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."
Martians (Score:3, Funny)
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:FCC regulations (Score:1)
Look like Pringles to me (Score:1, Funny)
Re:The Beagle (Score:5, Interesting)
Re:The Beagle (Score:2)
everybody's a comedian (Score:2)
Re:The Beagle (Score:2)
Actually, the Groundhog Day reference put me in mind of the Bill Murray movie [yanman.com], and the Beagle engineers. Every day, they wake up to the same thing, over and over again:
Can you hear me now? Crap!
Can you hear me now? Crap!
Can you hear me now? Crap!
X-band, and other matters. (Score:5, Informative)
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).
Re:X-band, and other matters. (Score:5, Informative)
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)
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).
Re:X-band, and other matters. (Score:3, Informative)
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)
--Stephen
Re:Important Question (Score:5, Funny)
First the moon landing, now this [photopile.com]. When is NASA going to come clean?
Re:Important Question (Score:2, Funny)
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
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?
Re:Important Question (Score:1)
loose lips sink ships, and could really fuck with a glorified RC car on mars, too.
so shhh...
Re:Important Question (Score:1)
Re:Important Question (Score:3, Funny)
Re:Important Question (Score:1)
Re:Important Question (Score:1)
Re:Important Question (Score:1)
Re:Important Question (Score:2, Insightful)
Re:Important Question (Score:1)
Re:Important Question (Score:2)
Re:Important Question (Score:2)
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)
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.
Re:Well, it is mars (Score:4, Funny)
They still have to watch out for the DMCA, tho.
Re:Well, it is mars (Score:2)
Oh, crap. I can kiss goodbye my copy of "The Greatest Hits from Mars Rovers - Live from Mars". This will certainly be copy-protected.
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:Well, it is mars (Score:1)
Re:Well, it is mars (Score:2)
I sure hope so (Score:2)
We'll see if they are so smug once that meet Val Kilmer's robot dog.
Re:Well, it is mars (Score:2)
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.
Re:Well, it is mars (Score:1)
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.
Re:You got it right in the first sentence, but... (Score:2)
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)
Re:From the Nasa website... (Score:1, Insightful)
Re:From the Nasa website... (Score:2)
Re:From the Nasa website... (Score:1)
My parents' house was such a house, too far from the central office. Luckily now we have cable, but our phone lines were such that I spent my days at 26.4 kbit. Others I know had similar experiences. So, I don't see anything wrong with NASA calling the typical home modem 32k, a more accurate representation of what people
It's the same kind used in mind control... (Score:2)
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
Cluing you in... (Score:2)
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)
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
Re:It's the same kind used in mind control... (Score:3, Informative)
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.
You can't get something for nothing... (Score:2)
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.
Replacable modules (Score:2, Funny)
Pringles on Mars (Score:4, Funny)
Micro passive phased array antenna (Score:5, Informative)
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.
Re:Micro passive phased array antenna (Score:3, Informative)
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
Re:Micro passive phased array antenna (Score:2, Interesting)
phased array (Score:5, Informative)
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.
Public R&D (Score:2)
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'
ansible / fatline? (Score:1)