Within the Bird project a small satellite was developed, which is equipped with infrared sensor technology to detect high temperature events on earth. DLR Simulation and Software Technology supported the DLR Institute of Space Sensor Technology and Planetary Exploration in this project with the software technical development of the satellite attitude control system.
The software technical development encloses the development and modelling of the software architecture with UML, the object-oriented implementation of architecture, and the control algorithms, and gives support for the test of the attitude control system. The BIRD project comprised the development of a micro-satellite with an infrared sensor for detecting high-temperature events on the ground. DLR Simulation and Software Technology is supporting the Institute of Space Sensor Technology and Planetary Exploration in developing the software for the satellite's attitude control system. This comprises the software architecture, the implementation, and the testing of the attitude control system.
The attitude control system is built by three software layers. The layer "command and housekeeping interface" provides the connection to other software applications running on the board computer of the Bird satellite. These are in detail the common application layer, the payload interface, and the onboard navigation system. The common application layer manages all applications on the board computer of the Bird satellite and provides interfaces for the commanding of applications and the data transmission over the telemetry line to the onboard computer applications. The onboard navigation system computes GPS based the satellite's orbit information and supplies the position of the satellite to the attitude control system. The payload interface provides the interfaces between the board computer and the payload computer system of the Bird satellite. The attitude control system sends via the payload interface the satellite position and attitude data to the payload. Furthermore this layer establishes a monitoring of the two subordinated layers of the attitude control system and initiates autonomously the appropriate handling of recognized malfunctions in order to assume further operations of the satellite.
The layer "estimation prediction control" contains the heart of the attitude control. The control is executed every 500 ms in four steps. In the first step a state machine controlled by the current status data and the commanded target attitude reconfigures the modules of the next three steps of the layer. This reconfiguration only happens if a reconfiguration was not forbidden by previous ground commands. In the step called estimation, the current attitude state is calculated from the sensors measured attitude. This calculation covers a plausibility check and the selection of trustworthy sensor data with a following filtering of the sensor data. In the step called prediction, the attitude is calculated after 500 ms from in the prior step estimated attitude and the self-dynamics if no control activities takes place. In the last step of a control cycle, the step control, the desired control torque is computed for the two actuator systems from the current attitude and in accordance with the commanded target attitude and the boundary conditions.
The lowest layer of the attitude control system is the layer low level actuator and sensor processing. This layer establishes the interfaces to the sensors and actuators needed for the attitude control. The attitude control system possesses four kinds of sensor types. These are a magnetic field sensor and two sun sensor systems as non-intelligent sensors coupled over an A/D interface to the main board computer. Furthermore one laser gyroscope and two star sensors as intelligent sensors are used. The laser gyroscope is coupled over a communication bus with the board computer and sends with a rate of 5 ms a packet with information for the rotation of the satellite. These data packetes are consolidated by the attitude control system to 500 ms cycles. Both star sensors are connected by two communication busses to the board computer. The attitude control system drives both star sensors in exact timing with two state machines and queries cyclically the attitude data from the cameras. As actuators four reaction wheels in a tetrahedic arrangement and a magnetic coil system with six coils are available. All actuators are coupled with the board computer over two communication busses. The lowest layer determines the necessary commanding for the actuators from the torques computed by the layer estimation prediction control. It performs the time critical communication with actuators and determinates the state of the actuators. For communication with both star sensors and the actuators over two communication busses the lowest layer provides a
The Bird satellite was launched successfully on 22.10.2001 at 04:53 UTC with an Indian PSLV-C3 launch vehicle from the SHAR Centre at Sriharikota together with the ESA satellite PROBA and the Indian satellite TES. The attitude control system works since the start as intended without larger incidents. In case of disturbances caused by bad space weather the hard mounted solar panels are intentionally oriented by the Safe Mode of the attitude control system to the sun. This will keep battery power high. The BIRD satellite is at present in nominal operation controlled by the GSOC at Oberpfaffenhofen and the DLR organization Optical Information Systems located at Berlin. Necessary changes to the attitude control software to increase the performance are supported by DLR Simulation and Software Technology and also the software configuration management for the attitude control software is accomplished.
DLR Optical Information Systems
(former Institute of Space Sensor Technology and Planetary Exploration)
01.03.2000 - 31.12.2002