Acta Astronautica Vol. 46, Nos. 2-6, pp. 321-333, 2000 Q 2000 Elsevier Science Ltd. All rights reserved

Printed in Great Britain PII: SOO94-5765(99)00227-l 0094-5765/00 $ - see front matter

CHAMP ATTITUDE AND ORBIT CONTROL SYSTEM

Christian Schmitt DaimlerChrysler Aerospace, Jena Optronik GmbH

07745 Jena, Priissingstr. 41 Hans Bauer

Fachhochschule Heilbronn,University of Applied Sciences 7408 1 Heilbronn, Max-Planck-Str. 39

ABSTRACT

The small satellite mission CHAMP was initiated by the German Aerospace Center (DLR) as a lead project for the East German space industry in order to consolidate their national and international position. After a short survey over the mission goals the main requirements on the Attitude and Orbit Control System (AOCS) of the satellite will be outlined. Furthermore the technical solution will be described which fulfils best the requirements on the AOCS. For that a functional block diagram is given, which shows the sensors and actuators in use. In the following the operational AOCS Modes and the sensor data processing up to the generation of actuator commands are described. Finally this paper outlines that the system is currently in the test status and that the system has been realised under strong cost and schedule constraints.

1. INTRODUCTION

CHAMP is an ambitious national satellite project for geoscientific applications. The mission goals and scientific experiments are defined by scientists of the GeoForschungsZentrum Potsdam (GFZ). The satellite will be developed by a company consortium under the leadership of Jena-Optronik GmbH. After completion of definition /specification phase CHAMP has entered Phase C/D in January 1997. The launch is scheduled for early 2000.

2. THE CHAMP SATELLITE AND ITS MISSION

CHAMP as a geoscientific mission with a multipurpose and complementary payload shall substantially contribute to one of the

0 2000 Elsevier Science Ltd. All rights reserved

basic research objectives of planet earth, that is, to the determination of the composition, structure, and dynamics of the solid planet, its oceans and atmosphere, and its surrounding envelope of charged particles and fields, especially l global earth gravity field mapping l global magnetic and electric field

mapping l atmosphere/ionosphere sounding 0 altimetry l orbit determination of LEO Satellites.

The satellite will have a life time of at least 5 years and will be launched in a 450 km orbit with an inclination of 87”.

CHAMP will fulfil the criteria of a small satellite mission, i.e. short development time and reduced costs through the usage of existing hardware, reduced quality standards and test effort and a protoflight development approach.

321

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Skin Connector

Small Satellites&w Earth Observation

Figure 1: Configuration of CHAMP Satellite with payload instruments.

2. REQUIREMENTS ON THE AOCS 4. AOCS BLOCK DIAGRAM

The scientific payload of CHAMP requires during nominal operation a three axes stabilised spacecraft with an Earth orientation. The z-axis of the body-fixed satellite system shall be Earth pointing. The x-axis is aligned with the orbit plane and is close to flight direction, whereas the y-axis completes a right hand system. During nominal operation attitude control shall be performed with a pointing accuracy of better than 5” and 0. lo/s for each satellite axis. In order to rise or decline the orbit height the satellite shall be capable to perform slew manoeuvres around the z-axis and shall employ special orbit control thrusters.

Figure 2 outlines that payload instruments will be used for Attitude and Orbit Control contrary to most satellite missions. The AOCS consists of:

A cold gas system with 12 thrusters with 20mN thrust each for attitude control and 2 thrusters with 40mN thrust each for execution of orbit manoeuvres. Three redundant magnetic coils, which produce control torques through interaction with the Earth Magnetic Field.

In case of reference attitude loss the spacecraft shall ensure a positive energy budget and a safe thermal control. This is performed by acquisition of the solar panels to the sun, whereas a coarse Earth orientation is maintained. The resulting disturbance torque, which acts on the payload accelerometer shall not exceed lOe-4 Nm. This is performed by using 6 pairs of thrusters with 2OmN thrust for each thruster. This design ensures force free actuator torques.

A Coarse Earth and Sun Sensor (CESS) which provides the measurement of the Earth and Sun vector in the S/C body fixed system with an accuracy of 2”. The appropriate accommodation of 6 heads provides an omnidirectional Field of View. Four Star Sensors i.e. Advanced Stellar Compass (ASC), which measure the inertial attitude of the satellite with respect to the S/C body axis with an

accuracy of 3arcsec (lo). A redundant GPS receiver for measuring position and velocity in the Earth Reference System WGS84 with an. accuracy of 60 m (lo) for the position measurement.

Small Satellites for Earth Observation 329

. A redundant Fluxgate Magnetometer (FGM) for measuring the Earth Magnetic Field in the sensor co ordinate system with an accuracy of 20nT

(1.0): .__................_........ .... .,,, .... ..._ .,,__ ,,_,,, ........_,, ___ _,_,,_,,_,

AOCS i ._.__.._.___.__...._..........,.............................................................

Figure 2: Functional AOCS Block Diagram

5. AOCS OPERATIONAL MODES

In order to ensure optimal payload operations and to cope with non-nominal situations the AOCS Modes as shown on Figure 3 have been introduced.

) Figure 3: AOCS Operational Modes (aoc-x: transition conditions)

5.1 Coarse Sun and Earth Pointing Mode (SEP)

The AOCS orients the Space Craft (S/C) z- axis towards the earth and in addition the y-z plane to the sun. Starting conditions can be any initial attitude and angular rates

of up to f3”/s. In case the measured sun vector is within a cone angle of 30” of the - z-axis the AOCS controller will disable the sun control in order to reduce the cold gas consumption. Attitude control is based on CESS measurements. This AOCS mode is the default mode after any On Board Computer start up (incl. after separation from the launcher).

5.2 Coarse Earth Pointing Mode (CEP)

Upon Telecommand the AOCS orients the S/C z-axis towards the earth, starting from any initial attitude and angular rates of up

to +3”/s. Rate Control is performed around z-axis. Attitude control is based on CESS measurements.

5.3 Rate Damping Mode (RDM)

The objective of this mode is to damp the spacecraft angular rates starting from up to +6”/s in all axes to the lowest level possible in order to achieve conditions allowing an ASC and GPS lock-in. Rate control is based on FGM measurements. This mode shall be entered in case monitoring and control of housekeeping data shows, that SEP is malfunctioning to a degree which is decided to be unacceptable, especially if the rates in SEP are increasing.

5.4 Nadir Transition Mode (NTM)

The objective of this mode is to perform the transition from RDM to FPM. This

330 Small Satellites for Earth Observation

mode will be selected by ground control, if recovery of the SEP Mode is not possible.

5.5 Fine Pointing Mode (FPM)

The satellite supports full payload operation in this mode. The reference attitude during FPM is such, that the y-axis (pitch) is perpendicular to the orbit plane and the x-axis (roll axis) is close to the flight direction and the z-axis (yaw axis) is parallel to the nadir direction. This mode has three major objectives:

attitude control with a pointing accuracy of better than 5” and O.l”/s for each satellite axis

slew manoeuvres about z-axis delta-V manoeuvres for increasing or decreasing the orbit, while controlling the attitude as mentioned before.

5.6 AOCS Mode Transitions

The following table defines with respect to figure 3 the transitions between the AOCS

ID

aoc- 1

aoc-2

aoc-3

aoc-4

Transition Conditions

b Default after Power-on Reset (On Board Computer Switch- on, re-boot, etc.)

D Normal Command by ground

b Normal Command by ground

D Normal Command by ground (As a pre-condition fine attitude measurement with respect to the Earth Orientation must be available) z

B Autonomous transition. An autonomous transition will be performed if it is enabled by ground and the attitude measurement with respect to Earth is available.

aoc-5

aoc-6

aoc-7

aoc-8

aoc-9

l Normal Command by ground

or l No valid attitude measurement

with respect to Earth Orientation for more than e.g. 2oos&r

l Loss of attitude

l Normal Command, if SEP (or CEP) is not working g

l Satellite rates exceed the limits for SEPKEP (default limit or limit set by ground command)

l Normal Command a

l Satellite rates below the limits for SEP (default limit or limit set by ground command)

l Normal Command when ASC and Orbit Propagator provide valid data

l Normal Command ok

l Autonomous transition, if Nadir and S/C z-axis coincide better than 30” for e.g. 200 cycles

Table 1: Transition between AOCS Modes

6. SPECIFICATION OF AOCS MODES

The following table on the next page gives a survey on the functional performance of the AOCS Modes and the usage respectively the constraints of the actuators and sensors in the different AOCS Modes.

Small Satellites for Earth Observation 331

sub-

v&&e ZEP

SEP

RDM

NTM

FPM

Function

Initial Condition

the SIC angular rates are c 3”ls in each axis (e.g. after separation)

the S/C attitude may be any (e.g. after separation)

m the S/C angular rates are c 3”/s in each axis (e.g. after separation)

l the S/C attitude may be any (e.g. after separation)

l the S/C angular rates are < 6”/s in each axis (e.g. after separation)

m the S/C attitude may be any (e.g. after separation)

l S/C angular rates < O,Y/s

l the attitude may be

any l ASC and Orbit data

available

l Same as Steady State for SEP (FPM controller requires < 30” and rates < 0. lo/s

9 Fine attitude data must be available (derived by ASC plus orbit position)

erformance

Steady State

Damping of S/C angular rates to < O.Y/s per axes (generally < 0.1 O/s is achieved)

Earth acquisition

Coarse Earth pointing of SIC z-axis (x- and y- axes free) with 15” (lo) per axis

Damping of S/C angular rates to < O.Y/s per axes (generally < 0.1 “Is is achieved)

Earth acquisition

Coarse Earth pointing of S/C z-axis with 15” (lo) per axis (tbc).

Coarse sun pointing of y- z-plane (if sun presence and sun vector is outside a cone of 30” from -z axis) with 15” (10) for 60% of sun presence (tbc).

1 Damping of S/C angular rates to < 0.5”/s per axes

1 The S/C attitude may be

any

1 End condition is: angle between nadir and S/C z-axis < 30” for 200 set

1 Control of S/C angular rates to < 0.1 O/s per axes

1 Fine attitude control to < 5” each axis

) Slew maneuvers on TC around yaw (180” f 10’) and pitch (*lo”)

T Equipmf

Sensors

1 Coarse Earth & Sun Sensor

1 FGMlor2

) Consider boom status: FGM transformation matrix must be selected accordingly

) Coarse Earth & Sun Sensor

1 FGMlor2

b Consider boom status: FGM transformation matrix must be selected accordingly

) FGMlor2 1 Cold gas system I

) Consider boom attitude thrusters

status: FGM 1 Consider boom status: transformation Thruster no. 5 disabled matrix must be as long as boom selected accordingly stowed

) same asFPM

b GPS (nominal or redundant) or on- board orbit predictor

m Advanced Stellar Compass, at least one head delivering valid data

B Fluxgate Magneto- meter (nom. or red)

mt Required

Actuators

Cold gas system / attitude thrusters

Consider boom status: Thruster no. 5 disabled as long as boom stowed

Magnetorquer (nom. or red.) however not as long as boom is stowed

Cold gas system / attitude thrusters

Consider boom status: Thruster no. 5 disabled as long as boom stowed

Magnetorquer (nom. or red.) however not as long as boom is stowed.

1 Magnetorquer (nom. or red.) however not as long as boom is stowed.

Cold gas system / attitude thrusters

Consider boom status: Thruster no. 5 disabled as long as boom stowed

Magnetorquer (nom. or red.)

) Cold gas system / attitude thrusters

1 Magnetorquers

) Cold gas system / orbit control thrusters are activated for orbit manoeuvres only

1

Table 2: Specification of AOCS Modes

332 Small Satellites Jbr Earth Observation

7. PROCESSING OF SENSOR DATA Sensor data pre-processing is performed, where the validity of the sensors

The following figure shows the handling of measurement will be proofed, using sensor data processing in FPM with respect plausibility checks and “Health Data” of to their measurement co-ordinate system the sensors itself. up to the generation of actuator commands.

ASC Measurement

,

of Epoch 2000 is set to TDS; failure negligible

GPS Measurement in Ground update WGS84 System ; in TDS System

’ Transformation of GPS- ‘, Measurement from WCiS84 ;

L. to TDS with Tiw ,I’

Computation of Tbi, which describes the trans- formation from TDS

GPS Measurement in TDS

~L!Y!

On-Board 54 Esttmarod Propagator Computation in TDS

b ~ r -- ~----

Computation of Tie, which descrtbes the tram formation from earth oriented system to TDS ~~_-~...-.___

set-point command in control system / _ with respect to earth-oriented svstem

FGM Measurement in Sensor System

body-fixed system

Computation of Tee, which describes the trans- formation from control-

between body fixed and ce _. _

AOCS controller computations are performed in the body-fixed-system; Dependant on set point, the inertial position/velocity and the current inertial attitude the control angles in the body-fixed system can be computed by using Tbc=Tbi*Tie*Tec, which describes the transformation from control to body-fixed system.

/ Output of controller thruster ON-time commands current commands for magnetorquer

+ where TDS: True of Date System (inertial)

Figure 4: Functional Handling of AOCS Input Data and their respective Co-ordinate Systems

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8. DEVELOPMENT STATUS

Performance Analyses for the different operational AOCS Modes have been performed. The control Software has been integrated on the target system. The AOCS relevant H/W has been delivered and will be integrated currently on the satellite.

Earth Observation 333

Subsystem testing of the AOCS has been started end of February 1999 within strong cost and schedule constraints. First results of closed loop testing show that a stable and robust AOCS system has been designed and developed which meets the functional requirements on the AOCS very well.