Slashdot Mirror
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.
would the strength of the signal they are using up there be alot stronger than that which we use here on earth? possibly they use a different range than 2.4GHz? surely there are no FCC regulations on mars ;-)
-- If I were a fish, I'd be wet
Paint some cans white and glue them together and voila! You've got yourself a nice set of omnidirectional antennae.
I have been pwned because my
Are they still looking for "The Beagle"? Or have the technicians basically declared "this mission has gone *BSD" and given up.
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.
Taken from: http://marsrovers.nasa.gov/mission/spacecraft_rove r_antennas.html </B>
The rover has both a low-gain and high-gain antenna that serve as both its "voice" and its "ears". They are located on the rover equipment deck (its "back").
The low-gain antenna sends and receives information in every direction; that is, it is "omni-directional." The antenna transmits radio waves at a low rate to the Deep Space Network (DSN) antennas on Earth. The high-gain antenna can send a "beam" of information in a specific direction and it is steerable, so the antenna can move to point itself directly to any antenna on Earth. The benefit of having a steerable antenna is that the entire rover doesn´t necessarily have to change positions to talk to Earth. Like turning your neck to talk to someone beside you rather than turning your entire body, the rover can save energy by moving only the antenna.
Not only can the rovers send messages directly to Earth, but they can uplink information to other spacecraft orbiting Mars, utilizing the 2001 Mars Odyssey and Mars Global Surveyor orbiters as messengers who can pass along news to Earth for the rovers. The orbiters can also send messages to the rovers.
The benefits of using the orbiting spacecraft are that the orbiters are closer to the rovers than the Deep Space Network antennas on Earth and the orbiters have Earth in their field of view for much longer time periods than the rovers on the ground.
The radio waves to and from the rover are sent through the orbiters using UHF antennas, which are close-range antennas which are like walky-talkies compared to the long range of the low-gain and high-gain antennas. One UHF antenna is on the rover and one is on the petal of the lander to aid in gaining information during the critical landing event. The Mars Global Surveyor will be in the appropriate location above Mars to track the landing process. (2001 Mars Odyssey will not be in the vicinity.)
<B>Also of note is the data rates and methods used for transmitting pictures back to Earth: </B>
The data rate direct-to-Earth varies from about 12,000 bits per second to 3,500 bits per second (roughly a third as fast as a standard home modem). The data rate to the orbiters is a constant 128,000 bits per second (4 times faster than a home modem). An orbiter passes over the rover and is in the vicinity of the sky to communicate with the rovers for about eight minutes at a time, per sol. In that time, about 60 megabits of data (about 1/100 of a CD) can be transmitted to an orbiter. That same 60 megabits would take between 1.5 and 5 hours to transmit direct to Earth. The rovers can only transmit direct-to-Earth for at most three hours a day due to power and thermal limitations, even though Earth may be in view much longer.
Mars is rotating on its own axis so Mars often "turns its back" to Earth, taking the rover with it. The rover is turned out of the field of view of Earth and goes "dark", just like nighttime on Earth, when the sun goes out of the field of view of Earth at a certain location when the Earth turns its "back" to the sun. The orbiters can see Earth for about 2/3 of each orbit, or about 16 hours a day. They can send much more data direct-to-Earth than the rovers, not only because they can see Earth longer, but because they can operate their radio for much longer since their solar panels get light most of the time, and they have bigger antennas than the rovers.
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 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 they said they should still get good data at up to 4 degrees off, and beyond that they would still get carrier.
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.
But we all know they're simply using the technology they've been using for years to practice mind control on us.
-Adam
Somebody set us up the rover! We get signal. All your Mars mission are belong to us.
I just hope the AE-35 doesn't blow.
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.
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.
Don't blame Durga. I voted for Centauri.
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 is always traded off against gain. Indeed, beamwidth is a pretty good function of gain.
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.
What it will do is to start sending its ID signal whenever the sun is overhead. Provided it made it down to the surface in one piece. Mars Global Surveyor should have seen evidence of its parachute near the landing ellipse. And they should probably make those 'chutes out of something nice and shiny so MGS and its successors can find 'em easier. "Brightest three pixels on Mars!"
"You might as well get your son a ticket to hell as give him a five string banjo." -unknown minister
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.
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't like the idea of Mars or doing what's right for the country, I just have reservations on how honest it all is.
"Enjoy what you're doing! If it becomes drudgery, you're doing it wrong!" - Jim Butterfield
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].
dishes are? On the moon?
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.
There is much pleasure to be gained in useless knowledge.
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?