News: Radio Astronomers Rescue Science Results for Huygens' Doppler Wind Experiment

Earth's radio astronomers have saved the day for one of the Huygens instrument teams. Today, the Doppler Wind Experiment (DWE) team announced their first science results, despite losing nearly all of their expected data. "We were not able to receive our Doppler Wind data on the channel from Huygens that we wanted," says Doppler Wind Experiment Principal Investigator Mike Bird, "but we were able to receive that radio signal at many stations on Earth, and we now have a profile of the winds on Titan."

The Doppler Wind Experiment has confirmed that Titan is a "super-rotator," meaning that its atmosphere rotates from west to east faster than the planet does. Bird explained that this is the expected situation on slowly rotating planets having atmospheres. Another such planet, Venus, is also a super-rotator, and evidence from ground-based and Cassini observations of the motion of Titan's clouds had already indicated that Titan was a super-rotator. In fact, throughout the zonal wind profile "the winds are basically what we expected," Bird says. "Particularly in the latter 60 kilometers (37 miles) of the descent, the winds are a little bit boring because they're just almost exactly what we expected."

The Doppler Wind Experiment has confirmed that Titan is a "super-rotator," meaning that its atmosphere rotates from west to east faster than the planet does. Bird explained that this is the expected situation on slowly rotating planets having atmospheres. Another such planet, Venus, is also a super-rotator, and evidence from ground-based and Cassini observations of the motion of Titan's clouds had already indicated that Titan was a super-rotator. In fact, throughout the zonal wind profile "the winds are basically what we expected," Bird says. "Particularly in the latter 60 kilometers (37 miles) of the descent, the winds are a little bit boring because they're just almost exactly what we expected."

But, Bird added, "There are some surprises in this profile. No one was really expecting anything other than a fairly simple linear increase [of wind speed] with altitude up to the point we started the mission. I'm glad we didn't measure just that, because you always like to have a little fun with these measurements, and I think we have enough to work with here, something for the theoreticians to think about."

What the Doppler Wind Experiment team found was that above an altitude of 60 kilometers (37 miles), Huygens' ride through the atmosphere was a lot bumpier than theoreticians had predicted, a fact that had already been observed in data from the other instrument teams. "The atmosphere of Titan was a bit unkind to Huygens because it was really quite turbulent," Huygens Project Scientist Jean-Pierre Lebreton says. "We could clearly see in the engineering data that there was a lot of swinging under the main parachute and a lot of turbulent motion of the probe, especially in the lower stratosphere. It suddenly became quieter when we went through the tropopause."

The tropopause is the boundary between Titan's troposphere and stratosphere. At that altitude, the Doppler Wind Experiment team observed large variations in their measurements, indicating a region of "very severe fluctuations in the wind velocity," according to Bird. Such rough winds are not present at the tropopause in Earth's atmosphere, so its origin is a puzzle to the science team.

The unexpectedly rough ride has caused serious difficulties for the other science teams, particularly for Huygens' cameras, the Descent Imager / Spectral Radiometer. The cameras depended upon a Sun sensor to determine which direction the spinning probe was facing when it took pictures. But "the probe was tipping so violently that the Sun would go out of the field of view," says camera co-investigator Lyn Doose.

The Descent Imager / Spectral Radiometer had three cameras, one pointed to the side, one at a roughly 45-degree angle downward, and one straight down. At one point in the descent, Doose reports, the camera pointing downward at a 45-degree angle actually had the Sun in its field of view! "The Sun was at 50 degrees elevation, so [Huygens] was turned 90 degrees, I think. It was unexpected. We were given specifications that said there may be a gust that would tip you more than 10 degrees a few times during the mission, but typically you should expect to be hanging vertically by 10 degrees or less. Nobody could have predicted this."

DWE Lost and Regained

That the Doppler Wind Experiment team can report results today is more amazing than the results themselves, because they come despite the devastating loss of most of their data. To understand what happened, we must take a look at how the Doppler Wind Experiment was designed.

A carrier signal is a continuous signal of a fixed frequency. The signal can be modulated in either amplitude or frequency to carry data. For example, a radio station at 89.3 FM broadcasts a carrier signal at 89.3 MHz, and then modulates the frequency of that signal minutely to transmit data that can be converted by a radio receiver into the sounds of voices and music. The carrier signal from Huygens was broadcast at 2040 MHz, a region of the electromagnetic spectrum known as S-band radio.

The goal of the Doppler Wind experiment was to track the horizontal motion of the Huygens probe as it descended through the atmosphere. It worked by observing the Doppler shift of the carrier signal transmitted from Huygens to Cassini. As Huygens traveled away from Cassini, the Doppler effect caused the frequency of her carrier signal to decrease, or "shift." As Huygens' motion carried the probe lower in Titan's atmosphere, winds of greater or lesser intensity caused the Doppler shift of Huygens' signal to increase and decrease. The Doppler shift of Huygens' signal over the course of the mission therefore provide a direct measurement of the velocity of the Huygens probe along the line of sight from the probe to the orbiter.

If there were no winds on Titan, Huygens would have descended straight down onto Titan under its parachutes. The velocity of the probe as measured by the orbiter under this circumstance can be modeled quite accurately. By examining how the measured velocity of the probe at different altitudes (measurements of the probe's altitude being provided by other instruments), the science team could determine how the actual winds on Titan made Huygens' descent diverge from the vertical.

In order to measure minute shifts in the frequency of Huygens' carrier signal, the Doppler Wind Experiment team needed to be assured that the signal from Huygens was extremely stable, never varying from its nominal 2040 Megahertz. Radio transmitters and receivers employ a device called an "oscillator" to generate and observe the carrier frequency. For the purposes of the Doppler Wind Experiment, the usual quartz oscillators would not be stable enough, especially under the extreme conditions of Huygens' deceleration in the first part of its descent. So the team designed and built Ultrastable Oscillators that contained a rubidium oscillator, the first time such oscillators were included on a deep space mission.

Both Huygens and Cassini were fitted with Ultrastable Oscillators provided by the Doppler Wind Experiment team in order to generate and receive Huygens' carrier signal. Huygens and Cassini had two redundant radio systems, one of which, "Channel A," had a signal provided by the Ultrastable Oscillators, and the other, "Channel B," had a signal provided by the standard quartz oscillators. Most of Huygens' instruments used both channels to transmit data to Cassini, but the Doppler Wind experiment depended entirely upon Channel A.

Unlike winds on Earth, Titanian winds are primarily “zonal,” traveling straight from east to west. With only one line-of-sight measurement, the science team would assume that all of the measured velocity was in the east-west direction. A second line-of-sight measurement would provide the second vector necessary to detect whether the winds also had a “meridional,” or north-south component.

There was some redundancy in the Doppler Wind Experiment, however. Huygens broadcast signals for the experiment only on Channel A, but those signals could be detected in two places. Cassini was the primary detector. The team also hoped that the weak signal from Huygens could be detected on Earth using some of the Earth's largest radio telescopes. Many team members were not confident that this detection would be possible. In fact, Bird says he had placed a 10-Euro bet with a colleague that it would not work (presumably figuring that either way, he would be a winner). If the direct-to-Earth measurement worked, it would allow the science team to determine not only the east-west component of Titan's wind speeds, but also the north-south component. This direct-to-Earth measurement was considered a bonus, not required for the success of the experiment, but having great potential value to add if it worked.

When Cassini transmitted the data from Huygens to Earth on the evening of January 14, no data came through on Channel A. Overnight, it became clear to the horrified mission operators that Cassini had never been instructed to turn on the Receiver Ultrastable Oscillator on Channel A. The entire Channel A data stream was lost, and with it the data for the Doppler Wind Experiment (as well as half of the images from the Descent Imager).

But the Earth's radio astronomers saved the day. Huygens' signal was detected loud and clear by at least 17 radio telescopes, Bird lost his 10-Euro bet, and he gained a priceless data set. Still, the Doppler tracking that was performed by the Earth-based telescopes only gives information about the east-west wind speeds. The second horizontal component of the wind speeds would have been impossible to determine with just the Doppler data. But recent improvements in Very Long Baseline Interferometry will eventually permit radio scientists to produce a three-dimensional record of motion for Huygens during its entire descent. For a blow-by-blow account of the loss, and recovery, of the Doppler Wind Experiment by the Earth's radio scientists, read the related story: "They Were the First, and the Last, to Hear from Huygens."

What's So Important About the Wind Profile, Anyway?

First of all, knowledge of the wind speeds on Titan at all altitudes is a critically important ingredient in understanding the structure and dynamics of Titan's atmosphere, and therefore its weather and climate. "The theoreticians were very anxious to get this example of Titan, which is an intermediate case between Venus and Earth, and of course they have a lot of pet theories that they want to test," says Bird. Based on the images returned from the Descent Imager, surface processes near the Huygens landing site are dominated -- as they are on Earth -- by weather from the atmosphere, not by cratering or volcanism. So understanding Titan's atmosphere is key to understanding its surface and recent geological history.

Another useful application of the Doppler Wind Experiment data is to figure out exactly where Huygens landed. Although maps showing a location for Huygens' landing site have been published, Cassini-Huygens scientists privately express doubt that the dot marks the right spot. The three-dimensional profile of Huygens' motion that will eventually be produced from the Very Long Baseline Interferometry observations should allow scientists to pinpoint exactly where Huygens landed by tracing its path from its entry point to the surface. Preliminary calculations from the zonal wind profile released today suggest that the landing site "is going to be a good 100 or 150 kilometers [60 to 90 miles] from where we went in," Bird says. "We dropped 150 kilometers, and I have a feeling that we drifted about that same amount in the easterly direction."

For Bird, though, the real result from the Doppler Wind Experiment was "just to see if we could do it! And we did. It's kind of remarkable that we can use this technique to determine the velocities of circulation and atmospheres as far away as Titan. And now we're doing it with less data than we had originally planned to have, and it is really a remarkable feat to be able to get all of this with just the data that we have from the radio telescopes on the ground."

Bird is philosophical about the loss of the Channel A data. "I've never felt such exhilarating highs and dispiriting lows than those experienced when we first detected the signal from the Green Bank Telescope, indicating 'all's well,' and then discovering that we had no signal at the operations center, indicating 'all's lost.' I have to say that I feel better now than I did just after the mission. It's behind us, and I've pretty much decided that I'm just going to let it ride, because there is nothing I can do about it now."