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Saving Huygens
TazMainiac writes "This months IEEE Spectrum is running an article on how a Swedish scientist discovered that the Titan probe Cassini had a communications flaw that would cause it to lose all data sent back from the Huygens lander as it plunges into Titan's atmosphere. The problem - Doppler effect. The fix: go read the article."
So wait, reading the article will fix the Cassini probe?
"Have you ever thought about just turning off the TV, sitting down with your kids, and hitting them?"
RTFA will fix the probe.
reversing the polarity of the transponder...
duh...
If you don't know what AltaVista is (was), get off my lawn.
It is amazing that the problem with the reciever was detected. It was more amazing to read what they went through to document and present the problem. It also says something about the relationship between NASA and it's subcontractors when they can accept a receiver design and not sign a standard non-disclosure agreement so that they can see the specific design elements. If they had done so, they would have been able to see the problem before launch. However, having read the article, the complexity of the mission is such that I am possibly more amazed that more didn't go wrong.
http://www.busyweather.com/
Installing the reentry sensor upside down, ignoring the Doppler effect - this rocket science stuff is so hard, they're missing all the easy stuff.
--
make install -not war
"We have a technical term for what went wrong here," one of Huygens's principal investigators, John Zarnecki of Britain's Open University, would later explain to reporters: "It's called a cock-up."
We Americans speak English, but this is proof positive that the British have had much more time to master the use of it
"If you think you have things under control, you're not going fast enough." --Mario Andretti
Oooooh! I love that technical jargon.
Spoiler Warning:
Now you know how they fixed it, so no need to read the article.
See what I've been reading.
a collaboration with the European Space Agency, Cassini, in addition to its own suite of scientific instruments designed to scan Saturn and its moons, carries a hitchhiker--a lander probe called Huygens.
http://www.busyweather.com/
I drove a Fiat for years. I could have told them an Italian radio wasn't gonna work. :)
12:50 - press return.
This is not a simple subcontractor arrangement. It is cooperation between government agencies in different governments, each of which has private contractors working for them.
Besides the obvious contractual nightmare this represents, there is also the issue of Export control between governments, which cannot be countermanded with a simple non-disclosure.
IMNAL, but I work on a similar project and you need to learn some of this stuff, sadly, to get your work done. I'm hopeful this incident will help to clear up these sort of cooperation issues in the future.
Good work in resolving this all involved! Remember Slashdotters, we explore to learn...
Certainly, satellites around the Earth qualify -- if you want to be truly successful working with amateur (ham) satellites, you need to adjust for the doppler shifts., especially at the higher frequencies. If you don't, you'll only be successful when the doppler shift is small -- basically, when the satellite is as high in the sky as it's going to get in this pass.
Suppose your signal is at 441 mHz, and the signal is only 20 kHz wide. It only takes a 0.005% shift in the frequency to move that signal 20 kHz so you can't detect it at all, and doppler shifts seen by objects in low Earth orbit satellites can be a good deal larger than that.
It looks like the relative velocity of Cassini to Huygens actually *was* high enough to lend a singificant Doppler shift, so correspondingly the data rate was massively compressed--like the frequency of a racecar coming towards you getting higher and higher, except in this case its bitrate instead of sound. The antenna was only designed to "listen" for a fairly static bitrate --like if once the car got close enough and the sound frequency high enough you just stopped hearing it. So instead they're altering the flightpaths so that Cassini is now far enough away from Huygens that the broadcast vector is mostly perpendicular, with minimal Doppler shift -- think about standing very far away from the racetrack instead of right in front of the car. The total distance between you and the car changes by much less, so you hear more of a constant hum than a higher and higher frequency, analogous to the drone of a jet plane passing far overhead. Because the Doppler shift is minimal, the antenna can now receive data at a nearly constant bitrate it can handle. Very nicely done.
What I find hard to beleive is that the data slicer for the radio was not a chunk of code running on a processor, rather than a hardwired circuit.
I do SDR (Software Defined Radio) for a living - doing a data slicer like this isn't very hard at all. Why they couldn't just reprogram the slicer to take into account the bit timing shift - or better still, why weren't they resyncing on the zero crossings of the signal so they could deal with bit timing errors automatically?
Hell, for that matter why don't they have an option to route the recovered signal verbatim to the main transmitter and send that to earth - and do the signal processing here? NASA *used* to have the philosophy of "all the bits to earth" - the wouldn't even use lossless data compression lest the signal be corrupted and unrecoverable.
www.eFax.com are spammers
While slashdot has category icons, it really REALLY needs a "hero" tag, like you'd see on Fark.
This engineer that found the problem and rallied against opposition to see that this gets fixed is, in my opinion, a total hero. The world would be a much better place if more people like him were around!
I have no problem with your religion until you decide it's reason to deprive others of the truth.
The relative velocity was quoted as 5.5km/sec which means v/c=0.000018. This is not such a small number.
Furthermore, even though the frequency changes little, the phase can shift a lot. The change in phase is proportional to v/c times the number of cycles in the segment you are examining - and there are a lot of cycles in 1/8192 second chunk of the microwave signal they are using to communicate.
Lastly, the length of the transmission also matters - 2 hour transmission of 1/8192 sec chunks amounts to approximately 60 million chunks. If you multiply the doppler shift above by the number of chunks you get approx 1000 - i.e. the chunk timing will shift through completely 1000 times during transmission. (In other words you will be drifting in and out of sync with transmission rate 1000 times during descent.. A sure way to get most data scrambled)
Sure, everyone knows that a)something Doppler related went wrong with Huygens and b) they fixed it with "fancy flying", but that's like saying don't bother to read a history of World War II because everyone knows a) Hitler started it and b) the Allies won.
The point of the story was to explain the problem with a level of accuracy and detail that was simply missing from most report and to tell the story of some stone-cold great work by an engineer, something of interest to most engineers, and I would hazard, to most slashdotters.
As far as I am aware, no-one else has told the story of how Boris Smeds pushed through the comms test that showed something was wrong, despite intial rejection and then later, modified it on the fly to reveal the problem was Doppler related, saving months of delay. Learning about his example of how to be a great engineers is the article's real utility, not teaching Spectrum readers how to fix Titan landers.
Disclaimer -- I edited this story for IEEE Spectrum
"Just once, I'd like to meet an alien menace that wasn't immune to bullets." -- The Brigadier, Dr. Who
I'm uptight because its annoying when you work on an article and people comment without bothering to read it -- yes, I know that's endemic to /., but it's still a pain in the ass.
/. decided to post the story.
The point is that we dug up an aspect of the story you're not going to see any where else, let alone a general overview program, but a really cool story of a guy who deserves a lot of credit, Boris Smeds. I would hate for anyone to not bother to find out about him because a related program on the telly happened to be braodcast the night before
"Just once, I'd like to meet an alien menace that wasn't immune to bullets." -- The Brigadier, Dr. Who
The amount of doppler shift is proportional to frequency and velocity. But it wasn't the change in signal frequency alone, it was the change in length of data timing as well.
The general equation is:
fdoppler = (frest * velocity )/ c
where:
fdop = frequency after doppler shift
frest = frequency before doppler shift
velocity = speed of object relative to oberver
c = speed of light
Although radio waves have a longer wavelength (kilohertz/megahertz) than light (terahertz+), the
effect is less noticable, but still significant.
According to the article, the doppler shift was +/-38 Kilohertz. Given the fact that data was being transmitted on an 8/16 Kilohertz carrier wave, that's a rather significant change.
This is enough difference to allow police speed radar traps to work, and for researchers to measure the wind speeds inside tornado's.
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Boris was key to finding the problem, but the solution was mostly a trajectory one. The article has a rather mangled explanation of the trajectory changes, and it treats the people who worked on it rather anonymously.... just calling them 'navigators'. The fact is that the trajectory changes done to save the probe mission were far from trivial.
Boris deserves a lot of the credit for saving Huygens, but several other people deserve credit but have been rather anonymous outside of ESA/JPL. 'Saving' Huygens was a team effort, and a lot of people played a part. There are a handful of other key people that the article doesn't mention at all.
Also there are a few factual errors in the article.... NASA couldn't simply sign a NDA to get the specs for the receiver, and there was a lot of effort by NASA to get the specs. Even after the problem was detected, Alenia resisted sharing information for many months.
There are 10 types of people in this world, those who can count in binary and those who can't.
I agree the recovery was a team effort, but the fact remains that what Smeds did was a rarity: a singular individual effort that, if it hadn't occured, would have resulted in disaster. Thus we felt he deserved some serious kudos and so the article focused on him.
I'm in no way deingrating the amazing and creative work that the trajectory guys did. But think of it like this: If any one of those guys were absent from the project, because of a sabbatical, or, God forbid, an accident, chances are that the mission still would have been salvaged.
The same cannot be said about Smeds during the period between being told to do a test and coming back to ESA with the results -- it's fair to say that many, if not most, engineers would have just developed a carrier wave only test as originally planned, or wouldn't have bothered to persist with the more complex test after being turned down (after all, who's looking to get into trouble to do extra work?), or might not have had the insight required to modify the test on-the-fly when the downlink started showing problems.
The situation is analogous to Apollo in some ways: a lot of people helped design and build the LEM (Tom Kelly is one of my personal engineering heroes), but John Houbolt deserves his place in history for pushing NASA onto the LOR architecture path in the first place.
As for what we said about NASA and the NDA, I'll just have to say we stand by Oberg's reporting. But if you have something that shows we really did get it wrong, I'd be more than happy to look at it and print a correction if warranted.
"Just once, I'd like to meet an alien menace that wasn't immune to bullets." -- The Brigadier, Dr. Who