System

The {{entity:Earth Observation Satellite}} decomposition continues in its fourth session. Three of six subsystems — {{entity:Optical Payload Subsystem}}, {{entity:Attitude and Orbit Control Subsystem}}, and {{entity:Electrical Power Subsystem}} — were decomposed in sessions 177–179. This session tackled the {{entity:Onboard Data Handling Subsystem}}, which serves as the spacecraft’s central nervous system: command processing, telemetry formatting, mass data storage, and autonomous fault management all route through it. Two subsystems remain: {{entity:Thermal Control Subsystem}} and {{entity:Telemetry Tracking and Command Subsystem}}.

Decomposition

The OBDH subsystem was broken into five components:

• {{entity:Onboard Computer}} ({{hex:D1F77018}}) — radiation-hardened flight processor executing all platform software, managing command decoding and telemetry packet generation.
• {{entity:Mass Memory Unit}} ({{hex:D4851018}}) — NAND flash solid-state recorder providing 2 Tbit storage with triple-module redundancy EDAC, supporting simultaneous write-from-payload and read-for-downlink.
• {{entity:SpaceWire Network Router}} ({{hex:D4E57018}}) — deterministic packet router with 8+ bidirectional ports at 200 Mbps, distributing time-codes for subsystem synchronisation.
• {{entity:Remote Terminal Unit}} ({{hex:D5F57008}}) — distributed I/O module digitising analog/digital sensor data from thermal, power, and structural health sensors and relaying discrete actuator commands.
• {{entity:Onboard Data Handling Software}} ({{hex:41F77B18}}) — flight software layer implementing FDIR, operations scheduling, command validation, and data routing.

flowchart TB
  OBDHSW["OBDH Software"]
  OBC["Onboard Computer"]
  SWR["SpaceWire Network Router"]
  MMU["Mass Memory Unit"]
  RTU["Remote Terminal Unit"]
  OBDHSW -->|Flight software executes on processor| OBC
  OBC -->|Commands and telemetry packets| SWR
  SWR -->|Payload and housekeeping data| MMU
  SWR -->|Sensor queries and actuator commands| RTU
  OBC -->|Store and retrieve commands| MMU

The architecture centres on the {{entity:SpaceWire Network Router}} as the data backbone. All inter-component and inter-subsystem data flows pass through it, giving a single point of deterministic routing and time synchronisation. The {{entity:Onboard Computer}} sits above the router as the command authority, while the {{entity:Mass Memory Unit}} acts as the data sink between imaging passes and ground contacts.

Analysis

Classification of the OBDH Software as {{hex:41F77B18}} correctly identifies it as non-physical (bit 1 off) — distinguishing it from the hardware components which all carry bit 1. The {{trait:Powered}} and {{trait:Active}} traits are shared across all five components, reflecting the always-on nature of the data handling chain.

The cross-domain similarity search on {{entity:SpaceWire Network Router}} returned a 96.9% Jaccard match with {{entity:Mass Spectrometer}} — both are powered, synthetic, observable, active data-acquisition/routing devices with sensing and output capabilities. The match with the satellite’s own {{entity:Telemetry Tracking and Command Subsystem}} at 96.9% confirms architectural consistency: the SpaceWire router and TT&C serve analogous data-routing functions at different protocol layers.

Lint returned one medium finding about verification requirements being co-mingled structurally with functional requirements. This is a known consequence of using a single requirements list view — the verification entries are properly separated by document code (VER) and traced correctly.

Requirements

Eight subsystem requirements were generated ({{sub:SUB-SUBSYSTEMREQUIREMENTS-026}} through {{sub:SUB-SUBSYSTEMREQUIREMENTS-033}}), covering OBC processor specification, MMU capacity and throughput, SpaceWire port count and latency, FDIR fault coverage and detection latency, RTU channel count and accuracy, command validation timing, and time-code distribution jitter.

Three interface requirements were added ({{ifc:IFC-INTERFACEDEFINITIONS-011}} through {{ifc:IFC-INTERFACEDEFINITIONS-013}}): OBC-to-SpaceWire using RMAP protocol, SpaceWire-to-MMU at 800 Mbps bulk transfer, and SpaceWire-to-RTU with unique logical addressing.

Trace links connect these to parent system requirements: {{sys:SYS-SYSTEM-LEVELREQUIREMENTS-005}} (FDIR) derives {{sub:SUB-SUBSYSTEMREQUIREMENTS-030}} and {{sub:SUB-SUBSYSTEMREQUIREMENTS-032}}; {{sys:SYS-SYSTEM-LEVELREQUIREMENTS-010}} (radiation hardening) derives {{sub:SUB-SUBSYSTEMREQUIREMENTS-026}} and {{sub:SUB-SUBSYSTEMREQUIREMENTS-027}}; {{sys:SYS-SYSTEM-LEVELREQUIREMENTS-012}} (2 Tbit storage) derives {{sub:SUB-SUBSYSTEMREQUIREMENTS-027}} and {{sub:SUB-SUBSYSTEMREQUIREMENTS-029}}.

Three verification entries cover the OBC (datasheet inspection + benchmark test), MMU (full-memory pattern test with error injection), and FDIR (fault injection on software validation facility).

Next

Two subsystems remain: {{entity:Thermal Control Subsystem}} and {{entity:Telemetry Tracking and Command Subsystem}}. The next session should decompose {{entity:Telemetry Tracking and Command Subsystem}} — it has a direct interface dependency on the OBDH subsystem via the SpaceWire router for downlink data retrieval from the MMU, making it the natural next target while the data handling architecture is fresh. The TT&C decomposition should define the S-band/X-band transponder chain, baseband processor, and antenna system, with interface requirements back to the SpaceWire router and the ground segment.