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Inner Workings of High-Gain Mars Rover Antennas?
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."
Naturally we're just piggy-backing on the already built martian wireless infrastructure.
The MERs use X-Band for high data rate communications back to earth-- which has a wavelength of 3cm, making high gain antennas considerably smaller and more practical.
It's my understanding that the high gain antenna on MER is a compact phased array design. Even parabolic antennas could be practical at the 3cm wavelength, though they wouldn't be flat (which was obviously preferable for footprint issues).
The important question is, what is the frequency of the transmissions being sent back to Earth, and can we figure out how to interpret the data being sent? We don't want any sort of NASA cover-up of the Martians, now do we?
--Stephen
Did you ever notice that *nix doesn't even cover Linux?
I'm not a radio expert so I don't really know what design they use, but you need to take into account two major points.
1. The rover is operating outside of FCC restrictions. So it can use as much bandwidth as it wants. Also, because there are few other sources of radio signals on mars there is likely no trouble with interference.
2. Because mars has a drastically different atmosphere than earth, the way the signals travel, etc will be different. From what I understand, much of earth based radio communication relies on bouncing signals off of the upper atmosphere and other "tricks". And of course if the atmosphere is thinner it will offer less resistance to the signal.
Beagle2 hasn't reported back. They're now trying silence to try to get it to go into "CSM2". In February, it's scheduled to go into broadcast mode [e.g. "Help! Can't hear you at all!"] on Groundhog Day or thereabouts.
"You might as well get your son a ticket to hell as give him a five string banjo." -unknown minister
Before Harris sold it to JetBlue, they developed LiveTV, a system to bring DirecTV to airliners in-flight. The receiver includes a phased array antenna that scans in elevation while sitting on a gimble that allows the beam to be scanned in azimuth.
Phased arrays use lots of power, but that's because each antenna element in the array requires its own amplifier(s) and phase shifter (or time delay unit). Fortunately, those amplifiers cam be much smaller than the monolithic amplifier required to drive a dish (since the signals from each amplifier in the array are summed together).
What NASA doesn't show you is the guy who takes Pringles cans, paints over 'em (after eating all the chips), and declares it "space ready" (for only $500k/unit!)
Invalid Checksum. Retrying.
The rover antenna appears to be an example of a flat-plate phased array antenna, which is a generalization of the "slot antenna". The basics are that you have a feedpoint where energy is coupled to/from a cable which goes to your transceiver. This feedpoint is coupled, either through transmission line divider/combiner networks of the appropriate impedance or the equivalent in waveguides, to each individual radiating element. In this case the radiating elements are segments of the surface of the disc, which happen to be connected electrically (which is not of great consequence). So long as each slot is at least a half-wavelength long, applying an RF voltage across its center lets it radiate just like a dipole perpendicular to the slot. Connecting a large number of slots via feedlines or waveguides so that they are all driven in phase gives you a nice, flat wavefront, which is also what you get from the reflection of a spherical wave off a parabola. The details differ, the result is more or less the same.
None of this would have been strange to a techno-geek of fifty years ago, because geeks of that time were into ham radio instead of computers.
Time is Nature's way of keeping everything from happening at once... the bitch.
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.
Time is Nature's way of keeping everything from happening at once... the bitch.
As others have pointed out, it's most likely a flat phased array antenna.
There's a couple attributes that would make it attractive for a extraterrestrial application. They're very compact for the gain they provide, and within the limits of the design they can be electronically steered (that is, no moving parts). I would imagine they probably have a mechanical coarse steering mechanism and electronic fine steering.
Sadly I can't seem to find any confirmation of this, just a few mentions of other spacecraft such as MESSENGER using phased array antennas.
If you're really a radio newbie you should know that gain is how well the antenna concentrates the signal. An isotropic radiator basically receives/transmits signals in a perfectly spherical manner. By sacrificing the directional coverage you can increase the gain. A great example is a flashlight bulb -- uncovered it radiates almost everywhere; with a parabolic reflector it radiates a beam. When they talk about using the low gain and high gain antennas they're basically talking about the radiation pattern.
You generally use low gain antennas for signal acquisition when you don't have control over where the antennas are going to be pointed. Once you know where everything is, you can point the high-gain antenna at the target. With more gain you have a better signal-to-noise ratio and can then crank up the data rates.
Phased array antennas work essentially by combining a large number (an array) of simple low-gain antennas such that they add their signals together (in phase) in a particular direction. In other directions the signals don't add the same way and there's much less gain. At microwave frequencies like X-band (about 8 GHz), a simple dipole antenna is only about an inch long, so it's easy to put a bunch of dipole-equivalents in a small space to make an array.
NASA gets its frequency allocations through the same process as other government agencies. The ITU makes international allocations. The FCC (civilian) and NTIA (military/government) make domestic allocations. The FCC and NTIA have to cooperate with each other on spectrum policy.
Mea navis aericumbens anguillis abundat
Most US laws apply everywhere: remember Sklyarov?
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].