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* dBi Is the antenna gain relative to an isotropic radiator expressed in decibels (dB). See Sec. 13.3.5

* dBi Is the antenna gain relative to an isotropic radiator expressed in decibels (dB). See Sec. 13.3.5

11J Command and Data Handling

Richard T. Berget, BF Goodrich Aerospace Data Systems Division

The command and data handling system, C&DH, performs two major functions. It receives, validates, decodes, and distributes commands to other spacecraft systems and gathers, processes, and formats spacecraft housekeeping and mission data for downlink or use by an onboard computer. This equipment often includes additional functions, such as spacecraft timekeeping, computer health monitoring (watchdog), and security interfaces.

While they normally provide independent functions, the combination of command and data handling into a single subsystem provides an efficient means for autonomous control of spacecraft functions. An onboard computer or microprocessor can send commands and monitor telemetry over a single interface with the C&DH system, allowing the control of multiple subsystems.

The C&DH system's size is directly proportional to spacecraft complexity. The more systems a spacecraft has, the more monitoring and configuration capability required. Reliability concerns alone may double the hardware's size if we require redundant C&DH subsystems.

The ideal C&DH system is one which has been proven on another spacecraft and which requires no modification for the mission under development However, new missions are usually supported by systems which evolve from older designs. We make small improvements in die performance of the systems from the viewpoint of speed, power, weight, volume, or other operating parameter. In the case of a new or custom design, extensive testing must simulate the strenuous environments involved in a space launch and flight

11.3.1 Introduction to C&DH

Figure 11-6 shows a typical command decoder. Command messages can originate from an onboard computer, uplink transponders, or a hardline test interface. An arbitration scheme is necessary for source selection which gives uplink commands priority. Commands from the computer are delayed until a time slot is available. The hardline test interface is not active during flight and when in use overrides the other command sources.

Several standards exist for command message formats. (See CCSDS 201.0-B-l, 1987.) Typically, a command consists of a synchronization code, spacecraft address bits, command message bits, and error check bits. Received commands are validated prior to execution. Validation consists of receiving synchronization code, checking command message length (correct number of bits), exactly matching the spacecraft address and any fixed-bit patterns (unused message bits), and detecting no errors in an error check polynomial code. Once the decoder validates a command, it increments a counter to record the number of executed commands. Then the message bits pass to a decoder for execution. Hie command decoder rejects commands that do not pass the validation criteria and it increments the command reject counter. The data handling system reads the accept and the reject counters and includes them in the downlink data to provide operational feedback.

Command Sources

Command Outputs

Uplink

Onboard Computer

Hardline Test

High-Level Discrete

Command

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