## 3 System Design and Platform Subsystems
### Attitude Determination and Control Subsystem (ADCS)  
The ADCS is the subsystem that is responsible for controlling the pointing of the satellite throughout the mission lifetime including de-tumbling the satellite after deployment, pointing the satellite as per the mission requirements and recovering it from any spin-ups. Given that the spectrometer used as the primary payload of the mission has a Field of View (FOV) of 0.15 degrees and to enable relatively accurate detection of GHG sources, a pointing accuracy of less than 1 degree is required. This requirement means that the passive attitude control mechanisms using magnetorquers usually used in CubeSats is not enough. Using reaction wheels and accurate attitude determination sensing (such as star sensors) would be required.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Payload** & & \\ \hline  & SXGA & VGA \\ RBG camera & 1,638,400 bytes & 384,000 bytes \\ \hline Spectrometer (10 images) & 5130 bytes & 5130 bytes \\ \hline
**Total** & **1,643,530** & **389,130** \\ \hline \hline \end{tabular}
\end{table}
Table 1: Data Volume Generated by Mission Payloads.  
Figure 6: MeznSat mechanical structure.  
The ADCS used for this mission is a CubeADCS 3-Axis integrated bundle from CubeSpace with a flight heritage of more than ten years (with the exception of the start tracker which has no flight heritage) [18]. Figure 7 shows an overview of the ADCS architecture and Figure 8 shows images of the ADCS hardware. The ADCS has three-axis control capability using three Reaction Wheels (with maximum momentum of 1.77 mNms and a maximum torque of 0.23 mNm), in addition to three magnetorquers (to be used mainly for desaturation). The sensor set includes a deployable magnetometer, two CMOS cameras (one used for fine sun sensing and the other for horizon detection), ten coarse sun sensors and a star tracker based on ARM Cortex-M3 architecture.  
Figure 9 shows the complete set of ADCS modes for MeznSat. Due to the injection from the deployer, it is expected that MeznSat would experience rotation rate up to 10 deg/s. \"No Control Mode,\" where the ADCS does not perform active control, as per the CubeSat specifications document [11], which states that no operations need to be performed by the satellite for the first 30 min after deployment. Next, MeznSat will be controlled to move into \"De-tumble/Attitude Acquisition Mode.\" The purpose of this mode is to reduce the rotation rate of MeznSat from 10 deg/s (worst case) to a slower angular rate of <3 deg/s. This will be followed by magnetometer deployment when ground contact with MeznSat has been established. As a result of the deployment, the Y-axis angular rate will increase. Y-Thomson  
Figure 8: Integrated CubeADCS (**left**) and star tracker (**right**).  
Figure 7: MeznSat Attitude Determination and Control Subsystem (ADCS) architecture.  
Controller will reduce X-axis and Z-axis angular rate and control Y-axis angular rate to reach \(<\)1 deg/s. During payload operation, XYZ-reaction wheel mode will be operational leading to decreasing the angular rate to \(<\)0.03 deg/s.  
Changes between one mode to another are done autonomously (no ground intervention needed, except the magnetometer deployment and Y-Thomson spin). Exceptions are made for the satellite to recover from \"Safe Mode\" to any other mode depending on the satellite's angular rate only when the satellite receives commands from the ground station.