# Interplanetary Student Nanospacecraft: Development of the LEO Demonstrator ESTCube-2
## 2 ESTCube-2 Mission
### Satellite Design
#### 2.3.2 Avionics Stack Systems
Depending on the activity and current amplitude for a given voltage level, the MPB voltage is converted by switching or linear regulators. A switching regulator on the battery management PCB provides 5 V for the high-speed communication system (HSCOM). Two low-dropout regulators connected in parallel provide 5 V to the CGP payload. The power budget of ESTCube-2 is visible in Tables 15 and 16 in Appendix B. The ESTCube-2 _communication system (COM)_ communicates between the ground station (GS) and the satellite. The system consists of two hot-redundant subsystems--primary communications (PCOM) and a backup subsystem called secondary communications (SCOM), housed on the same PCB. PCOM can receive and transmit signal on the radio amateur 70 cm band, and SCOM can receive on the radio amateur 2 m band and transmit on the 70 cm band, using AX.25-encapsulated data packets. SCOM serves the additional purpose of disabling the satellite's transmission ability upon receiving an appropriate telecommand. This requirement comes from the international amateur radio union, as ESTCube-2 transmits on radio amateur frequencies [44]. The PCOM system is tuned at the radio amateur 70 cm band with a planned frequency of 435.8 MHz and 9600 to 19,200 bit/s variable data rate. Its processing unit is an STMicroelectronics STM32L4 series MCU, and RF connectivity is achieved with a Silicon Labs Si4463 transceiver. SCOM is primarily used as a secondary receiver subsystem and uses a radio amateur 2 m band. The data processing unit is a Silicon Labs EZR32 WG330 series MCU with a built-in RF transceiver module. For transmission, the subsystem can change its carrier frequency to the 70 cm band and use PCOM's RF path to transmit the signal as a backup transmitter. The link budgets of PCOM and SCOM are visible in Tables 15-16 in Appendix C.  
The _onboard computer (OBC)_ coordinates the operations of all systems and payloads and runs the AOCS algorithms using an STM32F7 series MCU. The MCU features 512 kB of static random-access memory (SRAM) and 2 MB flash memory for housekeeping and telemetry data storage. The system can monitor the spacecraft's overall health, mitigate identified failure scenarios, schedule telecommands for execution at a specific date and time, collect housekeeping and experiment data from payloads and other systems, perform data compression, and store the data in its onboard memory. At least two images of firmware are stored on the processor for redundancy. In addition, the system is capable of full firmware and delta updates, including an update roll-back mechanism for safety. The OBC is central to all internal communications; the avionics stack is connected via three internal communication protocol (ICP) buses, and the payloads are connected via two CSP buses (see Figure 9). Although the OBC has a real-time clock (RTC), it has no uninterruptible power supply. Since the EPS is the only system that has a battery-backed RTC, the OBC synchronizes time with the EPS. The OBC provides a time synchronization service for the payload modules. The OBC board houses two redundant sets of microelectromechanical systems (MEMS) sensors: two magnetometers (LIS3 MDL), two gyroscopes (BMG160), and an accelerometer (FXOS8700 CQ) in each set for attitude determination purposes. For the ESTCube-2 mission, the _star tracker (ST)_ is an optional attitude determination system component. It was developed as part of the deep-space platform demonstration. It consists of STMicroelectronics STM32L4-series-based MCU to gather different analog measurements from the PCB, configure the field-programmable gate array (FPGA), and enable communication with the complementary metal-oxide-semiconductor (CMOS) sensor. An Altera Cyclone IV-based FPGA device processes the image, extracts the star location information, compares it to a database of known stars, and extracts the valuable attitude information.