Pinpointing Huygens: VLBI observations of the probe's descent

On 14 January 2005 the European Space Agency (ESA) Huygens probe descended on to the surface on 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 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 VLBI experiment, which was coordinated by the Joint Institute for VLBI in Europe (JIVE), directly observed the carrier signal of the transmission from Huygens to Cassini. The aim of the VLBI experiment was not to try and decode the transmitted data, as the received signal strength on earth was too weak to allow this even with the largest telescopes, but to pinpoint the exact location of the probe during its descent. Simulations by JIVE have shown that the position of the probe can be measured to an accuracy better than 1 km every minute and the velocity measured to better than 1 metre per second. Combined with measurements of the Dopper shift of the carrier signal (see the separate report on the Doppler Wind Experiment), this will allow the full three-dimensional trajectory of the descent to be re-constructed. As Titan has an appreciable atmosphere the probe's parachute was expected to be caught by these winds and the VLBI observations would allow a direct measurement of the wind speeds through a cross-section of the atmosphere. Because the signal was so weak the experiment depended crucially on having at least one large telescope in the array. At the start of the experiment this role was filled by the 100-m Green Bank Telescope (GBT) in the USA, but it could not track the probe for the entire experiment. A few minutes after Titan set at the GBT, it rose above the elevation limit at Parkes which tracked it for the rest of the experiment.

The basic observing mode used a standard phase referencing technique where the telescopes nodded between the Huygens probe and a near-by background radio galaxy. However, the specific observing set-up required significant changes and upgrades to accommodate the experiment. Because the processing of the Huygens signal will be done

using a software correlator (the required spectral resolution is around 1 Hz, hundreds of time higher than is achievable by normal VLBI correlators), the recording had to be made using disk-based recorders rather than the standard tape-based S2 system. Thanks to recent work by the University of Swinburne and the University of Tasmania, all Australian VLBI antennas were already equipped with appropriate disk-based recorders. The background radio galaxy is weak but a strong detection was required to provide the desired high-precision position measurements. This was done by recording as large a bandwidth as practical, a total of 128 MHz (a data rate of 512 Mbps, enough to fill a CD every 10 seconds) which was twice that possible with pre-existing hardware. A new Data Acquisition System (DAS) had to be assembled and others moved between telescopes. A further complication was that the frequency of the carrier, 2040 MHz, is non-standard and out of range of standard receivers. Some creative engineering was needed to extend the range of the receivers to this frequency. Unfortunately, the frequency was too low for the Compact Array antennas to be modified so the Compact Array could not be used to track the probe. However, the array was included in the VLBI experiment to observe the calibrator sources which were also observed at frequencies accessible to the Compact Array.

Because of the non-standard nature of this experiment and the one-shot nature of the observation, a number of practice observations were made before the event to iron out any problems that could occur. The first test did not look good for Mopra as a hard-disk failure on the VAX one hour before the experiment meant observations there had to be cancelled. Things worked much better for the second test in November and in January we were confident everything was working well.

On the night of the experiment, the signal was first picked up by the GBT confirming that the parachute was successfully deployed and the probe was transmitting. At 11:29 AEDT, when Titan became visible at Parkes, the signal could immediately be detected. Sixteen minutes later Parkes could easily see that the probe had landed on the surface based on the characteristic of the changed Doppler shift. It was not known how long the probe would last on the surface before the batteries went flat or froze. The experiment was scheduled to last almost four hours after the landing, which was considered extremely optimistic (a few minutes to an hour was considered more likely). At 2:56 am when Titan set at Parkes the probe was still transmitting strongly. This triggered a series of frantic emails to VLBI antennas in Europe trying to arrange an ad-hoc VLBI observation with no preparation time!

Because VLBI telescopes are not connected by network links with enough bandwidth required for real-time correlation of the data, in general we cannot know until the data are shipped to the central VLBI correlator if the observations are successful. However, the team at JIVE was keen to know 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. To allow this, at the end of the experiment at 3:30 am, Chris grabbed the disks from the Compact Array recorder and jumped in a taxi to Narrabri airport to board a waiting charter plane (Figure 1).

Figure 1: The twin-engine Cessna 310R chartered to bring back the Compact Array, Mopra and Parkes data to Marsfield as quickly as possible so a sample of the recorded data could be transferred immediately to JIVE to allow a "quick look" correlation of the VLBI data.
Photo credit: Chris Phillips


This flew him back to Sydney via Coonabarabran and Parkes airports, collecting the Parkes and Mopra data en route. Once back at ATNF headquarters in Marsfield the disks were re-loaded onto a waiting PC to be sent to JIVE. To facilitate a speedy transfer of data, nine research and education networks collaborated to give us a dedicated door-to-door gigabit connection from Marsfield to JIVE. The organisations involved included CeNTIE and AARNet within Australia, Pacific North West in the USA, CANARIE in Canada, and SURFnet in Europe. This allowed us to transmit a subset of the recorded Parkes and Mopra data for JIVE to process on their VLBI correlator. After a brief period translating the data format and copying to their local recording media, JIVE was able to confirm fringes on the calibrator sources between Parkes and Mopra.





Figure 2: Huygens carrier signal detected in the Parkes VLBI data. Plots – courtesy Sergei Pogrebenko, JIVE.

The disks have now all been sent to JIVE and they have been busy translating the data format and doing initial correlation. So far everything looks like it has worked well. They have been able to successfully detect the probe signal in the Parkes VLBI data (Figure 2). The full 17-station correlation is expected to start soon. Then the complicated job of detection of the probe signal in the interferometry data and modeling the probe velocity will commence. As processing of this experiment is even more difficult than the observations, it will be a while before the full results are available.

The VLBI observations were originally envisaged to complement the Doppler Wind Experiment (see separate report from Parkes), which used special purpose hardware installed on Cassini as well as Parkes and the GBT. However one of the two receiving channels on Cassini was misconfigured, meaning all the Doppler measurements from Cassini were lost. While the DWE data from Parkes and GBT can replace the Cassini data, 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.

We would like to thank everyone who has been involved in the LBA observations of the Huygens descent onto Titan. This has been a large and complicated experiment and would not have been successful without the hard work of many people.

Chris Phillips and Tasso Tzioumis
(Chris.Phillips@csiro.au, Tasso.Tzioumis@csiro.au)