Attitude & Orbit Control Subsystem 26 April 2007

Contents Key Requirements AOCS Design Description –Functional block diagram –AOCS modes AOCS Hardware Description –Hardware Functions/ characterization –Interface Summary (Power, Bi-level, Discrete, analog, serial bus) AOCS Software Development

Contents (cont’d) Major Trade-offs –Star camera orientation –Thruster configuration –Jitter analysis (rigid body) –Sun Sensor configuration Design and Analysis –ASH mode –Navigation filter –Attitude estimator –Off loading –Guidance –Normal mode

AOCS Key Requirements Orbit Altitude Orbit Inclination Equator Crossing Time Attitude Control Accuracy Attitude Control Accuracy Goal Attitude Control Bandwidth Attitude Knowledge

AOCS Key Requirements (cont’d) Attitude Maneuvers Spacecraft Jitter On-board Orbit Determination Satellite Autonomous Operations Over-sampling Maneuver Agility

AOCS Design Description: Functional block diagram Telecommand estimation Attitude estimation Orbit Normal Mode control - Reaction Whl command - Magnetoquer command 1: Attitude acquisition and Safe-hold (ASH) sub-mode: Stabilize (STAB) Sun tracked (STRA) Sun locked (SLO) 2: Normal mode (NM) sub-mode: Geocentric attitude pointing (GAP) Maneuver (MAN) Fine imaging pointing (FIP) Sun pointing (SUP) 3: Orbit control mode (OCM) Commanded quaternion NM Mode manager : GAP, MAN, FIP, SUP satellite ASH Mode manager : STAB, STRA, SLO MAG Star camera Sun sensor GPS IMU OCM Mode control - Thruster command 3 other ASH Mode control - Reaction Whl command - Magnetoquer command

AOCS Design Description: AOCS modes FIP MA N SUP GA P OC M ARO TC A A A : Automatic transition TC : Telecommanded transition ARO : Attitude Reconfiguration Order (from any submode) STRA SLO STAB A A ASH Mode Normal Mode

AOCS Design Description: Mode Function ModeFunction Acquisition and Safe -Hold Mode (ASH) Rate damping after launcher separation (tip-off rates < 2.5deg/sec). After rate damping the AOCS shall provide shall be possible without time and orbit information STAB Submode Perform initial rate damping for initial rates < 2.5 deg/sec each axis within 3 (TBR) orbits. Shall be able to perform safe mode control with one wheel failure STRA SubmodeThe AOCS will be able to perform sun tracking during eclipse with rotation rate of two times orbit-rate in pitch axis SLO SubmodeThe AOCS will be able to achieve Sun acquisition in less than 10 minutes after eclipse. Backup ASH Mode Provide 3-axis pointing capability with an accuracy as required for imaging The Sun pointing accuracy shall be better than 35 degrees

AOCS Design Description: Mode Function (cont’d) ModeFunction Normal Mode (NM) Provide 3-axis pointing capability with an accuracy as required for imaging Provide attitude knowledge as required for imaging Provide nadir-pointing, sun-pointing and imaging maneuver attitude control GAP Submode In eclipse, the AOCS shall provide geocentric pointing capability with an accuracy of 0.5 degrees. FIP Submode The AOCS shall provide imaging pointing capability with an accuracy of 0.1 degree (TBR )(3 sigma in all three axes). SUP Submode In sunlight, the AOCS shall provide the capability to point the normal of the solar array to the sun with an accuracy of 5 degrees. MAN Submode Provide the capability to autonomously transition the spacecraft between geocentric attitude and sun pointing attitude. Each transition shall take place in eclipse and shall be finished within 5 minutes (TBR). Orbit Control Mode (OCM) Perform orbit correction, orbit transfer (inclination changes, orbit raising), and orbit maintenance (“station keeping”).

AOCS Hardware Description: Sensors: –Sun Sensors –Magnetometers –Inertial Measurement Unit (IMU) –Star Camera with two Camera Heads Actuators: –Reaction Wheels –3 Magnetic Torquer –1 RCS (cold gas) with 4 thrusters

Major Trade-offs : Maneuver Agility Attitude maneuver performed by a cluster of 4 whls Wheel capacity 20 deg/min for each axis based on current whl capacity Possible to increase agility for specific axis from (, ) 25 % torque margin 

Major Trade-offs : Magnetorquer sizing ,, S/C (nadir /Sun pointing) Wheel Control wheel off-loading control law Preliminary analysis shows: Wheel unloading control in NM mode, Maximum command magnetic command shall be able to retain wheels angular momentum variation induced by the environment disturbing torques Detumbling control In ASH mode, maximum command magnetic command shall be able to stabilize the spacecraft within 2 orbits  Cross denote wheel control has been absent from the control loop and enforced S/C with nadir attitude in eclipse and sun pointing attitude in sunlight   H was calculated by integrating T off-loading + T dist instead of feeding from wheel speeds

Major Trade-offs : Star camera orientation Sun is a point source, Sun masking angle: 39 deg Earth is an extended source, Earth masking angle (from Earth limb): 23 deg Earth Sun direction 7.5 deg 39 deg Sun masking 23 deg Earth limb masking CHU los 28.6 deg +Ysc -Zsc Available for roll maneuver: 59.8 deg Xsc Ysc Zsc CHU los Rx Rr CHU A los CHU B los +Y +X +Z

Major Trade-offs : Star camera orientation (cont’d) Conclusion: Based on the simulation results, at least one of the two CHUs will be always kept out from blinding. To extend roll maneuver capacity from +/- 25 deg to +/- 35 deg, elimination of 10 deg either in Sun or Earth exclusion angle is needed

Major Trade-offs : Thruster configuration z y x COM Four thrusters configuration Only one of the two thruster branches is used after 1 failure Propulsion module is centred around centre of mass (COM), the thruster configuration cannot create any torque aligned on Y axis. Orbit control –On Y axis: No capacity around Y, Y axis is always controlled by wheels. –On X and Z axes: In the nominal case, the thruster is performed by firing the 4 thrusters simultaneously. In a degraded case (one thruster failure), the pair that includes the failure thruster is no longer used and the thruster is performed with the remaining thrusters. The X or Z axis is therefore control by wheels Off-modulating Control. The pair (1,2) control Z axis, the pair (3,4) control X axis

Argo PDR – AOCS Jitter Analysis

Preliminary Performance Analysis: Jitter analysis (rigid body) Objective: Analyze whether pointing req. for 0.5” ∀ freq > Hz is achievable. Method: Frequency domain analysis. Results: Normal Mode (FIP, MAN sub-modes) + time delay

Jitter Conclusion Required specification achievable. Given Hz cl-BW, Relative Accuracy: 47.20” + 2 nd order LPF with 4 Hz sampling rate  output: pointing error ~ 0.19”, for freq > Hz.

Argo PDR – AOCS Omni-directional Sun Sensor (OSS)

OSS Conclusions Maximum OSS sun direction error < 12 deg. Sensitivity analysis will be done after PDR. Those including: variation of mean albedo, unequal cell degrade, mismatch of measurement resistors, head misalignment, and variation of backside radiation.

Preliminary Performance Analysis: ASH mode Objective: –To reduce the initial rate, after that to track Sun and control the solar array toward Sun while it is in eclipse or daylight. –To keep the satellite in safe state once any contingency or anomaly happened. Method: STRA STAB SLO B-dot control law B-dot control law (X,Z) Sun acquisition control law (Y) Wheel off-loading control law (Y) automatic Sun presence Normal Mode TC ASH Mode Eclipse

Preliminary Performance Analysis: ASH mode (cont’d) Conclusion: Control law works. –The satellite spins down from the initial rate of 2.5°/s at each axis within 2 orbits, then transits from STAB to STRA. –STRA/SLO cyclic transition demonstrates Sun acquisition function well. –Angular momentum of each wheel is in the designed working range.

Argo PDR – AOCS Navigation Filter Design (NAV)

NAV requirement Orbit determination (Normal mode) –Position: 25 m (3D-3  ) –Velocity: 1.8 m/s (3D-3  ),

Argo PDR – AOCS Inertial Attitude Estimation (IAE)

Hardware: –Star camera (ASC) –Gyro (IRU) Measurements: q  ProsCons ASCdirect output deduced from q accurateblinding, expensive IRUdeduced from  direct output cheap, robust drift Inertial Attitude Estimation (IAE)

LPF is good enough + fast & easy to design/implement. Angular error < 40 arc-second, rate error < 0.5 deg/hr. Data fusion – camera head misalignment IAE Conclusions