# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 7 Attitude and Orbit Control System of the FireBIRD Satellites
### The Attitude and Orbit Control Subsystem of the FireBIRD Satellites  
This subsection will focus on the attitude control system and will mention the orbit control system only if necessary. The satellite's motion and orientation directly influence the generation of the remote sensor data as well as the off-line processing of the sensor data to obtain defined data products. The processing of data from level 0 (raw data) to higher levels requires auxiliary information from the AOCS such as position, orientation, and time. The used AOCS coordinate frames have to be connected to the sensor's internal frames by a \"geometric calibration\" allowing for geo-coding, which is usually performed during the off-line data processing on the ground. During the data acquisition cycles conducted by the FireBIRD multi-spectral camera systems, their Lines of Sight (LoS) must be directed to the Earth's center, which is secured by the so-called Earth-pointing attitude mode of the AOCS. To perform this attitude mode and further main attitude modes, briefly described in Table 4, the AOCS uses different hardware devices: sensors and actuators. Figure 16 gives an overview of these devices and their connections to the BIROS Satellite Bus Computer (SBC). The sensors of the BIROS AOCS are:  
* two redundant Autonomous Star Cameras (ASCs), i.e., the \"Star Sensor Systems\" shown in Figure 15;
* two redundant Inertial Measurement Units (IMUs);
* two redundant flux-gate Magnetic Field Sensors (MFSs);
* four redundant Coarse Sun Sensors (CSSs); and
* two redundant Geo Positioning System (GPS) receivers. The Autonomous Star Cameras (ASCs) and the Inertial Measurement Units (IMUs) provide the precise AOCS orientation data in the body-fixed coordinate system with an accuracy of 24 arcsec and 5 arcsec for TET-1 and BIROS, respectively. The flux-gate Magnetic Field Sensors (MFSs) and the Coarse Sun Sensors (CSSs) provide the coarse AOCS orientation data used, for instance, to align the -Z-axis of the satellite to the Sun to secure the best illumination of the solar arrays. The actuators of the BIROS AOCS are:  
* four Reaction Wheels (RWs); and
* three redundant Magnetic torquer Coils (MCs). \begin{table}
\begin{tabular}{c c} \hline
**Attitude Mode** & **Brief Characterization and Function** \\ \hline Inertial pointing mode & Align the satellite with an inertial frame \\ \hline Earth pointing mode & Align the satellite with an orbital fixed frame \\ \hline Target pointing mode & Align a satellite axis with the direction \\  & to a fixed target on the ground \\ \hline Sun pointing mode & Align a satellite axis with the direction to the Sun \\ \hline Client observation mode & Align a satellite axis with the direction to another spacecraft \\ \hline \end{tabular}
\end{table} 
Table 4: Main attitude modes of the FireBIRD satellites. The Reaction Wheels (RWs) and the Magnetic torquer Coils (MCs) allow us to turn and to stabilize the orientation of the satellite in a preselected direction in the body-fixed coordinate system with an accuracy of 30 arcsec. Comment: 5 arcmin = 1/12 angular degrees is approximately one-sixth of the angular extension of the Sun or Moon seen in the Earth's sky. The following Table 5 summarizes the abbreviations give in Figure 16.  
The different clocks used by the AOCS and by the remote sensing image sensor have to be linked in a defined way. The FireBIRD satellites TET-1 and BIROS use some additional status information, such as eclipse state, the availability of AOCS devices during the data take, and the Sun's direction. In combination with the time-stamped remote sensing data,  
Figure 16: Overview of the BIROS AOCS subsystem.