Comparison of the Huygens mission and the SM2 test flight for Huygens attitude reconstruction

The Huygens probe is the ESA’s main contribution to the Cassini/Huygens mission, carried out jointly by NASA, ESA and ASI. It was designed to descend into the atmosphere of Titan on January 14, 2005, providing surface images and scientific data to study the ground and the atmosphere of Saturn’s largest moon.
In the framework of the reconstruction of the probe’s motions during the descent based on the engineering data, additional information was needed to investigate the attitude and an anomaly in the spin direction.
Two years before the launch of the Cassini/Huygens spacecraft, in May 1995, a test probe called SM2 (Special Model 2) was dropped in the Earth’s atmosphere from the balloon launch site of Kiruna, Sweden, to verify proper operation during the descent and especially the parachute deployment sequence. It featured a flight standard structure and DCSS (Descent Control SubSystem) and, unlike the Huygens probe, was fully instrumented to monitor the orientation of the descent module (3-axes accelerometers and gyroscopes).
We describe how a comparison between the SM2 test flight and the Huygens mission provides some useful information about the Huygens probe’s behavior. After discussing the spin direction, we focus on the tip and tilt.
The final conclusions of this comparison at the time of writing are still of qualitative nature, but the results are a starting point for better interpretation of the engineering data in terms of attitude to derive the probe’s orientation.