Last June, scientists were thrilled when NASA's Cassini probe successfully began orbiting Saturn after a 3.5-billion-kilometer, seven-year journey across the solar system. The 6-ton spacecraft immediately started returning spectacular pictures of the planet, its rings, and its 30-plus moons. It was just the beginning of Cassini's four-year tour of Saturn's neighborhood, and while scientists expect amazing discoveries in the years to come, the most dramatic chapter in the mission's history will happen this January, when scientists attempt to peek beneath the atmospheric veil that surrounds Saturn's largest moon, Titan--a chapter that might have ended in disaster, save for one persistent engineer.
In 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. A stubby cone 3 meters across, Huygens was built for a single purpose: to pierce the cloaking methane atmosphere of Titan and report its findings back to Cassini for relay to Earth.
So it was quite a shock when Boris Smeds, a graying, Swedish, 26-year ESA veteran [see photo, ]., who normally specializes in solving problems related to the agency's network of ground stations, discovered in early 2000 that Cassini's receiver was in danger of scrambling Huygens's data beyond recognition.
Making that discovery would lead Smeds from his desk in Darmstadt, Germany, to an antenna farm deep in California's Mojave Desert, after he and his allies battled bureaucracy and disbelief to push through a test program tough enough to reveal the existence of Cassini-Huygens's communications problem. In doing so, Smeds continued a glorious engineering tradition of rescuing deep-space missions from doom with sheer persistence, insight, and lots of improvisation.
Larger than the planet Mercury, Titan appeared to the Voyager probes in the 1980s as a mysterious yellow-orange globe, its surface hidden by its soupy methane atmosphere [see image, ]. Cassini is equipped to peer through those clouds with special camera filters and radar, but really getting up close and personal with this enigmatic world is the job of ESA's Huygens.
Launched from Cassini, Huygens will soon slam into Titan's atmosphere at 21 000 kilometers per hour and begin a one-way, two-and-a-half hour descent to the surface, slowed by parachutes. The lander is fitted with cameras pointing down and sideways, instruments designed to unlock the atmosphere's chemical secrets, and a microphone to pick up wind sounds. Investigators have speculated there might be seas of liquid methane and ethane on Titan, so Huygens has been designed to float. Although its batteries will be nearly exhausted by the time it finally reaches the surface, researchers hope it will be able to make a few measurements of the physical composition of the landing site [see illustrations, ] and ].
Scientists believe the information gathered during the descent will open not only a window onto a mysterious world at the far end of the solar system but one onto the past as well, since Titan's atmosphere is believed to be similar to that of the primordial Earth.
Getting Huygens's once-in-a-lifetime readings and observations back to Earth is a two-stage process. Huygens is too small to be equipped with a radio transmitter powerful enough to reach Earth, so instead a receiver onboard Cassini will pick up Huygens's transmissions. With its powerful 4-meter main antenna, Cassini will then relay the data back to a small army of researchers, some of whom have been waiting decades for the insights they hope Huygens will provide.
When the Cassini-Huygens mission blasted off from Cape Canaveral in October 1997, no one suspected that a critical design flaw was lurking deep within the telemetry system onboard Cassini that was dedicated to harvesting Huygens's broadcast. Uncorrected, the flaw meant the data flowing from the hardy lander was in danger of being hopelessly scrambled, its seven-year odyssey across the solar system in vain.
"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."
But back in 1998, as Cassini was swinging past Venus and the Earth to build up speed for its run out to Saturn, Zarnecki and the other scientists and engineers at ESA and NASA were still blissfully unaware of any problem.
In fact, everything was working fine. The mission builders felt confident in their work: both the Cassini orbiter and the Huygens lander had been extensively tested on the ground, both separately and together. However, a proposal for a so-called full-up high-fidelity test of the radio link between the probes (where every system is subjected to a simulation of the exact signals and conditions it will experience during flight) had been rejected because it would have required disassembly of some of the communications components.
"Budget was a key part" of this decision, explained Robert Mitchell, program manager for the Cassini-Huygens Mission at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. The reassembled spacecraft would then have had to undergo exhaustive and expensive recertification. In hindsight, these testing failures were embarrassing. "We had three safety nets set up to catch things like Cassini-Huygens's communications problem," said John Credland, head of ESA's scientific projects, "and it now appears that we fell through all three."
Fortunately, Claudio Sollazzo, Huygens's ground operations manager at ESA's European Space Operation Centre (ESOC) in Darmstadt, Germany, had a nagging worry about the lack of a full-up communications systems test. Sollazzo knew there was time to run some tests during Cassini's long, uneventful stretches between the planets. So he approached Smeds in January 1998 with an unusual request: design a test to send a signal from Earth toward Cassini that would mimic a radio transmission from Huygens during its landing.
Smeds normally works on the communications links between ESA's global ground antenna network and its 11 active science spacecraft. Most of these are satellites that never stray more than tens of thousands of kilometers from Earth, a far cry from the Huygens probe, which was designed to plunge into an atmosphere 1.2 billion km away. But Smeds's experience with ground antennas was just what Sollazzo needed.
It was impossible to test the Huygens-to-Cassini radio link during the cruise using the spacecraft themselves: they were firmly mated together, communicating not by radio but via a cable. And even if Huygens could be made to transmit to Cassini, successfully sending a radio signal a few centimeters would hardly inspire confidence for the difficult Titan descent.
The Cassini-Huygens mission plan had Cassini jettisoning Huygens toward Titan before Cassini began a low-altitude, high-velocity fly-by of the mysterious moon. Huygens would reach Titan well in advance of Cassini, and as Cassini streaked along at some 21 000 km/h relative to Titan, Huygens would be descending on parachutes through the moon's soupy atmosphere at a comparatively leisurely 18 to 22 km/h. The relative velocity of Huygens to Cassini was expected to be about 5.5 kilometers per second, increasing the frequency of Huygens's transmitter by about 38 kilohertz as seen by Cassini because of Doppler shift.
If you've ever heard a screaming ambulance or whistling train pass by, you're familiar with Doppler shift. When an acoustic or radio wave is emitted by a moving object, an observer in front of the object will notice an increase in the wave's frequency as the wave's peaks and troughs are compressed by the object's motion, and an observer behind it will notice a decrease in the wave's frequency as the wave is stretched--hence the familiar rise and fall in the pitch of an ambulance's siren as the vehicle speeds by.
In the case of Huygens, its signal will vary not only in frequency but also in strength as the probe is buffeted by the atmosphere, changing the orientation of its transmitter. When Smeds was brought into the picture, the plan to test Cassini's receivers was to transmit a signal from Earth that would duplicate Huygens's carrier signal without modulating it with any simulated telemetry from the lander's instruments. If the Cassini receiver could pick up a fluctuating, Doppler-shifted carrier wave, all should be well. But Smeds wanted to do better. "If I do a test like this, I want to do it properly and simulate everything, not just a part of it," he told IEEE Spectrum.
Smeds used ESOC's engineering model of Huygens--an exact duplicate of the lander down to the last bolt and transistor--to generate a stream of typical telemetry. Then he developed a test signal pattern on his office computer that could modulate a carrier wave with telemetry as Huygens would. His plan was to broadcast the simulated Huygens telemetry from Earth to Cassini and have Cassini echo what it received back to Earth.
Huygens is designed to generate telemetry at a rate of 8192 bits per second. Using a common modulation technique known as binary phase-shift keying, Huygens's transmission system represents 1s and 0s by varying the phase of the outgoing carrier wave. Recovering these bits requires precise timing: in simple terms, Cassini's receiver is designed to break the incoming signal into 8192 chunks every second. It determines the phase of each chunk compared with an unmodulated wave and outputs a 0 or a 1 accordingly [see chart, ].
Smeds's scheme required that his test signal pattern be broadcast from Earth in a sequence of varying power levels to simulate the effect of Huygens and its transmitter's being swung around in Titan's atmosphere. The test signal's frequency would also be adjusted at broadcast so that when it arrived at Cassini, it would match the Doppler-shifted signal expected from Huygens. The echoed signal could then be decoded and verified by matching it against the original telemetry used to create the test signal.
In proposing this more complex test with simulated telemetry, Smeds "had to argue with those who didn't think it was necessary," recalled JPL's Mitchell. Smeds was persistent and continued championing the test even after it was initially rejected. In the end, with the backing of Sollazzo and Huygens's project scientist, Jean-Pierre Lebreton, Smeds's plan was accepted because it was easy to do, even though hardly anybody seemed to think it was worth doing. On such seeming trivia US $300 million missions can turn: the simpler carrier-signal-only test, Mitchell noted, would never have uncovered any problems.
So it was that in early February 2000, a jet-lagged Smeds found himself sitting in a windowless, fluorescent-lit, concrete basement below one of NASA's Deep Space Network (DSN) 34-meter dish antennas in Goldstone, Calif. [see photo, ]. He had been scheduled for two test sequences during consecutive days, when Cassini would be above the horizon and in view of the dish. The test signal Smeds had devised on his office computer was loaded into Goldstone's signal-processing center, located at the far end of the sprawling Mojave Desert complex, which would adjust the frequency to simulate Huygens's Doppler shift.
Smeds and a DSN technician couldn't stay in the relative comfort of the processing center. They had to be present there in the bowels with the noisy signal generators to adjust the power of the outgoing transmission during the test. Smeds and the technician set up shop, ready to swap in and out a series of laptop-controlled attenuators to simulate the signal strength fluctuations that were expected from Huygens.
When Cassini appeared over the horizon, the test sequence began. Smeds's test signal was transmitted to Cassini at a given power level for 5 minutes at a time before moving on to another power level. Cassini was now so far from Earth--430 million km away, somewhere in the asteroid belt--that it took 48 minutes for the signal to reach the probe and be relayed back to Goldstone. The signal from Cassini was then sent to ESOC in Darmstadt for decoding and verification; the center kept in touch with Smeds during the test by fax and phone.
Soon, it became obvious that something was very wrong. Darmstadt reported that it was picking up the carrier signal, but none of the simulated telemetry was coming through. The data in the decoded signal was a mess. As Smeds worked through his test sequence, the situation grew even more puzzling, as Darmstadt would occasionally get short bursts of good data. "Specific things were very confusing. When you increase the power, you expect the signal to get better. Initially it did, but then when I increased the power even more, the data was corrupted again," Smeds told Spectrum.
After the day's test sequence, Smeds kept thinking about the scrambled data during the hour-long drive back to Barstow, Calif., the nearest town to Goldstone with a motel. He started to get a hunch that the problem didn't have anything to do with signal strength but with Doppler shift. He was running out of time, however, to test theories--he had only a few hours the next day at Goldstone before the communications pass would be over. It would be months before another test could be arranged, because other investigators were in line to communicate with their equipment onboard Cassini.
Smeds decided to carve out some more time for himself. The next day he cut each step in the official test sequence from 5 minutes to 2, allowing him to finish early.
Now he could act on his intuition. He called up Goldstone's signal-processing center and had it reduce the simulated Doppler shift of the signal reaching Cassini to zero. Forty-eight minutes later--light speed to the asteroid belt and back--Smeds's hunch paid off. "Suddenly I got better results. I knew then that there was something wrong in the data-detection system and that it was sensitive to Doppler shift," said Smeds.
Even with the test results in hand, Smeds was greeted with some skepticism on his return to Darmstadt. "Some people didn't believe me," he chuckles. They thought that "something was wrong with the test setup. But I had the engineering model, and I continued doing tests on the ground and doing more investigations. I could demonstrate the effect of the Doppler shift and the effect it had on the data reception."
By September 2000, Smeds and his allies had managed to convince ESA that the problem was real and that it was time to tell NASA. "Without Smeds, we wouldn't have known we had a problem," says JPL's Mitchell. Adds Zarnecki, "The guys who pushed the original test through are heroes."
But what had gone wrong
ESA immediately convened an inquiry board, with two NASA observers. One of them was Richard Horttor, who was then JPL's telecommunications system engineer for the Cassini project. He recalls, "We worked our way out by being totally candid from top to bottom once we detected the problem. There was no hesitancy or lack of resources. Nor was there any 'nation-to-nation finger-pointing.' "
The board discovered that Alenia Spazio SpA, the Rome-based company that built the radio link, had properly anticipated the need to make the receiver sensitive over a wide enough range of frequencies to detect Huygens's carrier signal even when Doppler shifted. But it had overlooked another subtle consequence: Doppler shift would affect not just the frequency of the carrier wave that the probe's vital observations would be transmitted on but also the digitally encoded signal itself. In effect, the shift would push the signal out of synch with the timing scheme used to recover data from the phase-modulated carrier.
Because of Doppler shift, the frequency at which bits would be arriving from Huygens would be significantly different from the nominal data rate of 8192 bits per second. As the radio wave from the lander was compressed by Doppler shift, the data rate would increase as the length of each bit was reduced.
Although the receiver's decoder could accommodate small shifts in the received data rate, it was completely out of its league here. The incoming signal was doomed to be chopped up into chunks that didn't correspond to the actual data being sent, and as a result the signal decoder would produce a stream of binary junk. The situation would be like trying to watch a scrambled TV channel--the TV's tuned in fine, but you still can't make out the picture.
Alenia Spazio wasn't alone in missing the impact Doppler shift would have on the decoder. All the design reviews of the communications link, including those conducted with NASA participation, also failed to notice the error that would threaten to turn Huygens's moment of glory into an embarrassing failure.
Alenia Spazio's insistence on confidentiality may have played a role in this oversight. NASA reviewers were never given the specs of the receiver. As JPL's Mitchell explained to Spectrum, "Alenia Spazio considered JPL to be a competitor and treated the radio design as proprietary data." JPL's Horttor admitted that NASA probably could have insisted on seeing the design if it had agreed to sign standard nondisclosure agreements, but NASA didn't consider the effort worthwhile, automatically assuming Alenia Spazio would compensate for the changing data rate.
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.)
Frustratingly, engineers discovered that the timing scheme was implemented by firmware loaded in Cassini's receiver; a trivial change to some operating parameters would have fixed Cassini's comprehension problem. But the firmware could not be altered after launch.
Now, the question remained: how to save Huygens's mission
From a variety of proposed fixes, 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, Cassini will be much farther from Titan when Huygens enters its atmosphere. As a result of this geometrical rearrangement, the probe's major deceleration component will be perpendicular to the Huygens-Cassini line of sight rather than mostly along it. This simple change literally sidesteps the Doppler shift problem, as the radio waves coming out perpendicular to Huygens's direction of motion will be neither stretched nor compressed.
By the time NASA and ESA realized a rearrangement was needed, interplanetary navigation experts had already laboriously developed Cassini's multiyear flight plan to maximize the number of visits to Saturn's moons. There were to be 44 close fly-by passes of Titan, 8 close passes of smaller moons, and between 50 and 100 more distant passes of these other moons. Reconstructing this celestial ballet from scratch would have been prohibitively expensive.
So the navigators designed a trajectory in which Cassini initially enters a lower and faster orbit around Saturn, drops off Huygens, and then hits a specific point in space that coincides with a point on the previously planned path. There Cassini fires its rocket engine again to get back on the original course. During this altered period, it will make three orbits of Saturn instead of the original two, but the extra rocket fuel needed to make the changes is available because Cassini's navigation has been so precise that a lot of fuel allocated to course corrections has not been used.
The upshot of this maneuvering is that instead of landing on Titan in November 2004, Huygens will now be deployed on 24 December 2004 for a 14 January 2005 landing. The lander still faces enormous engineering challenges as it ventures into the unknown conditions of Titan's atmosphere and surface. But at least now it has a fighting chance to transmit its findings back to Earth.
As for Smeds, ESA's staff association awarded him and some of his colleagues a plaque and a small cash prize for their role in saving the $300 million mission, though Smeds told Spectrum that he is still looking forward to his real reward: "I hope to sit in Darmstadt and see the data coming in on the screen in January."
About the author
James Oberg is a 22-year veteran of NASA mission control. He is nowa writer and consultant in Houston. His last article for IEEE Spectrum was in August, about the first private suborbitalspacecraft, SpaceshipOne.
Stephen Cass contributed additional reporting for this article.
To Probe Further
For more information on the Cassini-Huygens Mission, go to http://saturn.jpl.nasa.gov/home/index.cfm.