Attitude Control

1
HALE
Attitude Control Dave Olney Jim
Morrissey Jennifer Bracken
7/9/98
1
NASA/GSFC/IMDC
2
ACS Coordinate System
Az
Yaw
North Celestial Pole
Roll
Pitch
Sun
7/9/98
2
NASA/GSFC/IMDC
3
Pointing Error Definitions
target desired pointing direction true actual
pointing direction (mean) estimate estimate of
true (instantaneous) a pointing accuracy
(long-term) s stability (peak-peak
motion) k knowledge error c control error
7/9/98
3
NASA/GSFC/IMDC
4
Science Pointing Requirements
Pitch and Yaw attitude following sun
acquisition, the instruments active tracker
provides pointing and stabilization to 1.5
arc-sec, rms Roll attitude within 0.05 of
preferred roll attitude real-time (0.01 in post
processing). 10 arc-sec, rms, roll stability
over a 30 second period
7/9/98
4
NASA/GSFC/IMDC
5
Modes
Acquisition Reduce tip-off rates (this could be
as high as 8 deg/sec) Coarse acquisition of the
sun using RWs, IRU, and CSSs Fine acquisition of
the sun Roll about the sun line to acquire
pre-determined star field with the Star
Tracker Mission Mode Pitch yaw fine pointing
using the instruments sun tracker Roll fine
pointing using the star tracker Thruster
Mode Uses hydrazine thrusters Wheel momentum
due to solar torques - unloaded every 1 to 4
weeks Orbit adjust maneuvers - 3 to 4 times per
year Safe Hold Re-acquire and maintain coarse
sun pointing during anomalous operations
7/9/98
5
NASA/GSFC/IMDC
6
ACS Component Recommendations
Coarse Sun Sensors (5) 2 accuracy with a ? 85
FOV Digital Sun Sensor (1) 0.5 accuracy with
a ? 64 FOV Gyroscope (1) Three two-axis gyros
in a single package with 1.2433x 10-6 resolution
1.5 arc-sec/sec (1?) noise and 0.6 deg/hour
drift. (Note directly from SMEX-Lite SC Bus
specifications) Possible candidates DRIRU II,
Honeywell RL6, or Litton LN2000 Reaction Wheels
(3 or 4) 14 N-m-s (maximum) momentum
storage Star Tracker (1) 5.5 arc-sec (1?)
accuracy/star with an 8 x8 FOV Hydrazine
Thrusters (TBD) Unloading in all 3 axes required
7/9/98
6
NASA/GSFC/IMDC
7
Attitude Control System
Solar Torques
Ref Attitude Axis Pointing X
RA,dec sun center Z
Azimuth angle (from north)
about sun
Actual Inertial Attitude
Hydrazine Thrusters
Spacecraft Dynamics
?
Pointing Control Laws
?
Reaction Wheel Rotors
Reaction Wheel Motors
On-Board Inertial Attitude Determination
Momentum Management
Tachometers
Inertial Star Position
?
Gyros
Post Processing Inertial Attitude Determination (
Ground)
Star Tracker
X
Spacecraft Sun Sensors
X
Inertial Sun Position
Error Signals from the Experiment
MDI
TRACE
7/9/98
7
NASA/GSFC/IMDC
8
Magnetic Field at GeoSynch Orbit
1.5
Magnitude
Y-Axis
Z-Axis
Magnetic Field (mGauss)
0.0
X-Axis
-1.5
0
0.2
0.4
0.6
0.8
1
Orbit Fraction
7/9/98
8
NASA/GSFC/IMDC
9
Issues
3 Vs 4 Reaction Wheels Dynamic effects of
antenna motion require analysis
7/9/98
9
NASA/GSFC/IMDC