Parkes tracking of the Huygens probe

As well as playing a major role in the VLBI tracking of the Huygens probe, Parkes was involved in a second complementary experiment, the "Doppler Wind Experiment" (DWE), coordinated by the Jet Propulsion Laboratory (JPL, USA). This experiment involved using the Greenbank (USA) and Parkes radio telescopes to detect the Doppler shift of the Huygens 2040-MHz carrier signal in real-time. From the Doppler-shift measurements the line-of-sight velocity of the probe could be determined. The data from the DWE and VLBI tracking will be combined to allow a more comprehensive understanding of the direction and strength of the zonal winds of Titan's atmosphere.

For the first part of the descent, the Huygens probe was visible from Greenbank while the second part was visible at Parkes. However, twenty minutes before the probe was scheduled to land, it set at Greenbank. At Parkes, the expected landing time of the probe was just four minutes after it was scheduled to rise above the local horizon.

Inside the Parkes control room the team set about calmly preparing for the track. Dion Lewis, checked and double checked the VLBI recording equipment and cleared lots of disk space for the expected flood of data. John Reynolds performed numerous focus and pointing checks of the receiver, fine-tuning the system to get the strongest possible signal. Jim Border and Doug Johnston, engineers from JPL, set up their equipment and established a continuous telephone link to JPL's mission control in Pasadena.

When Huygens entered the atmosphere of Titan, the giant Greenbank telescope in West Virginia, USA, was poised to detect the signal when the transmitters sprang to life. Right on schedule at 9:18 pm (AEDT), Greenbank reported detecting the signal. A quiet cheer went up in the Parkes control room — we knew we had a mission.

Both Greenbank and Parkes were equipped with spectrum analysers that allowed them to see the signal as a small spike in the pass-band of the receiver. It was this spike that Greenbank reported seeing. At Parkes, Doug Johnston had the capability to further process the data to produce plots of the carrier's Doppler-shift variations. The radio science receivers at Greenbank and Parkes were capable of measuring the Doppler shift of the signal in real-time. Doug could compare these to the predicted Doppler shifts based on a smooth atmospheric descent model. Any variation from these predictions was an indication of winds in the atmosphere of Titan. Within just three minutes of the initial signal reception, Doug transferred the Greenbank data over the JPL network and processed it at Parkes. The difference between the real and predicted Doppler shifts was plotted. This showed that the probe was deviating significantly from the expected descent profile. At first the detected signal was just 35 Hz off the predicted value, but as it descended further, the deviations increased and fluctuated. The winds on Titan were much stronger than expected. It was an amazing feeling to realise that we were the first people ever to "see" the winds of Titan.

At 9:32 pm Doug reported seeing the glitch in the Doppler shift that indicated the main parachute had deployed on schedule.

Meanwhile, the tension at Parkes was quietly rising as the time slowly approached for Parkes to take over. One hour before the beginning of the track, John Reynolds switched over to the generator to prevent an unforseen power loss from disrupting the track. At 11:12 pm the Parkes dish was slewed over to the horizon to wait for Titan to rise. At the same time the probe set at Greenbank.

As the minutes ticked by, we all gathered in front of Doug's console waiting for the beginning of the track and confirmation that we were receiving the signal. Right on schedule at 11:29 pm the dish began tracking and the signal appeared on the spectrum analyser. The signal was 4 dB or 2.5 times stronger than expected.

Doug had been viewing the plot of the Doppler shift variations, which only showed glitches when it departed from the predictions. Jim Border, decided to plot the sky frequency, that is, the actual frequency received. Sure enough, there was a large glitch at the suspected landing time of 11:45 pm. This confirmed the landing of the probe on Titan. Shortly after midnight, Jim and Doug alerted JPL and ESOC and the word quickly spread around the world that the probe had landed. Cheering erupted in the control room and congratulations were exchanged. The landing had been a much softer touchdown than expected and occurred sometime between 11:45 and 11:46 pm, 12 or 13 minutes later than expected. It was a second moon landing for Parkes.

The dish continued to perform flawlessly throughout the track until at 2:56 am Huygens finally set at Parkes, still transmitting strongly. The champagne was duly popped open in celebration. For this moon landing, the high winds were thankfully on Titan and not at Parkes.

For photographs of the event see: www.parkes.atnf.csiro.au/events/huygens_track/

John M Sarkissian, Operations Scientist
(John.Sarkissian@csiro.au)