Titan Flyby Altitude – Tour Updates
Upcoming Observations in 2006
C. J. Hansen
6 January 2005
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
Titan Flybys in 2006-2008
21 low
altitude
Titan
flybys
Tit an Flyby
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
H [k m]
1 8 55
1 8 79
1 9 06
950
950
950
950
950
950
1 5 00
950
2 7 26
950
950
950
950
950
950
2 4 26
950
1 9 44
1 3 00
3 2 12
950
950
1 3 00
950
950
950
950
950
1 3 48
V [k m/ s]
5 .8
5 .8
5 .8
6 .0
6 .0
6 .0
6 .0
6 .0
6 .0
5 .9
6 .0
5 .8
6 .3
6 .3
6 .3
6 .3
6 .3
6 .3
6 .1
6 .3
6 .2
6 .2
6 .1
6 .3
6 .3
6 .3
6 .3
6 .3
6 .4
6 .4
6 .4
6 .3
Planned Dat e
4/ 3 0/0 6
5/ 2 0/0 6
7/ 0 2/0 6
7/ 2 2/0 6
9/ 0 7/0 6
9/ 2 3/0 6
1 0 /09/ 0 6
1 0 /25/ 0 6
1 2 /12/ 0 6
1 2 /28/ 0 6
1/ 1 3/0 7
1/ 2 9/0 7
2/ 2 2/0 7
3/ 1 0/0 7
3/ 2 6/0 7
4/ 1 0/0 7
4/ 2 6/0 7
5/ 1 2/0 7
5/ 2 8/0 7
6/ 1 3/0 7
6/ 2 9/0 7
7/ 1 9/0 7
8/ 3 1/0 7
1 0 /02/ 0 7
1 1 /19/ 0 7
1 2 /05/ 0 7
1 2 /20/ 0 7
1/ 0 5/0 8
2/ 2 2/0 8
3/ 2 5/0 8
5/ 1 2/0 8
5/ 2 8/0 8
2006
2007
2008
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
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
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
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
Flyby recommendation summary
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
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
Upcoming Observations in 2006
Petal Rotation
Rev 20 – 25 Summary
Titan Flybys
•
•
•
•
•
•
•
T10
T11
T12
T13
T14
T15
T16
15 January
27 February
18 March
30 April
20 May
2 July
22 July
UVIS solar occultation
RSS gravity science
RSS earth occultation
RADAR
RSS earth occultation
MAPS – unique local time
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
Backup Charts
Safe Density (Phase A)
[But without the 5% DKDK margin]
Safe = 0.932Tumble
Titan
Flyby
C/A
Instrument(s)
S/C Orientation
16
17
18
19
20
20
21
23
RADAR
INMS, RADAR
INMS, RADAR
RADAR
ORS
RADAR
INMS, RADAR
RADAR
-Z to Titan, -X to ram
-Z to Titan, -X to ram
-Z to Titan, -X to ram
-Z to Titan, -X to ram
-Y to Titan, +X to pole
-Z to Titan, -X to ram
-Z to Titan, -X to ram
-Z to Titan, -X to ram
Tumbling
Density
[10-10
kg/m3]
40.57
40.66
40.45
35.13
25.30
34.75
34.35
36.45
Safe
Density
[10-10
kg/m3]
37.81
37.90
37.70
32.74
23.58
32.39
32.02
33.97
Safe Density (Phase B) [1]
[But without the 5% DKDK margin]
Safe = 0.932Tumble
Titan
Flyby
C/A
Instrument(s)
S/C
Orientation
25
RADAR
26
INMS
27
RSS
28
RADAR
29
RADAR
30
RADAR
32
INMS
32
UVIS/VIMS
-Z to Titan,
-X to ram
-Y to Titan,
-X to ram
-Z to earth,
-Y to ram
-Z to Titan,
-X to ram
-Z to Titan,
-X to ram
-Z to Titan,
-X to ram
-Y to Titan,
-X to ram
-Y to sun,
-X to ram
Tumbling
Density
[10-10
kg/m3]
32.70
Safe
Density
[10-10
kg/m3]
30.48
35.32
32.92
27.06
25.22
32.17
29.98
30.47
28.40
32.37
30.17
34.52
32.17
29.23
27.24
Safe Density (Phase B) [2]
[But without the 5% DKDK margin]
Safe = 0.932Tumble
Titan
C/A
Flyby Instrument(s)
36
INMS
37
INMS, ORS
39
RADAR
40
INMS
41
RADAR
42
INMS
43
RADAR
S/C
Orientation
-Z to Titan,
-X to ram
-Y to Titan,
-X to ram
-Z to Titan,
-X to ram
-Y to Titan,
-X to ram
-Z to Titan,
-X to ram
-Y to Titan,
-X to ram
-Z to Titan,
-X to ram
Tumbling
Density
[10-10
kg/m3]
32.65
Safe
Density
[10-10
kg/m3]
30.43
32.72
30.50
31.19
29.07
32.22
30.03
31.00
28.89
31.17
29.05
30.45
28.38