This project is an On Board Computer board for the ISTNanosat-1, a small Portuguese CubeSat. The main goals of the system are to collect data that allow to determine and control the satellite’s attitude and to monitor and control the remaining subsystems. The hardware design has taken into account the overall objectives of the system and is divided into several modules, as depicted in the block diagram.
| {gallery} LogicalDiagram |
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One of the first things to consider is the sensors required by the satellite. The purpose of the inertial sensors is to determine the satellite’s relative position to a coordinates system, based on an Inertial Measurement Unit (IMU), using existing MEMS sensors in the market. Combining a magnetometer, an accelerometer and a gyroscope, with 3-axis each, creates a system with 9 DOF. The SPI protocol was chosen to connect these sensors, because it is oriented towards centralized communications. However, to be able to determine the satellite’s location in its orbit, it necessary to add an absolute positioning sensor. This requires using a GNSS that communicate with this module by an asynchronous serial communication protocol (UART). Since this system will host the housekeeping task of the whole satellite, it is necessary to ensure that it communicates with other subsystems. It was decided to use the I2C protocol because it allows to address various subsystems and perform bi-directional communications, requiring only two lines (data and clock). For redundancy, two rails are used, and different physical hardware drivers. To control the satellite’s attitude, it will be used magnetorquers controlled by PWM signals. Its drivers will be on the photovoltaic panels. The system also have two connectors for expansion: one for the Beacon module and another for a GMSK modem.
One big concern in this project, which was improved in this review, was the power buses, which are common to all subsystems. For example, if there is a short circuit, none of the other subsystems will be powered. To minimize the impact of this topology, it was decided that each subsystem will have a protection circuit that will measure the current draw. If this current exceeds the normal consumption, it automatically turns off the power. Regarding of this mechanism, it also added an independent power regulator directly from one photovoltaic panel.