Inner Workings of High-Gain Mars Rover Antennas? 63
cavac asks: "I've been searching for detailed info on how the high gain antennas on the Mars Rovers work, but did not find much useful information except that they DO work. I've been wondering: they are disc-shaped and are approximately the size of a CD. They somehow reassemble parabolic antennas but actually aren't, are they? Anyway, how much use would a parabolic antenna that size have? When I first saw them, they reminded me of the old antennas[*] (enclosed in plastic) used on vacuum tube based radio projects[*]. So, what's really inside the Mars Rovers high gain antennas? Note: Links marked with [*] are german language but the pictures should be self explaining."
Important Question (Score:4, Interesting)
--Stephen
Re:Well, it is mars (Score:4, Interesting)
Ionospheric refraction (or bounce) is really only applicable to longer wavelengths. The MER radios are operating in the X-band region, therefore there would be little ionospheric interaction in this region. Moreover, I don't think Mars has an ionosphere. Earth's ionosphere won't be an issue since the signal's angle of incidence will be arbitrarily large at a point in time over the reception window.
Re:The Beagle (Score:5, Interesting)
Re:Important Question (Score:5, Interesting)
2) reverse-engineer the thing so that they could drive the Rover
That's funny, but do they actually bother with encryption/authorization stuff? I would think that the lander/rover already has such a limited bandwidth that they wouldn't want to waste any of it with hash or authorization codes--on the other hand, you don't want a 14 year old taking control of a $400M rover either. Do they just keep the frequency secret? Does the control apparatus require NSF type gear? Even at that, how do you keep the Russians from sabotaging a lunar landing to maintain nationalistic prestige?
You got it right in the first sentence, but... (Score:5, Interesting)
Yes, the rover is operating outside the jurisdiction of the FCC (though not outside of international treaties regulating interference between space probes). Yes, the rover can use as much bandwidth "as it wants". But how much is that?
The answer is, not much. The problem is that you're trying to get a tiny signal across a very large distance back to Earth, and even though Earth is listening with dishes up to 70 meters across you still have serious limits. That squeak of signal coming in has to compete against the rush of thermal noise coming from everything, including the receiver itself. (The first stages of the receivers are cryogenically cooled to reduce thermal noise.) The amount of noise you have to listen to is more or less proportional to the width of the channel you're demodulating (the noise power spectrum varies with frequency, but it's a thermal curve that varies slowly across small frequency ranges). The more bandwidth you use, the wider your receiver filters have to be set, and the more noise comes in with your signal. Once you get to -1.7 dB signal/noise ratio, in principle your ability to tell signal from noise disappears (in practice we don't use encodings which give such a sharp cutoff, so your error rate starts heading up well above that).
Using more bandwidth is pointless unless you have more power to push a signal. On a platform as power-limited as Spirit, ten KHz or so is about all that they appear to be able to use productively over the interplanetary link.
FCC (Score:5, Interesting)
FCC regulations (Score:3, Interesting)
Mars now joins Venus as one of the few places where the US has a positive trade balance. [This is serious: when NASA imported the diamond window for one of the Mariner Venus spacecraft, they claimed exemption from customs duty because they were going to re-export it to Venus; and they got it, too].
Re:Micro passive phased array antenna (Score:2, Interesting)