The basic function of all but the simplest spacecraft requires extensive contact with ground stations for control, command, communication, and data return, and sufficient computer processing power to run all spacecraft subsystems with, in many cases, a high degree of autonomy.
A spacecraft Command and Data Handling (C&DH) system handles all data sent and received by the spacecraft, including science data and spacecraft or payload operations. The system is connected to RF transmitter and receiver units that are the sole point of passage for data entering or leaving the spacecraft. A space link is a communications link between a spacecraft and its associated ground system or between two spacecraft. A space link protocol is a communications protocol designed to be used over a space link, or in a network that contains one or multiple space links. The basic data flow over a space link is made of Telemetry(TM) and Telecommand (TC) data. Thus, the TM downlink and TC uplink provide a communication channel between the spacecraft and the ground operators.
On the uplink, the C&DH system receives and decodes all commands and data for both platform and payload operations from the communications system. These commands (TCs) are then directed to the appropriate subsystem or executed directly at platform level. The handling of payload commands would generally not be done by the C&DH system, but would instead be passed, fully encapsulated, directly to the payload. TC is split into:
- Direct commands to the spacecraft for reconfiguration
- Application-specific commands
On the downlink, the C&DH system collects various types of data either acquired from the platform subsystems or generated by science payloads, and multiplexes it into transfer frames for transmission to ground. TM data can be split into:
- Spacecraft HK data
- Orbit (position) data
- Payload data (science data)
- Telecommand reception status (CLCW)
- Memory dump data