Command and Data Handling (C&DH)
The C&DH subsystem executes spacecraft commands, collects and stores telemetry, and interfaces with payload data.
Core Functions
- Receive ground commands via TT&C.
- Store and forward telemetry, mission data.
- Host flight software (FSW) and critical control loops.
Key Design Drivers
| Driver | Driven By | Impact |
|---|---|---|
| Processing Speed | Autonomy & data rates | CPU selection, memory bus |
| Memory Size | Data storage needs | Non-volatile + RAM capacity |
| Bus Architecture | Payload I/O requirements | Interfaces (1553, SpaceWire, custom) |
| Reliability | Mission critical ops | Rad-hard CPU, redundancy |
Processor & Software
-
Flight Processor
- Often rad-hard: e.g., Rad750, GR712.
- Runs real-time OS for deterministic scheduling.
-
Flight Software
- Attitude control algorithms, autonomy scripts.
- Stored in PROM/EEPROM with boot code in case of resets.
Modular Approach
- Partition data buses by subsystem to simplify integration (e.g., separate ADC and payload buses).
- Use standard interfaces (SpaceWire, 1553) for flexible payload integration.
Data Interfaces
- Low-rate: MIL-STD-1553 for robust control.
- High-rate: SpaceWire or custom if >100 kbps.
- Handshaking crucial to prevent data loss at high speeds.
Timekeeping
- Onboard mission-elapsed timer (MET) plus an epoch offset (e.g., seconds since Jan 1, 2001).
- Oscillator stability & calibrations are critical for accurate time-tagging.
Cross-Links
- See Telemetry Tracking and Command on command reception and telemetry downlink.
- See Spacecraft Power System for C&DH board power needs and bus voltage considerations.