Attitude Control System(Observing Spacecraft)

Katsuhiko YamadaDepartment of Aerospace Engineering

Nagoya University

Spacecraft Attitude

太陽同期軌道

軌道面と太陽の方向のなす角がつねに一定

昼間 夜間

Sun-synchronous orbit

JAXA

day night

Angle between the orbital plane normal and the direction to the sun is constant

zBT

yBT

xBT

地球

ロールφ：進行方向まわり

ピッチθ：軌道面法線まわり

ヨーψ　：地球方向まわり

衛星の姿勢角の定義

Roll Φ: around direction of flightPitch θ: around normal vector of orbital planeYaw ψ: around direction to the Earth

Definition of Spacecraft Attitude

後藤，市川：人工衛星の三軸姿勢制御，宇宙開発事業団報告TR-22，1988年12月

Earth

Active Attitude Control of Spacecraft

Feedback control of spacecraft attitude toward some direction

JAXA

Zero-momentum stabilization

Wheel

Gas Jet

Attitude Control System of Spacecraft

Attitude control logic Actuator Spacecraft

Sensor

Target value of attitude

control

torque

AttitudeAngular velocity

+

－

Detect attitude by sensors and give control torques by actuators

Method Contents

Gravity-Gradient Stabilization

Passive control to utilize gravity-gradient torque

Spin Stabilization Passive control to utilize spinning effects (gyro torque)

Bias-momentum Stabilization

Semi-active control to utilize wheel control torque and gyro stiffness

Zero-momentum Stabilization

Active control without gyro stiffness

Attitude Control Methods

Gravity-Gradient Torque

宇宙開発と設計技術, 大河出版, 1982

Gravity

Resultant of centrifugal force : acts on center of massResultant of gravity : acts on center of gravityThis difference causes gravity-gradient torque

Center of Gravity

Resultant of Centrifugal Force

Resultant of Gravity

Centrifugal Force

Centrifugal Force

Center of Mass

Gravity

Earth

Gravity-gradient Torque

Sensors and Actuators

Type Outline

Earth Sensor Detect relative angles of spacecraft (roll and pitch) with respect to the Earth

Sun Sensor Detect relative angles of spacecraft with respect to the Sun

Star Sensor Detect spacecraft attitude in the inertial coordinates by finding fixed stars

RF SensorDetect relative angles of spacecraft (roll and pitch) with respect to the Earth by radio wave from the Earth

Gyro Compass Detect relative angle yaw with respect to the Earth by utilizing orbital motion

GPS Detect spacecraft position (spacecraft attitude in some cases)

Rate Gyro Detect spacecraft angular velocity in the inertial coordinates

Spacecraft Sensors

Arrangement of GPS spacecraft

Kinematic positioning（attitude detection）

Point positioning

24 spacecraft, altitude 20200km, orbital period 12hours

GPS

Type Outline

Thruster Inject gas such as hydrazine from nozzle

Wheel Utilize reaction torque of disk rotation

CMG(Control Moment Gyro)

Utilize gyro stiffness of momentum wheel

Magnetic TorquerGenerate torque by interaction of electromagnet and the Earth magnetic field

Spacecraft Actuators

Spacecraft Attitude Control by Wheels

Spacecraft

Wheel Accelerate wheel in the direction of disturbance torque

Spacecraft attitude is recovered by reaction torque of wheel rotation

Control spacecraft attitude by the reaction torque of wheel drive torque

Disturbance Torque

Spacecraft attitude is affected by disturbance torque

宇宙開発と設計技術, 大河出版, 1982

磁気トルカによるトルクの発生

北極（Ｎ）

南極（Ｓ）

Ｎ

Ｎ

Ｓ

Ｓ

TM

TM

BE

BE

TM M BE

Torque by Magnetic Torquer

後藤，市川：人工衛星の三軸姿勢制御，宇宙開発事業団報告TR-22，1988年12月

North Pole (N)

South Pole (S)

Type Outline

Solar Pressure Torque generated by solar radiation pressure

Gravity GradientGravity-gradient torque becomes disturbance when equilibrium point differs from the desired attitude.

Earth Magnetic FieldTorque by interaction between magnetic moment in spacecraft and the Earth magnetic field

Aerodynamic Force Force generated by airflow around spacecraft

Spacecraft Disturbance

1m1m

2m5m

10-1

10-2

10-3

10-4

10-5

10-6

10-7100 1000 10000 100000

衛星の高度 [km]

外乱

トル

ク[Nm]

Disturbance Torque Example

後藤，市川：人工衛星の三軸姿勢制御，宇宙開発事業団報告TR-22，1988年12月

Dis

turb

ance

tor

que

[Nm

]

Spacecraft altitude [km]

Earth magnetic torque

(Residual magnetic moment M=10-5 Wbm)

Geo

stat

iona

ry

Gravity-gradient torque (mass 500kg)

Solar pressure torque (Cross section 7m2, CP-CG distance 0.35m)

Aerodynamic torque (Cross section 7m2, CP-CG distance 0.35m)

Debris Observation

Debris

Spacecraft

Target Time

Attitude Maneuver

OrbitPropagation

Debris Imaging

Pleiades CNES，2011Launch，Resolution 0.7m

2012.4.15 Envisat ©EADS Astrium

Imaging Example

Optical Sensor Alignment

Optical Sensor Pointing Error

Orbit

GroundTarget

sensor axis

determined attitudedetermined position

true attitudetrue position

pointing error

sensor alignment error

attitude error

position error

Pointing error = position error + attitude error + misalignment

orbit

GCP 2

GCP 1

spacecraft

Earth-fixed coordinates (WGS84)

Inertially-fixed coordinates (J2000)

Body-fixed coordinates

Sensor-fixed coordinates

Ground Control Point (GCP)

GCP Example

©Astrium

Optical Sensor Alignment Estimation

GCPObservation

determined attitudedetermined position

true attitudetrue position

GCPEstimation

+

－

AlignmentEstimation

GCP error

GCPEstimation

determined attitudedetermined position

Estimated alignment

GCP observation

+GCP error

sensor misalignment = constant + variation with

orbital period

Estimation

Verification

Alignment model

－