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Subsystem Description

Functional Architecture

Conceptually, the On-board Computer (OBC) acts as the brain governing the spacecraft, serving as the central component within the overall architecture of this system. It plays a pivotal role in a complex system that accommodates the following five distinct sub-modules: Electrical Power Supply (EPS), Communication System (COMMS), Attitude Determination and Control System (ADCS), and the payload (P/L).

In other words, the main purpose of the subsystem is to perform the housekeeping of the overall satellite. In this regard, performing both data processing and storage in a timely manner while maintaining a low power consumption is crucial so as to guarantee an efficient and robust performance of the PocketQube. In light of this, the Flight Software (FSW) running on the OBC needs then to be carefully designed by the application writter in order to meet the time requirements linked to the varied tasks that the satellite is expected to perform. A common technique used in embedded systems in order to fix this time constraint is implementing Real-Time Operating Systems (RTOS). In the PoCat project, FreeRTOS has been the open-source RTOS selected to manage the on-board FSW.

On the other hand, the OBC hardware needs to accommodate the FSW by providing enough resources for a correct execution. In summary, the breakdown of the OBC from the hardware perspective consists of a low-power but high-performance ARM Cortex M4 microprocessor, volatile and non-volatile memory banks for data storage, interfaces that reach out to peripherals and data buses interconnecting the whole system architecture.

To ensure the desired behaviour of the PocketQube, it is necessary for both software and hardware to converge seamlessly. This will enable the OBC to efficiently manage various tasks that play a critical role in determining the behavior of the picosatellite. In summary, the tasks that are performed by the On-Board Computer for the PoCat space mission are listed below:

  • Task scheduling.
  • Inter-task communication.
  • Power management.
  • Telecommand processing.
  • Telemetry data handling.
  • Payload data acquisition.
  • Attitude determination and control.
  • Error handling.

Requirements

SubsystemIDRequirement
OBCOBC-0010The OBC shall monitor all spacecraft subsystems.
OBCOBC-0020The OBC shall have a Scheduler which determines the execution of different tasks through time.
OBCOBC-0030The OBC shall provide and store the following housekeeping data: Satellite mode, Boot count, OBC error events, Internal satellite communication error events, RAM memory usage.
OBCOBC-0040The OBC shall retrieve and store housekeeping data for all spacecraft subsystems.
OBCOBC-0050The OBC shall monitor all satellite subsystems in order to verify their nominal behavior.
OBCOBC-0060The OBC shall execute TC received from the GSeg.
OBCOBC-0070The OBC shall be able to control and command all subsystems via its interfaces.
OBCOBC-0080The OBC shall retrieve and store scientific data from the Payload.
OBCOBC-0090The OBC shall have data interfaces with all subsystems.
OBCOBC-0100The OBC power supply voltage shall be 3.3 V.
OBCOBC-0110The OBC shall enable the manual transition between satellite modes if a TC from the ground is received.
OBCOBC-0120The OBC shall automatically transition between satellite modes based on battery levels.
OBCOBC-0130The OBC should allow in-orbit changes of its configuration.
OBCOBC-0140The OBC shall implement a command-less timer that triggers a recovery routine if a telecommand from the GS is not received after a certain period.
OBCOBC-0150The spacecraft shall allow modifications to the OBC Software after the satellite assembly is complete and while on ground.
OBCOBC-0160The spacecraft shall have a timer, set to a minimum of 30 minutes, before operations or deployment of the antennas.
OBCOBC-0170No radio emission shall be allowed after the spacecraft has been integrated within the PocketQube deployer until 45 minutes after deployment.
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