Attitude Control of the Ørsted Satellite

The satellite main body during
magnetic calibration.

The Danish Ørsted Geomagnetic Research Satellite is launched in 1998 from Vandenberg, California into a polar low Earth orbit. The scientific mission during the 14 months mission is to investigate the Earth's magnetic field and the influence from the Solar wind. The satellite is named after the Danish physisist Hans Christian Ørsted that found the relationship between an electric current and magnetism.

The department of Control Engineering is responsible for the entire satellite attitude control system (ACS) and this page describe mainly ACS activities. A list of other participants in the project is available below.

Objectives

The purpose of the Ørsted ACS project is to develop a complete attitude determination and control system for the 65 kg microsatellite. This involves algorithm development, software design, implementation, integration, and test. For the purpose of final tests and validation of the theoretical approaches, a mechanical simulation facility (SatLab) has been constructed.

Description

The satellite carries as the primary scientific payload a three-axis fluxgate magnetometer and a star camera for measurements of the geomagnetic field. The position is acquired with Global Positioning System (GPS) receivers. The orbit is inclined 98 degrees from the earth's equator resulting in precession of the orbit plane relative the direction to Sun. This precession allows a mapping of almost the entire globe as the Earth rotates.

The satellite main body carries electronics and an eight meter boom hosts the scientific instruments. Stabilization of the Ørsted satellite is accomplished by active use of a set of mutually perpendicular electromagnetic coils (magnetorquers). Interaction between the Earth's magnetic field and the magnetic field generated by the magnetorquers produces a control torque.

Aurora. Picture by
Børre Holmeslet, Norway



Ørsted with boom deployed and at 800 km above Denmark

Ørsted Operational Properties
Orbit	Inclination Apogee altitude Perigee altitude Orbital period	96.1 degrees 850 km 450 km 100 minutes
Attitude	Stabilization Control Orientation	Three axes stabilized Gravity gradient and magnetorquers Local orbit plane
Size	Main body Boom	45 x 34 x 68 cm 6.0 + 2.0 m
Mass properties	Total mass Moments of inertia, boom stowed Moments of inertia, boom deployed	60.7 kg X: 3.17 Y: 3.61 Z: 1.53 X: 181.8 Y: 181.3 Z: 1.53
Communication	Down link Up link Ground station in Denmark Mean communication time	2039.5 MHz 2215 MHz 56^o N 12^o E 73 minutes/day
Power	Solar cells on satellite body Maximum power Batery capacity	1.1 m² 60 W 6 Ah

Hardware Development

In order to estimate the satellite attitude relative to the Sun a coarse Sun-sensor has been developped. The development process includes basic design, manufacturing, and detailed test. Every single sensor is tested according to specifications to verify their ability to withstand the demanding space environment.

Attitude Control System Design

The ACS development has been divided into three subsystems: Supervisory control, Attitude Estimation, and Attitude Control. These independent tasks are described in the following sections.

Supervisory Control

The satellite will be left without ground contact for long periods. This demands for on-board autonomy. The control system must be able to handle different operational phases, reconfigure between several sensor and actuator configurations, and close down in case of severe faults.

The augmented control system is designed following a three layer structure. The bottom layer comprises all control configurations, which have been implemented and tested individually. An inference machine is implemented in the top layer to manage the bottom layer and handle communications with the satellite data management system. The supervisor rule base is designed using computer aided tools that guarantees completeness and consistency.

Attitude Estimation

Small satellite attitude determination has traditionally been based on single-frame solutions that offer simple robust attitude estimation but are not easily adapted to include fault tolerance or periods of incomplete observability. The Ørsted attitude estimation approach is based on extended Kalman filtering of magnetometer and sun sensor readings in combination with a star camera filter. The attitude is estimated for onboard attitude control purposes and the precision requirements are limited.

Supplementing star camera attitude estimates with attitude estimates based on magnetometer and Sun sensor data increases fault tolerance and estimates are provided even during anomalous periods (ei. camera blackouts, eclipse or sensor faults). Combined with detection algorithms the attitude determination algorithms introduces a significant degree of autonomy since fault can be handled without ground interaction.
Results from computer simulations integrating models of the sensors, the spacecraft, and its on-orbit environment demonstrate a projected performance capability of 0.1 degree in accuracy.

Attitude Control

Magnetic control systems are relatively lightweight, require low power and are inexpensive. These were the main reasons to suggest this actuation principle for the Ørsted satellite mission in early phases when a spin-stabilized mission, i.e. two-axis control was foreseen. Later redefinition of the scientific objectives required an alternation of the control requirements to three-axis stabilization. The actuation principle, however, only provide controlability in two degrees of freedom because the control torque can only be generated perpendicular to the local magnetic field of the Earth.The control system design has employed Lyapunov stability theory and control theory for periodic systems. As a result, three control principles has been developed:

Detumbling of the initial random tumbling after release from the launcher, based on Lyapunov theory,
Linear quasi optimal three-axis stabilization for normal operational phase (science observation),
Energy based approach for boom upside down recovery.

Project Staff

The Ørsted project is a collaboration between universities and space companies in Denmark. The project office resides at CRI in Birkerød. Below is a list of principle participants, both at AAU and the other institutions.

AAU Department of Control Engineering
Mogens Blanke, Prof., Ph.D.	Manager
Rafal Wisniewski, M.Sc.EE, Ph.D	Attitude Control Algorithms
Thomas Bak, M.Sc.EE	Attitude Determination
Søren Abildsten Bøgh, M.Sc.EE	Supervisory Control
Gitte Madsen, M.Sc	Academic Administrator

Main External Partners
Industry	CRI A/S Danish Meteorological Institute Terma Elektronik A/S Per Udsen Co
Danish Institutes	Danish Space Research Institute Niels Bohr Institute Danish Technical University
International	NASA Goddard Space Flight Center Jet Propulsion Laboratory CNES (France) ESA IABG

Related Publications

Bøgh S.A.: Fault Tolerant Control Systems - A Development Method and Real-Life Case Study. Aalborg University. Ph.D. Thesis 1997. Department of Control Engineering, Denmark.
Bøgh S.A. and Blanke M.: Fault Tolerant Control - A Case Study of the Ørsted Satellite. Proceedings of Fault Diagnosis in Process Systems. Colloquium in York, April 1997.
Wisniewski R.: Satellite Attitude Control Using Only Electromagnetic Actuation. Aalborg University. Ph.D. Thesis 1997. Department of Control Engineering, Denmark.
Bøgh S., R. Wisniewski, T. Bak: Autonomous Attitude Control System for the Ørsted Satellite. IFAC Workshop on Control of Small Spacecraft, Breckenridge, USA, 5. February 1997.
Bak T., R.Wisniewski, and M.Blanke: Autonomous Attitude Determination and Control System for the Ørsted Satellite . 1996 IEEE Aerospace Application Conference. Colorado, Feb.1996.
Bak T., M. Blanke, S.A.Bøgh, and R. Wisniewski: Dependable Attitude Control for the Ørsted Satellite . Poster session, EURACO Young Researchers Week, Dublin, Aug. 1994.
Wisniewski R., M. Blanke: Three-axis satellite attitude control based on Magnetic Torquing . 13th IFAC World Congress, San Francisco, USA, June 30-July 5, 1996.
Bak T.: Onboard Attitude Determination for a Small Satellite. In proceedings 3rd ESA International Conference on Spacecraft Guidance, Navigation and Control Systems, Noordwijk, 26-28 November 1996.
Bak T., M. Blanke, S.A. Bøgh, and R. Wisniewski: Dependable Attitude Control for the Ørsted Satellite. Poster session, EURACO Young Researchers Week, Dublin, Aug. 1994.
Bøgh, S.A., Roozbeh Izadi Zamanabadi, Mogens Blanke, "Onboard Supervisor for the Ørsted Satellite Attitude Control System", Proceedings to ESTEC 5th Workshop on Artificial Intelligence and Knowledge Based Systems for Space, pp.137-152. ESTEC, Nordwijk, The Netherlands, 10-11 October 1995.
Bøgh, S.A and M. Blanke, "Fault-Tolerant Control - a Case Study of the Ørsted Satellite", IEE Colloquium on Fault Diagnosis in Process Systems, York, UK, April 1997

Software in The Project

The Ørsted attitude control flight software is integrated into a fully atonomous real time environment. This demands a high level of security and robustness. This has been achieved using the object oriented design method HRT-HOOD (Hard Real Time - Hierachical Object Oriented Design), which guarantees a structural approach to robust real time software development. The software is coded in Ada because it supports the development of software with intergrated real time kernels where reliability is crucially important.