transmissions are digital, it might be easier to ignore the noise using digital signal processing techniques, which are designed to facilitate communications in noisy environments.
But if the transmissions are intended to be proof against eavesdropping (e.g. military communications) it gets harder again, because they'll use a variety of techniques like encryption, steganography and frequency hopping to avoid interception. That's not necessarily a stopper, since some military intelligence can be gained from simply observing the volume of communications between source and destination desapite not understanding the content; e.g. if you see a sudden increase in communications between Bad Guy in London and Bad Guy's Handler in New York, you know something's going off in London.
That said, many or most satellite transmissions are encrypted anyway, because telecom companies want to assure some degree of confidentiality to their subscribers and TV stations want to charge money for their broadcasts; an illicit receiver would probably need decryption keys for most satellite broadcasts.
Which of course is why Ursula LG invented ansibles.
posted
The question as it relates to my plot isn't one of military intelligence - just people looking at an Earth-like planet and trying to figure out if there are any people on it.
The planet has a "colony" of outlaws, who would prefer that nobody know they are there. Hence, no satellites. I was just wondering if they would have an outright ban on, say, AM radio for communication, because that would give away their presence.
Also was wondering if my intrepid heroes, who basically just point their receivers at the planet to see if anyone is home, would notice the signals before they were right on top of it. I currently have a city that has radio comm, and the heroes don't detect the signal until they are in orbit (at which point they have been noticed).
posted
From the end of that article, it sounds like less-aggressively broadcast signals would be indistinguishable from background noise a planet or so away, if I Love Lucy would be toast outside the solar system.
posted
The Voyager spacecraft are transmitting back to Earth from the edge of the solar system. Let's see that's, um, way stinkin' out there. And they're doing it with 23 watt transmitters! Thats a little more than 5 times the output of your standard CB radio.
posted
If you are using a noise-canceling algorithm for white noise, you can improve the Signal to Noise ratio. Yes, radio waves can communicate with spacecraft near other planets. How do you think we got back all those images from Voyager and other probes?
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posted
Quoting directly from the article you referenced,
quote: The antennas that the Voyager spacecraft use are big. You may have seen people who have large satellite dish antennas in their yards. These are typically 2 or 3 meters (6 to 10 feet) in diameter. The Voyager spacecraft has an antenna that is 3.7 meters (14 feet) in diameter, and it transmits to a 34 meter (100 feet or so) antenna on Earth. The Voyager antenna and the Earth antenna are pointed right at each other. When you compare your phone's stubby, little omni-directional antenna to a 34 meter directional antenna, you can see the main thing that makes a difference!
Sounds like you have to be trying to be heard from that far away.
In my instance, the people on the ship are pointing their antennas at the planet, but the people on the planet are very interested in not pointing antennas into space. They would use omni-directional broadcasting, and low-powered broadcasting at that.
I'm not an expert on the matter, but from this superficial research, it sounds like people on the planet could safely communicate via radio without worries that they'd be giving up their secrets to casual passers-by. It also sounds like they could have low-powered radio stations without much worry. They aren't trying to do everything possible to avoid people genuinely examining the planet (there is a city... which kind of gives up the game if you are looking). They just don't want anyone to notice by accident.
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