Telemetry, Tracking, and Command (TT&C)
The TT&C subsystem provides the RF link for spacecraft telemetry downlink, command uplink, and tracking (ranging/Doppler).
Core Functions
- Transmit spacecraft telemetry to ground.
- Receive ground commands.
- Provide ranging and Doppler data for orbit determination.
Key Design Drivers
| Driver | Driven By | Impact |
|---|---|---|
| Data Rate & Distance | Payload & Ops concept | Tx power, antenna gain |
| Frequency | Regulatory & data rate | Hardware design, antenna size |
| Ground Station EIRP | Ground architecture | Spacecraft Rx sensitivity |
| Duty Cycle | Orbit & ops concept | Power budget, heat dissipation |
RF Link Considerations
- Inverse-Square Law: Doubling distance = x4 the required Tx power (if antenna gain is fixed).
- Antenna Gain (\propto) Aperture Area / Beamwidth:
- High-gain antennas reduce Tx power but need precise pointing.
- Omnis are simpler but limit data rate, especially at large distances.
- Coding & Standards: CCSDS frames, robust forward error correction.
Common Frequencies
- S-band (~2 GHz): Lower data rates, smaller antennas for LEO or near-Earth.
- X-band (~8 GHz): Higher data rates for deep-space.
- Ka-band (~26 GHz): Even higher rates but more susceptible to rain fade.
Ground Tracking
- Ranging: Measure round-trip delay of signals.
- Doppler: Two-way coherent transponder is typical; velocity is deduced from frequency shift.
Power & Thermal
- Tx can consume 6-7x its RF power in electrical input (e.g., 5 W out may need ~30-35 W in).
- The receiver must typically remain powered for safe commanding.
Cross-Links
- See Command and Data Handling for how commands are processed after reception.
- See Spacecraft Power System for transmitter power budgets.