On Board Command and Data Handling
The command and data handler forms the brain of the STUDSAT. It is responsible for controlling the satellite from the time it is ejected into the space. The command and data handling unit STUDSAT consists of a central controller to control all the subsystems of the satellite. The controller that will be used for this purpose is a 32 bit RISC controller with features like; very low power consumption and DSP instruction set. Some of the major objectives of the OBC&DH are as listed below:
1. Antenna Deployment: As the satellite is ejected into the orbit, the first major task is to deploy the antennas. This task is initiated the OBC&DH subsystem. The Onboard computer commands the antenna system to deploy, so that that satellite can successfully establish a link with the ground station.
2. Stabilize the satellite in the orbit: Once the satellite is ejected into the orbit, the satellite will be highly unstable due to the various forces acting on it. Hence the satellite's first task is to get stabilized and orient the antennas towards the earth. This process is called the attitude determination and correction (ADC) and it is carried out by the OBC.
3. Establish a communication link with ground station: when the satellite is stable, it should be in a position to communicate with ground station. Hence after attaining stability, the satellite will wait till it reaches the communication window and start establishing a communication link with the ground station. The communication protocol used for communicating with the ground station is the modified version of the AX.25 protocol and frames used are the AX.25 UI frames. The OBC&DH system is responsible for the implementation of the complete communication protocol.
4. Monitor the health of the satellite: It is very important to monitor the health of the satellite and coordinate the activities in the satellite so that no subsystems eats away all the power and the satellite is left with no power to perform any further tasks. So the OBC performs the task of monitoring the health of the satellite by acquiring the data from various sensors and comparing their values with the previously set thresholds. Based on the result of the comparison appropriate actions are taken.
5. To capture images of the earth: The main objective of the satellite is to capture the images of the earth. This function is performed by the camera placed in the satellite. The actions of the camera are completely controlled by the OBC. The captured image is stored onboard in a parallel FRAM and is transmitted to the ground station as the link is established.