The geometrical configuration and sequence of events during Titan descent are of vital importance to the execution of DWE. The present mission baseline (Lebreton 1995) foresees probe release on the Orbiter's first inbound pass after Saturn Orbit Insertion (SOI). The nominal date for the probe descent is 27 November 2004, some 150 days after SOI and more than 7 years after launch. Titan, which orbits Saturn at a constant distance of 20.4 Saturn radii, is located very close to Saturn's noon meridian on this date.
Probe separation from the Orbiter occurs ca. 22 days prior to Titan encounter, at which point the Probe is targeted for entry into Titan's atmosphere. A final orbiter deflection maneuver is performed 2 days after probe separation, targeting the Orbiter such that it will fly very nearly over the Probe, but at a safe altitude of 1500 km above Titan's surface. The closest approach of the Orbiter at Titan is delayed by about four hours with respect to the probe atmospheric entry. This Orbiter Delay Time (ODT) is short enough to provide adequate margin of the PRL at the beginning of descent (maximum range) and yet long enough to avoid orbiter High Gain Antenna (HGA) pointing problems toward the end of the Huygens mission (minimum range).
The probe target is characterized by parameters in the Titan B-plane, defined by the asymptotic probe approach velocity as shown in Fig. 2. Aim points in the B-plane are defined by the magnitude of the impact parameter vector and its associated azimuthal angle with respect to the T-axis. Equivalently, the atmospheric entry angle at a given altitude could be used instead of the impact parameter B. This alternative was adopted for Huygens mission planning purposes.
Figure 2: Definition of the Titan B-plane. The normal to the B-plane (origin at Titan center) is given by the direction of the asymptotic probe approach velocity . The intersection of this plane with the Titan equator defines the direction of the vector . , also lying in the B-plane, completes the 3-axis system.
It is planned to target the probe to the position shown in Fig. 3 at an entry angle = 64°, and B-plane azimuth angle = -60°. The probe delivery accuracy (3) in the B-plane is given by the ellipse encircling the tip of the target vector ( = -60°, = 64°) in Fig. 3. The dimensions of the ellipse are 452 km in the horizontal (longitudinal) direction and 59 km in the vertical (latitudinal) direction. The targeted value of , defined as the angle between the nadir direction and the Probe's velocity vector at a reference height of 1270 km, was selected to ensure a safe atmospheric entry and to guarantee successful probe radio communications via the PRL. Contours of constant are drawn as concentric dotted circles in Fig.3. The targeted latitude of the Probe on Titan is 18°N.
Figure 3: Huygens target in the upper right quadrant of the Titan B-plane.
Due to the extreme elongation of the error ellipse in the horizontal direction, dispersion in the entry angle is small for values of -90°, but quite large for 0°. In order to minimize the entry angle dispersion, the Huygens B-plane target azimuth was selected as = -80° in the early stages of the mission planning process. Considerable trade-off analyses, however, were conducted to address scientific preferences for the impact latitude, solar zenith angle (SZA), and, specifically for DWE, the angle between the east-west direction on Titan and the line-of-sight direction from Probe to Orbiter. This angle, referred to as the Doppler Wind Component (DWC) angle, was very unfavorable to DWE for = -80°. The cosine of the angle DWC, which is the "zonal wind projection" (ZWP) onto the PRL ray path, regulates the magnitude of the Doppler shift from zonal winds. The nearly vertical solid curves in Fig. 3 are contours of constant ZWP.
A study of the DWE wind recovery algorithm under various Probe/Orbiter geometries showed that a very good representation of the input wind was derived for ZWP > 0.5. This is shown as Region (a) in Fig. 3. Less precise, but still satisfactory recoveries could be obtained in Region (b), where 0.3 < ZWP < 0.5. The discrepancy between the input wind and the recovered profile begins to increase dramatically, however, when ZWP < 0.3. The Region (c), where the zonal wind recovery error became unacceptably large, was thus declared a "zone of avoidance" by DWE.
The original Huygens target with = 64°, = -80° was, in fact, located in Region (c). The DWE request to move the B-plane azimuth angle to = -60° was granted after carefully reassessing the consequences for the overall mission performance. Only a very small portion of the 3 targeting ellipse in Fig. 3 is now located in Region (c). The calculations of ZWP were performed for the "100% nominal" input wind model (prograde, linearly increasing from zero at the surface to 100 m/s at 200 km altitude; Flasar et al. 1981). Different contours are obtained for other wind models. A prograde zonal wind significantly shifts the final touchdown site to the east of the Probe's atmospheric injection point, thereby improving the ZWP. The situation is the reverse for retrograde winds, which tend to increase the recovery errors. For all reasonable cases tested, however, the recovery algorithm never becomes indeterminate (i.e. when ZWP 0).