1. Titan Flyby Altitude – Tour Updates Upcoming Observations in 2006
C. J. Hansen
6 January 2005

2. Background: Safe Titan Flyby Altitude
The atmospheric density experienced by the s/c on the Ta flyby was higher than expected, leading to concerns about low altitude flybys possibly putting the s/c in safe mode
The thrusters fire to overcome torque on the s/c from the atmosphere
If the density is too high the thrusters cannot compensate and the s/c goes off point
If the thrusters fire continuously for >7 sec the s/c enters safe mode
The altitude of the T3, T5, and T7 flybys was raised by lowering the altitudes of the T4, T6, and T8 flybys
Starting with T16 we won’t have that option because we have long series’ of 950 km flybys – the tour must be re-designed to fly at higher altitudes
The AACS team has been working to define the density at which the s/c will lose control
This density is dependent on the orientation of the s/c at closest approach, also on the type of activity (slewing) taking place
TAMWG has been meeting to develop an atmospheric model that predicts density as a function of altitude, given the differing values measured to-date

3. Titan Flybys in
2006
2007
21 low
altitude
Titan
flybys
2008

4. Safe Titan Flyby Density
Low-altitude Titan flyby are all performed with thrusters. Thrusters are used:
To overcome Titan atmospheric torque
To perform target motion compensation (TMC)
To slew the S/C about X, Y, Z-axis (or multi-axis) for science observation
To maintain a set of commanded per-axis attitude controller dead-band
The safe Titan flyby density is highly scenario dependent. It is a function of many variables:
Thruster magnitude
Spacecraft’s attitude time history
Spacecraft’s projected area and c.p. location near TCA
Spacecraft’s inertia properties (including c.m.)
RCS controller dead-band
Per-axis slew rates near TCA
Titan-relative S/C flyby velocity
AACS team used a test bed named Flight Software Development System (FSDS) to determine the tumbling densities of all low-altitude flybys

5. Reference Trajectory Modifications
The project started development of the new tour in December
Changes won’t be huge – petal rotation, icy satellite flybys, occultations will still happen, but times may move, sometimes considerably
The AACS team has used the current TOST plan to define a safe density for each flyby based on the s/c attitude
If the tour is designed at precisely those altitudes / attitudes we lose some amount of flexibility – any attitude which is not safe at that altitude will not be possible
Discussed at length at TOST November workshop
TOST made recommendations to Project
Altitude decisions made at Project meeting November 30
Used INMS density multiplied by AACS-driven fudge factor, with cosine latitude dependence
General policy was to be conservative on early inbound Titan flybys, take more risk on later outbound flybys (but thruster torque dropping so these still turn out to be relatively high

6. TOST Input to Process
The correlation of s/c attitude with safe flyby altitude is a concern to TOST planning
A workshop was held 18 Nov. to discuss the impact on TOST plans and flexibility of fixing an altitude and thereby fixing the s/c attitude
One particular outlier was T20 option a
Attitude of the spacecraft at closest approach is predominantly either –Z to Titan, -X to ram (RADAR + INMS) or –Y to Titan, -X to ram (ORS + INMS)
Comparison of T25 and T26 results (see Frautnick presentation) shows that –Y to Titan, -X to ram differs from –Z to Titan, -X to ram by only 5 km
T25 and T26 best for comparison because at ~ same latitude, timeframe

7. TOST “Flexibility” Recommendation
Preserve the capability to switch between ORS + INMS and RADAR + INMS
5 km difference in altitude
TOST recommends that this be subsumed in overall project margin, not added on top
Preserve the capability to fly either the a or b options of T20 and T32
Choose the higher altitude, or
Subsume difference in project margin
For some flybys science gain is worth additional risk
MAG, INMS would like to go low to get data deeper in atmosphere (INMS) and search for an internal field (MAG)
On many flybys however outbound science is too important to risk

8. Flyby recommendation summary

9. Implementation: Tour Modifications
Tour redesign is underway
New tour plus limited case studies are out for review
Team review due January 5
Comments to TWT and OST’s, TWT and OST feedback due January 11
UVIS is more impacted than any other team because so much of our science comes from occultations
Timing shifts necessitate re-integration of science sequences already on the shelf
Actual occultation geometry may change
Initial response from UVIS was to spend delta-v to maintain timing of old tour
Based on initial NAV analysis of a cost of 35 m/sec
That turned out to be incorrect – cost is > 100 m/sec
New strategy is to emphasize importance of occ’s to the project, try to get commitment to do re-integration

10. Implementation schedule
Feedback on the new reference trajectory will be reviewed at the PSG
New (final) tour developed February 3 – 13
Modifications begin with T17 (S23), September ’06
Sequence re-integration will be handled in aftermarket process

11. Upcoming Observations in 2006

12. Petal Rotation

13. Rev 20 – 25 Summary Titan Flybys T10 15 January UVIS solar occultation
T February RSS gravity science
T March RSS earth occultation
T April RADAR
T May RSS earth occultation
T15 2 July MAPS – unique local time
T July RADAR, UVIS stellar occultation
Tour phase: petal rotation to magnetotail
In equatorial plane until July
Only one relatively close icy satellite flyby (no targeted flybys)
Rhea 21 March 82,000 km

14. Backup Charts

15. Safe Density (Phase A) [But without the 5% DKDK margin]
Safe = 0.932Tumble

16. Safe Density (Phase B) [1] [But without the 5% DKDK margin]
Safe = 0.932Tumble

17. Safe Density (Phase B) [2] [But without the 5% DKDK margin]
Safe = 0.932Tumble