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."

Comments

We have a technical term for it

[RealAlaskan]

"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."

Oooooh! I love that technical jargon.

Spoiler Warning:

... the Cassini team crafted a response plan that centered on reducing the Doppler shift sufficiently to keep the data signal within the recognition range of the receiver. They accomplished this trick by altering the planned trajectory of Cassini.

Now you know how they fixed it, so no need to read the article.

Clever Solution

[SparksMcGee]

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.

Re:SDR

[Anonymous Coward]

The signal strength is very weak. We'll be using the 100 meter Greenbanks telescope, as well as the VLA, Parkes and Mauna Kea radio telescopes to record the signal on the ground, but the primary plan is still the Cassini orbiter.

This isn't the only screw-up for Alenia this mission. Look for articles involving the Ka Band Translator if you're interested. You may not find many, it hasn't been covered very publicly. Basically, we can no longer send a Ka band uplink to the spacecraft becasue the Alenia built receiver broke. See Paragraph 10 here

Re:RTFM is the fix?

[twiddlingbits]

NASA Has an Independant Verification and Validation Center to provide technical oversight. I worked there (it's in West F'ing Virgina of all places). The folks there do a great job with what data they are given. Often to save costs, this IV&V team is not even allowed to participate in the Design Reviews, and when they are and discover issue the Project Offices sweep them under the rug. No use admitting to problems that might show someone is not thinking correctly or is not managing the project well. The theory is "Let's avoid the problems by witholding information and communication from anyone who might find a problem". Solid testing? Thats a joke too, it costs money to test things well, and who knows they might break!. Contractors will make mistakes, after all they employ humans, but the mistakes can be corrected BEFORE flight if they are found. Having an extra set of eyes, and doing extensive testing is valuable but costly. In FACT having IV&V on manned systems is the LAW since the Challenger disaster, it's just commonly disregarded at NASA for anything but ISS. Even STS has no IV&V, after all it's a "mature" system and there are no bugs left. Regardless of what you hear about NASA "changing" after Columbia it really isn't. I fully expect another STS disaster, and several more mini-disasters in unmanned systems in the near future.