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)

Commandedquaternion

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

MAN

SUP

GAP OCM

ARO

TC TC

TC

TCA TC

TC

A

TC

TC

A : Automatic transitionTC : Telecommanded transitionARO : Attitude Reconfiguration Order (from any submode)

STRA

SLO

STAB

A

A

ASH Mode

Normal Mode

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

Angle Duration Acceleration Max Rate Inertia Uncertainty Used Inertia(deg) (s) (deg/s2) (deg/s) (kgm2) (%) (kgm2)

roll 20 60 Spec 0.022 0.667 100 20 120

pitch 20 60 Spec 0.022 0.667 80 20 96

yaw 20 120 Spec 0.006 0.333 100 20 120

Analysis of bang-bang profile +Y

-X

Z

RW2

RW1

RW4

RW3 X

cc

Time

T

Torque Max H Avail. Torque Avail. H(Nm) (Nms) (Nm) (Nms)

0.047 1.164 0.052 4.607

0.037 0.931 0.046 4.114

0.012 0.582 0.019 1.677

alpha (deg)beta (deg)

Wheel Torque (Nm)Wheel H (Nms)

Major Trade-offs : Magnetorquer sizing

,

S/C

(nadir /Sun pointing)

Wheel

Control

2B

BHΔKM

BMT loadingoff

H

distloadingoff TT

distT

wheel off-loading control law

loadingoffT

H

Preliminary analysis shows:• Wheel unloading control in NM mode, Maximum command magnetic comm

and 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 + Tdist 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 degSun masking

23 deg Earth limb masking

CHU los28.6 deg

+Ysc

-Zsc

Available for roll maneuver: 59.8 deg

Xsc

YscZsc

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

4

312 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 > 0.015 Hz is achievable. ∀

Method: Frequency domain analysis.

Results: Normal Mode (FIP, MAN sub-modes) + time delay

Jitter Conclusion

Required specification achievable. Given 0.0061 Hz cl-BW, Relative Accuracy: 47.20” + 2nd order LPF with 4 Hz sampling rate output: pointing error ~ 0.19”, for freq > 0.015 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:

STRASTAB SLO

B-dot control law B-dot control law B-dot control law (X,Z)

Sun acquisition control law (Y)

Wheel off-loading control law (Y)

automaticautomatic

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 Pros Cons

ASC direct output

deduced from q

accurate blinding, expensive

IRU deduced 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