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Inner Workings of High-Gain Mars Rover Antennas? 63

Posted by Cliff
from the how-do-they-work dept.
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."
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Inner Workings of High-Gain Mars Rover Antennas?

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  • Martians (Score:3, Funny)

    by Anonymous Coward on Monday January 12, 2004 @07:54PM (#7958098)
    Naturally we're just piggy-backing on the already built martian wireless infrastructure.
  • Paint some cans white and glue them together and voila! You've got yourself a nice set of omnidirectional antennae.
  • by mlyle (148697) on Monday January 12, 2004 @08:07PM (#7958209)
    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).
    • by stienman (51024) <adavis AT ubasics DOT com> on Monday January 12, 2004 @08:58PM (#7958588) Homepage Journal
      The X-Band Phased Array Antenna [nasa.gov] has one major benefit. Phased array antennas are meant to mimic the directivity and gain of a parabolic antenna, with the ability to aim it, in an array of antennas that does not move and is flat.

      So basicly you take a bunch of flat antennas, do some 'magic' between the array and the signal source (or destination) and you can effectively aim the antenna as though you were actually moving a parabolic antenna.

      Since the antenna on spirit is aimed mechanically, and phased array antennas are, IIRC, still pretty power hungry, then it may be that they are not using a phased array. However, it would make a lot of sense to use a phased array for fine control aiming and the machanical link for coarse control.

      -Adam
      • You mean like this? (Score:5, Informative)

        by JCMay (158033) <JeffMayNO@SPAMearthlink.net> on Monday January 12, 2004 @09:28PM (#7958797) Homepage
        Before Harris [harris.com] sold it to JetBlue [jetblue.com], they developed LiveTV [livetvifs.com], a system to bring DirecTV to airliners in-flight. The receiver includes a phased array antenna [livetvifs.com] 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).
      • Yes, there are plenty of advanced technology phased array antennas where delay lines and elements are dynamically swapped in and out.

        But it is possible to build a statically aimed phased array-- and this is what most patch antennas are in practice.
  • Important Question (Score:4, Interesting)

    by illuminatedwax (537131) <stdrange@@@alumni...uchicago...edu> on Monday January 12, 2004 @08:12PM (#7958256) Journal
    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
    • by eclectro (227083) on Monday January 12, 2004 @09:54PM (#7958999)

      First the moon landing, now this [photopile.com]. When is NASA going to come clean?
    • Dunno the frequency, but I'm not sure I'd want it posted on /. anyway. If it were, about 90% of the geeks seeing it would:
      1) immediately have orgasms -- this stuff is SO leet, ya know?;
      2) reverse-engineer the thing so that they could drive the Rover;
      3) using the results from step 2, play Martian Quake, or Planetary Doom 3, and probably run over lots of shit, including (quite likely, since there's bugger all down here) the only intelligent forms of life in the known universe. Luckily, those lif

      • 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?
        • I don't think they need encryption for the transmissions to the rover. First you need the right frequency, then you have to know exactly where (and when) to aim the signal, then figure out the exact command codes and finaly get your hands on a couple of BIG dishes with megawatt transmission powers like the Deep Space Network.

        • As the British now know, sometimes you DON'T keep the russians from hacking a mission. This whole "we haven't heard from it, we'll keep looking" thing is just a cover story. The CIA knows that a couple of 1337 h4x0r kiddies in Moscow are using the probe to play tic-tac-toe in the Martian dust as we speak.
      • 13 year old geeks with dial-up should be able to handle that sort of lag easily :-)
    • by Anonymous Coward
      Suppose you knew that the frequency was 8.123456GHz, the encoding was QPSK, the protocol was CCCSP, the error correction was Reed-Solomon, etc. Now all you need to interpret data coming down from the lander is a big freakin' huge 34m dish.
      • If it's this tough for someone out of the loop to get mars rover data. I think the chances of seti discovering a usable alien signal are next to nothing.
        • yeah, we rely on the aliens having way better technology than us, and wanting to be seen.
        • If it's this tough for someone out of the loop to get mars rover data. I think the chances of seti discovering a usable alien signal are next to nothing.

          There's a major difference between the two: SETI is just looking for the existence of a signal. With the Mars rovers, you'd be trying to interpret the signal as well. SETI would have no trouble finding the signal from Spirit and recognizing it as being sent from an intelligent source, if it looked in the right place at the right time.
  • Well, it is mars (Score:4, Insightful)

    by Descartes (124922) on Monday January 12, 2004 @08:16PM (#7958282) Homepage
    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.
    • by NanoGator (522640) on Monday January 12, 2004 @08:22PM (#7958323) Homepage Journal
      "1. The rover is operating outside of FCC restrictions."

      They still have to watch out for the DMCA, tho.
    • Re:Well, it is mars (Score:4, Interesting)

      by muonzoo (106581) on Monday January 12, 2004 @08:31PM (#7958375) Homepage
      ... From what I understand, much of earth based radio communication relies on bouncing signals off of the upper atmosphere and other "tricks". ...

      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.
      • Moreover, I don't think Mars has an ionosphere.
        Of course it does. You get an ionosphere from having your atmosphere hit by solar radiation. Mars, at 1.5 AU from the sun, gets almost half as much radiation (to a given area) as the earth does. More information [google.com].
    • Maybe that manned Mars mission can include the "Can you hear me now? Good." guy from the Verizon ads.
      • I sure hope so. It had better be one way, and skimp on the oxygen. Might as well send that Sprint guy in the trenchcoat with him.

        We'll see if they are so smug once that meet Val Kilmer's robot dog.
    • yes but the signal still has to get back here. It can't interfere with OUR signals when it gets here (or rather, they don't want our signals interfering with it) and it has to make it through OUR atmosphere.

      I know some of the signals are being relayed when it 's on the "dark side" of mars, but I wouldn't have thought they'd bother to mention that if ALL of the signals were being relayed the same way.
      • Actually they wouldn't HAVE to reach earth, they could be collected by the deep space network. As for the dark side they can probably relay through any number of probes in orbit including the ones that went at the same time as the landers.
    • by Tau Zero (75868) on Monday January 12, 2004 @10:30PM (#7959368) Journal
      you should have stopped there.

      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.

      • Sorry, as I said I'm not a radio expert.

        Anyway, if bandwidth isn't the limiting factor, you can choose signal strength or whatever. I assume the regulations regarding interference between space probes are somewhat more relaxed than FCC regulations.

        What I mean is they can pick whatever part of the radio spectrum best fits their atmospheric needs and just use that, although that is just conjecture.

        Also I was under the impression that they were relaying the signal from the rover through a satellite in orbi
    • FCC (Score:5, Interesting)

      by Detritus (11846) on Monday January 12, 2004 @11:49PM (#7959911) Homepage
      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.
  • I suspect that it's simply a patch antenna. For the size and weight, it's hard to beat the gain of a patch antenna.

    Here [geocities.com] is an example for 802.11b of which the author notes, "What's nice about the patch antenna over the "cantenna" is its broad beamwidth. The cantenna has to be pointed very precisely at the AP to get anything at that range, but the patch can be tilted several degrees and still get a signal." The Spirit's antenna was estimated to be 2 degrees off aim at the initial connect attempt, but t
    • The best "patch" antenna on that page [hyperlinktech.com] has 14 dB of gain over isotropic (which almost nobody bothers to make because isotropic antennas are not generally useful on Earth; a much more realistic assessment is gain over a dipole). That same page lists 24 dB "grid" (non-solid parabolic reflector) and 20 dB "panel" (apparently flat-panel phased array) antennas.

      Energy is conserved; you are not going to get a stronger signal across one part of the sphere without taking signal away from some other part. Beam width

      • Re:Cluing you in... (Score:3, Informative)

        by fwc (168330)
        > The best "patch" antenna on that page has 14 > dB of gain over isotropic (which almost nobody > bothers to make because isotropic antennas are > not generally useful on Earth; a much more realistic assessment is gain over a dipole)

        The reason why almost all non-ham-radio antennas are specified in dBi's (decibels over isotropic) instead of dBd's (decibels over a dipole) is that you use dBi's when computing a link margin instead of dBd's. If you use dBd's you will be off by at least 2dB per en

    • What's nice about the patch antenna over the "cantenna" is its broad beamwidth.
      Just for the record, `broad beamwidth' is just another way of saying `low gain'.

      One attains `high gain' by having a narrow beamwidth. That's all `gain' means when referring to an antenna -- the narrower the beam, the higher the gain.

    • While the frequency is different, you'll find that these people sell patch antennas which compare favorably in signal strength with their parabolic antennas, but with a wider beam spread.

      You can't have it both ways. If you want high gain, you have to have narrow beamwidths. If you want wide beamwidths, you can't have high gain. It's a conservation of energy thing.

  • by jdawg (21639)
    I just hope the AE-35 doesn't blow.
  • by kmahan (80459) on Monday January 12, 2004 @09:43PM (#7958894)
    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!)
  • by Tau Zero (75868) on Monday January 12, 2004 @09:54PM (#7959006) Journal
    Claimer: I am an electrical engineer, I have studied wave mechanics, and I have seen devices like this antenna built into devices such as aircraft weather radars.

    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.

    • Yes, I think that is correct, but let me add something that might be confused here.

      Just because this appears to be a phased array does not mean it is an electrically steered phased array (as other postings have suggested). Look at a picture of the rover [http://marsrovers.nasa.gov/mission/images/rover1 _ detail_500.jpg]. The high gain antenna looks quite steerable. It is possible that it is also electrically steered to fine tune the aim, but it doesn't look like a high enough gain antenna to need tha
    • And some of those geeks are still radio hams, and some are indeed listening to the Mars effort. They've been tracking Mars Express into orbit, and are now planning their own ham radio mission to the red planet - AMSAT Phase 5A - which will be an independently built communications and science spacecraft to go into orbit sometime towards the end of the decade. Now that's what I call ham radio. More -- including helpful hints as to how you too can pick up signals from Mars (g'wan, admit it, it beats beaming W
  • phased array (Score:5, Informative)

    by mercuryresearch (680293) * on Monday January 12, 2004 @10:52PM (#7959520) Journal
    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.
  • Not only that it's also a bit of public funded R&D, remember all the neat stuff we kids grew up with that were offshoots of technology to get those lucky guys to the moon to drive around crater rims and hit golf balls. :-D

    But let's think of it from Bush's perspective, he's almost finished feeding those military supp...er contractors with the search for a war or something.. Now he needs to payb... er get america's tech industry going by landing some more guys all the way on Mars.

    It's not that I don'

  • How close are humans to implementing Orson Scott Card's ansible or Dan Simmons's fatline? Will it emerge from the space, military or telecommunications industry?

Never tell people how to do things. Tell them WHAT to do and they will surprise you with their ingenuity. -- Gen. George S. Patton, Jr.

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