TABLE OF CONTENTS The command and data system (CDS), sometimes called the command and data handling (C&DH) system, is the central nervous system for the spacecraft. The system manages three digital data streams, each critical to the spacecraft and each with distinctive characteristics. These data streams are as folllows:
Science or payload data. This is the information the spacecraft was sent to get. It is gathered by the spacecraft, processed onboard, and downlinked to the ground. It is usually the highest data rate on the spacecraft.
Engineering data. This is the information with which the ground operations team can determine the status and health of each subsystem on the spacecraft. Engineering data are usually downlinked at a medium data rate, substantially lower than the science rate.
Commands. The command link is an uplink from the ground at a low data rate. Commands are instructions to the spacecraft dictating configuration, attitude, and actions at particular times. Accuracy is the primary requirement in handling this data stream. A single bit error in the science data stream or the engineering data stream is not serious. It may cause some temporary consternation among the ground team but it is not dangerous. An error in the command stream is dangerous and can be life threatening. Extreme care is taken in the preparation, testing, and transmission of the command stream.
The CDS and the communications systems are closely related, as shown in Fig. 8.1, the block diagram for a typical CDS. The computer is central to the...
