# Interplanetary Student Nanospacecraft: Development of the LEO Demonstrator ESTCube-2
## 2 ESTCube-2 Mission
### Satellite Design
#### 2.3.2 Avionics Stack Systems  
The avionics stack systems are the heart of the satellite. It consists of the onboard computer (OBC) system for task scheduling, data logging, and storage; the electrical power system (EPS) for energy storage and distribution; the communication system (COM) for  
Figure 7: Exploded view of the 3U satellite, which showcases the satellite and its systems’ arrangement. The attitude and orbit control system (AOCS) components are distributed throughout the satellite—in the avionics stack (see Figure 8), cold gas propulsion, magnetorquers on side panels of X\(-\) axis Y+ axis directions, and sensors on the six side panel PCBs. telemetry and remote commands; the star tracker (ST); and the attitude and orbit control system (AOCS). The aforementioned systems are integrated into a 96 \(\times\) 96 \(\times\) 60 mm\({}^{3}\) volume (see Figures 8 and 9). Each system is located on a different PCB and is implemented to use the allocated space in the satellite efficiently. The _electrical power system (EPS)_ is one of the most critical systems upon which the mission's success depends greatly. The system ensures energy storage, voltage regulation, and power distribution on two boards. The EPS battery management board controls and monitors the charging/discharging of the battery packs. The battery management PCB autonomously controls the charge and discharge currents depending on the voltage levels present. Temperature measurements of individual cells are performed to ensure the battery packs' correct charge/discharge profile. The battery pack is configured in a 2s2p configuration and stores the energy generated  
Figure 8: A render of the satellite avionics stack electronics module. System names starting from the top: battery management printed circuit board (PCB) with batteries; electrical power system; communication system; onboard computer; star tracker. The AOCS components in the avionics stack—reaction wheels, star tracker, magnetorquer at the bottom of the stack, and sensors on the onboard computer (OBC) board. Figure 9: The inter-system electrical and the data-connections architecture. SP: side panels,CTS: Corrosion Testing in Space, HSCOM: high-speed communications, CGP: Coulomb drag propulsion (Ionospheric Plasma Brake), EPS: electrical power system, COM: communications system, OBC: onboard computer, ST: star tracker, EOP: Earth observation payload, CGP: cold–gas propulsion. by the solar arrays. Each series block of the pack consists of two commercial Panasonic 103,450 lithium-ion battery cells. Depending on the discharge state and temperature of the battery cells, this configuration ensures that the battery pack's voltage ranges from \(\sim\)6.0 V to \(\sim\)8.4 V. Each series block is connected to the main power bus (MPB), an unregulated voltage rail supplied directly from the battery pack, and is responsible for the power path between the avionics stack and payloads. The EPS main board is the core of the system. It incorporates hardware for an external Electrical Ground Support Equipment (EGSE) communication interface and uses a dedicated STMicroelectronics STM32L4 series microcontroller (MCU) to ensure power management functions. If required, it can override the autonomous control of the battery management board. External analog-to-digital converters (ADCs) perform various voltage measurements for different current, voltage, and temperature level monitoring, which are stored on an external FRAM. Multiple avionics stack systems require a 3.3 V supply produced by two switching regulators. They operate in a hot-redundant configuration and convert floating MPB voltage with \(\sim\)90% conversion efficiency. A power switch for each avionics stack system provides power distribution functionality. The EPS MCU controls these switches and can sense over-current conditions that automatically trigger the off-state of the system. Some satellite COTS systems cannot be powered from the MPB and require regulated voltage.