Chris Phillips (ATNF) on behalf of the Huygens team

On 14 January 2005 the European Space Agency (ESA) Huygens probe descended on to the surface of Saturn's moon, Titan. During the descent and for the three hours that the probe continued transmitting after it landed on the surface, the ATNF radio telescopes - Parkes, the Compact Array and Mopra, along with the University of Tasmania's telescopes in Hobart and Ceduna and 12 other telescopes in the USA, China and Japan were used to track the trajectory of the descent using the technique of Very Long Baseline Interferometry (VLBI).

As the probe parachuted to the surface of Titan, the data gathered by its on-board science packages was transmitted back to the Cassini spacecraft which stored the data and then re-transmitted it back to Earth after the descent. The ground based observations consisted of two experiments which eavesdropped on this transmission from Huygens.

The Doppler Wind Experiment (DWE) used ultra-precise spectrum analysers at both Parkes and the Greenbank telescope (in the USA) to measure the Doppler shift of the carrier signal. This allowed the line-of-site velocity of the carrier signal to be measured. This was to complement a similar onboard instrument allowing the velocity to be measured in two directions.

The second part of the ground based observations was a coordinated Very Long Baseline Interferometry experiment, coordinated by the Joint Institute for VLBI in Europe (JIVE), to accurately measure the location in the sky of the probe during its descent. Simulations by JIVE had shown that the position of the probe can be measured to an accuracy better than one kilometre every minute and the velocity measured to better than one metre per second. Combined with the DWE, the full three-dimensional trajectory of the descent could be reconstructed.

Because the signal from the probe was so weak, and the VLBI data needed to be processed using specially designed software, the standard tape based S2 recording systems were not suitable. Instead newly commissioned "LBADR" (LBA Data Recorder) systems were used which record the raw VLBI data directly onto computer hard disk drives, at a data rate four times faster than that achievable with the S2 recorders (512 Mbps, enough to fill a CD every 10 seconds).

On the night of the experiment, the signal was first picked up by Greenbank confirming that the parachute had successfully deployed and the probe was transmitting. At 11:29 ADST, when Titan became visible at Parkes, the signal could immediately be detected. Sixteen minutes later Parkes could see that the probe had landed on the surface based on the characteristic of the changed Doppler shift. Three and a half hours later, when Titan was no longer visible at Parkes, the probe was still transmitting strongly. The probe had been expected to only survive a few minutes on the cold surface.

Because the participating VLBI telescopes were not connected by network links with enough bandwidth required for real-time correlation of the data, it was impossible to know if the observations were successful until the data were shipped to the central VLBI correlator. However, the team at JIVE was keen to determine the success of the experiment quickly, so it was decided to get the data from the ATNF telescopes to JIVE, in the Netherlands, as quickly as possible. At the end of the experiment, at 3:30 am local time, a twin-engine Cessna 310R was chartered to bring back the Compact Array, Mopra and Parkes data disks to Sydney. To facilitate a speedy transfer of data, nine research and education networks collaborated to give a dedicated door-to-door gigabit network connection from Sydney to JIVE in the Netherlands. The organisations involved included CeNTIE and AARNet within Australia, Pacific North West in the USA, CANARIE in Canada, and SURFnet in Europe. This allowed a subset of the recorded Parkes and Mopra data to be transmitted to JIVE to process on their VLBI correlator. After a brief period translating the data format, JIVE was able to confirm fringes on the calibrator sources between Parkes and Mopra.

The ground based observations were originally envisaged to complement the Doppler Wind Experiment on Cassini. However one of the two receiving channels on Cassini was misconfigured, meaning all the spacecraft Doppler measurements were lost. The DWE data from Parkes and Greenbank can replace the Cassini data, but there was a 20-min gap where Titan was below the horizon at both Parkes and Greenbank. It now looks like JIVE will be able to use the VLBI data from the smaller telescopes to bridge this gap and allow Doppler measurements for the entire descent. Processing of the VLBI experiment is ongoing.

Figure 1: A scan-averaged power spectrum showing the Huygens carrier signal detected in the Parkes VLBI data.

Figure 2: A tired-looking Tasso Tzioumis handing over Mopra observatory data disks to Chris Phillips at Coonabrrabran airport in the early hours of the morning. Photo: Liz Cutts