Explanation
of the Model
This is a model of the rotation of Huygens during the
descent. It includes a rough estimate of
the tilt of the probe. The model is an
updated version of the previous similar model released on
The maximum azimuth error for Sun Sensor pulses is only 11 degrees and the RMS error is 4 degrees, because the Sun Sensor pulse triplet around 3643.6782 s corresponds most likely to a triple crossing of the first slit.
The two data files are in the same format as before:
Rotations are listed as clockwise from the azimuth of the sun. The arbitrary integer count of rotations was set so that the switch from counterclockwise to clockwise rotation near 542 s Mission Time has a rotation number between 0 and 1.
The first table is a summary listing the following data for each solar azimuth crossing according to the model: Mission Time, rotations per minute smoothed over 30 seconds, rotations per vertical kilometer, Sun Sensor time if transmitted, and rotation according to the model at the Sun Sensor time. The equilibrium for the rotations per vertical kilometer seems to be between 3 and 4. Note that Sun Sensor timings inferred from the times of error messages were included here.
The second table lists the rotation every 2 seconds of Mission Time. Linear interpolation is sufficiently accurate, since the strongest accelerations, near 4000 s Mission Time are about 2.5 RPM/min, which gives a maximum error of linear interpolation of 0.0004 rotations or 0.1 degree in azimuth, far less than the typical uncertainty of a few degrees or about 0.01 rotation.
The second table also lists the tilt inferred from smoothed tracking data of the radio doppler shift received on Earth. The tilt is around an axis pointed horizonally to the north, given in degrees. A positive value means that the Huygens rotation axis was pointed to the east of the zenith and to the west of the nadir. On the time scale of minutes, this tilt is closer to the truth than the assumption of zero tilt. However, for any individual exposure, the given tilt is probably far off due to rapid swing motions not included here.