Description of the table of Huygens Descent Parameters (HUYGENS_DESCENT_PARAMETERS_DESCRIPTION.TXT).

The file named HUYGENS_DESCENT_PARAMETERS.TXT contains 8 columns of information describing the Huygens Probe's position and attitude during the Titan descent, from the DISR sensor head's point of view.  The data are presented at mission times from Descent Data Broadcast (DDB) T0 (i.e. T=0) to probe impact at T=8870 seconds, with one final, end-of-mission point added at T=13050 to include the on-surface segment.


The first column is, of course, the mission time in seconds after T0 as described by Huygen's channel B Descent Data Broadcast (DDB) time.  This timer began at 09:10:21 hms UTC on 14 January 2006 +/- 0.3 sec.

The second column lists the probe rotation number.  Rotations are listed as clockwise from the azimuth of the sun (sun crossing is on the integer with the fraction of integer, multiplied by 360, indicating the number of degrees from the sun's azimuth).  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 (i.e. the rotation number indicates the number of solar crossings seen from the time the probe stopped rotating).  The rotation are listed for 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 third column contains the probes altitude above Titan's surface in Kilometers.  These numbers should be consistent with the DTWG altitude profile.

The fourth and fifth columns report the position of the sun in the sky relative to the sub-Huygens point during the descent.  The fourth column is the azimuth angle of the sun, in degrees east of north.  The fifth is the angle of the sun from zenith in degrees.   

Columns 6 & 7 show the position of the sub-Huygens point with respect to the mission time.  The 6th column is the east longitude (from Saturn), the 7th is the north latitude, both in degrees and with an accuracy of about 0.1 deg.

The last two columns contain information on the tilt of the probe.  Column 8 lists the tilt inferred from smoothed tracking data of the radio Doppler shift received on Earth (i.e. the AGC measurements).  The tilt is around an axis pointed horizontally 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 a 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.  The last column (9), shows the number of seconds till the next tilt measurement, thus giving an indication of the time variation and granularity of the data.