374 Introduction/Background:  HST Pointing Accuracy using High -Performance/Low IV RWAs  

Spacecraft Attitude Control Systems (ACSs) use Inertial Measurement Units , along with other sensor data  inputs (Star Tracker, Sun Sensor, Fine Guidance Sensor (FGS), etc.) to Command and Data Handling or  Line-Of-Sight Computer  control algorithms to command inertia wheel counter -torque outputs in a traditional  3-axis momentu m-management scheme (Fig. 1)1,2. HST’s PCS uses 4 RWAs ( i.e., variable- rate, fixed- spinaxis units) in an overall canted- pyramidal arrangement to generate torques for large target -to-target   maneuvers/attitude changes (~6° /min) as well as for highest -precision fine- pointing  operation.  

HST LOS Critical Performance System Jitter  Depend ence due to RWA  Induced Vibrations  

HST achieves stringent LOS pointing stability (top- level specified at <7 milli- arcseconds over  24 hours) on   celestial objects  for long exposures using  a structured flow-down budgeting  methodology3. Analogous  means  have been refined and applied to other  space platforms  (e.g., International Space Station  microgravity4). Imaging can take hundreds of hours via 20- minute exposures ; typically 2 per orbit .  Repeatable LOS pointing accuracy  is <0.01 arc sec (over 100 hours ). FGSs detect angular deviations  due  to on-orbit disturbances (see Table 1 sources5) to generate countering minute RWA rotation speed changes  to maintain  PCA LOS/attitude control ; down to <~2 milli- arcseconds as sub-allocated. Other fine- pointing  satelli tes obtain LOS levels via alternate ACS architectures /units  (see Table 25). Note that there is a   fundamental ACS/PCS tradeoff (in normal attitude hold mode) between smaller RWA s (correspondingly  small rotor mass and potentially  less stringent residual un balance)  operating at  higher maximum  rotation   rates  (much higher IV) ; or using a larger RWA, that, if driven to tight  residual unbalance limits , spins   significantly slower ( typically much lower IV).  It may be  more upfront affordable to impose tighter  tolerances/balancing in smaller RWA rotor -bearing assemblies , but, as discussed below, may not be overall  optimal . RWA IV  limits get significant  attention due to  several inherent  aspects:  1) rotation  rate can assume  any value in a wide spin-rate range,  and 2) commanded rates reached and held ( i.e., steady -state) dwell   sufficiently  long for fully -saturated resonance excitation at structural dynamic mode frequencies .  Table 1.  Fine-Pointing Satellite Distur bance Examples with Reaction Wheels Highlighted5  Source  On-Orbit Satellite Disturbance Type  Mitigation Approaches   Orbital   Torques  Atmospheric Drag, Gravity Gradient, Solar Pressure/Impingement, Residual Magnetic Momentum, Thermal -Radiant  Field Changes  Orbit Choice, Minimizing Drag Area,  Align Center -of-Pressure/Center -ofMass   Operations  Commanded Maneuvers/Slews  Stimulating Low -Damped Structures  Profiling, Ops Constraints/Stay -Outs   Mechanical  Gimballed Antennas, Solar Array Drives, Thermal Electric Coolers,  Cryocoolers, Maneuver -Induced Excitations, Thermal Creak/Snap  Body -Fixed Antennas/Solar Arrays, Stiff  Structures, Isolation, Profiled  Maneuvers   Sensor  Star Tracker Errors  – Noise Equivalent Angle, Spatial/Centroiding Errors, Star  Catalog Errors, Velocity Aberration, Alignment Variations/Errors   Gyro Errors - White Noise, Random Walk, Bias, Scale Factor, Al ignment  Higher Quality/Performing Sensors,  Better Filters/Estimators   Actuator  Reaction Wheels , Command Noise, Command Quantization, Residual  Static Imbalance, Dynamic Imbalance, Bearing/Drive Harmonics  Higher Quality/Perf . Wheels, Higher  Control Bandwidth, Isolation  Approache s 

RWA Concept -of-Operations (CONOPS) and Basic Induced -Vibration Test Data Considerations    Most HST disturbers (Fig. 3), if threatening jitter limits, can ty pically adjust their operations,  i.e., selectively  power off, switch operating modes/profiles, modify  duty cycles , and/or transition to alternate capabilities.   However,  using RWAs creates a unique situation due to operating constantly to maintain HST stability 6.  Also, since constantly commanded torques react to small  always -changing external torque   effects /phenomenon,  RWA  rotational rates do not have a priori predictability during each orbit’s phases. As  a result, there arose a focused demand for IV analytical investigations along with other disturbance source