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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
The problem was discovered years ago, took 6 months to investigate and nearly 2 years to resolve. The BBC told us all about it with nice graphics the other night.
Do not try to read the dupe, thats impossible. Instead, only try to realize the truth
What truth?
There is no dupe
"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/
As you answered in the previous paragraph - it's the non-scientist administrators.
A fine is a tax you pay for doing wrong and a tax is a fine you pay for doing all right.
The article says he is a 26-year EASA veteran, it does not say that he is 26 years old. Though, I thought the same thing on my first pass and had to re-read it.
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.
Given this is Slashdot I'd bet there are some that view Star Trek as absolute reality. Aparently some of them have mod points as well. *shudder*
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...
[NASA's] Horttor never got an explanation of why Alenia Spazio's telemetry system was built with a timing system that couldn't accommodate the Doppler shift in Huygens's telemetry. "It is a design feature of another application in Earth orbit, and they just reused it," he told Spectrum, adding, "I don't know why anyone would ever want to build it that way." (An Alenia Spazio spokeswoman said that none of the company's officials were available to comment because of a company-wide summer vacation period.)
Anyone think that the "company-wide summer vacation" may extend a little longer than originally expected?
"Hey, Tony! Glad to hear you ready for work. But why don't you go ahead and stay in Verona another month or two? Check out this web site while you're there. Ciao!"
Stressed? Me? Of course not. Stress is what a rubber band feels before it breaks, silly.
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
A shining example of the promblems with proprietary design. No one can see what's wrong with it without expending a huge amount of effort. I'm just glad someone did decide to spend the effort.
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)
So I guess they'd have had to run a billion-mile cable first. Line noise would be a bitch, wouldn't it?
I figure by 2030 or so my 6-digit UID will be something to brag about.
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
You mean "What is wrong with proprietary?" right? After all, the entirety of this problem was because NASA bought a black box proprietary technology, and without access to its specs could only pray that it performed as advertised.
In this case, the black box didn't meet the required standards, but there was no way NASA could have known that this company built the black box out of off-the-shelf terrestrial design principles unfit for cosmic use.
If I have been able to see further than others, it is because I bought a pair of binoculars.
Is *IS* a "software" radio and not hardware. It's implemented in "firmware" and they say they could have changed it easily, except there's apparently no way to do so after launch.
The problem is they didn't find this problem until AFTER launch. good timing, right?
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.
Vintage computer adverts: http://www.vintageadbrowser.com/computers-and-software-ads
Boris Smeds did a great job in replacing lots of expensive tests with a series of trivial, yet critical tests.
Why weren't simple tests like these used while the spacecraft was on the ground?
These are obvious problems. When you take a transmitter and throw it into a planetary descent, this is what should be expected.
It is shocking to me that a transceiver pair isn't tested by the team assembling the spacecraft before launch!
If it can be tested in 2 days when it's in space, 48 light-speed minutes away, why can't it be tested on the ground, fully assembled?
Engineering isn't a science, but I expect that engineers desigining projects like this should be using thorough unbiased scientific testing, not only thorough design.
If they slip up like this in non-destructive tests, one has to wonder about how tests on the resistance to physical damage are carried out?
Do they simply make assumptions that all nuts & bolts are manufactured to spec? Do they assume that all parts will withstand the forces that they are requesting in spec sheets?
How can a cost of hundreds of millions of dollars be justified in creating such craft, when basic, inexpensive testing isn't being carried out? If the test would cost 30,000$ (a few days of well-paid outside experts time, plus expenses and travel), as a critical portion of a 300,000,000$ mission, how is it not done?
The only agreement that should be accepted by an agency purchasing a part is that they won't use the specifications of that part to replicate exactly the same device. I'm sure that they paid a high premium for the transceiver. Why wouldn't they have access to the documentation and spec sheets? This use of NDA's is dangerous.