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10.4.4 Command and Data Handling Subsystem

The command and data handling subsystem, (C&DH), receives and distributes commands and collects, formats, and delivers telemetry for standard spacecraft operations (housekeeping) and payload operations. We usually handle housekeeping data intermittently and at rates below 1,000 bits/s. Rates for payload commanding and telemetry depend on the pay load's design. They may require very high data rates (10 kb/s to 500 Mb/s) and storage of payload data.

The C&DH subsystem may include encryptors, deciyptors, a sequencer or timer, a computer for data processing, and equipment for data storage. It interfaces with the communications subsystem from which it receives commands and to which it sends the formatted telemetry stream. It also delivers commands to and receives telemetry from the other spacecraft subsystems and may have similar interfaces with the payload.

The decoding of command signals is peculiar to the detailed design of the system. A typical command is a serial binary word containing a preamble, a user address, and the command word. The preamble allows the spacecraft to identify and authenticate the command. The command unit decodes the user addresses and then routes actual commands to the correct users, either by separate wires or by a data bus. Each user then decodes and executes the command.

In some cases, we need several commands to do something. If the function is time critical, we can send precursor commands and verify their receipt before sending a precisely timed execute or a time-tagged execute. A sequencer or onboard computer can execute time-tagged commands. If we need to send commands over time, we can time-tag them or use a timer-execute command followed by a sequence of timer-reset commands. If the command system fails, the command halts when the timer times out Ordnance normally requires separate arm and fire commands.

Telemetry signals tell us about the spacecraft's health and provide operational data needed to control the spacecraft Normally, we convert telemetry measurements to digital signals, serially multiplex them in a telemetry frame using a repetitive pattern, and transmit the frame using a main frame word for frame synchronization. We may use a main frame word as a subcommutated channel or we may supercommutate signals if we need a sampling rate higher than the frame rate. Finally, we may place the telemetry frames in packets and multiplex them with other downlink data.

Commands and telemetry signals depend on the spacecraft's operation. Each of the spacecraft's commandable functions needs a separate command. In addition, we must provide enough telemetry to define the spacecraft's state of health, as well as data for operational control.

Table 10-24 lists the steps to size the C&DH subsystem. In its simplest form, this subsystem consists of a command decoder and a telemetry multiplexer. More complex systems distribute the command decoding function by routing serial commands to user subsystems for final decoding. Telemetry multiplexing can also be distributed. High-performance subsystems use central digital computers for data processing. Table 10-25 presents typical characteristics. Chapter 11 describes more complex subsystems.

10.4.5 Thermal Subsystem

The thermal design of a spacecraft involves identifying the sources of heat and designing paths for transporting and rejecting heat, so components will stay within

TABLE 10-24. Steps in Sizing the Command and Data Handling Subsystem.

Step

What's Involved

Reference

prepare Command List

Prepare a complete list of commands for the payload and each spacecraft bus subsystem. Include commands for each redundancy option and each commandable operation.

Sees. 10.4,11.3

prepare Telemetry List

Analyze spacecraft operation to select telemetry measurement points that completely characterize It Include signals to identify redundancy configuration and command receipt

Sees. 10.4,11.3

Analyze Timing

Analyze spacecraft operation to identify time-critical operations, and timeliness needed for telemetry data.

Sec. 10.3, Chap. 16

Select Data Rates

Choose data rates that support command and telemetry requirements and time-critical operations.

Sec. 13.3

Identify Processing Requirements

Examine need for encryption, decryption, sequencing, and processing of commands and telemetry.

Sec. 11.3

Identify Storage Requirement

Compare data rales of payload and spacecraft to the communications subsystem's ability.

Sec. 11.3

Select Equipment

Configure the subsystem and select components to meet requirements.

Sec. 11.3

TABLE 10-25. Typical Characteristics of Basic Components for Command and Data Handling.

Component

Weight (kg)

Power (W)

Comments

Command Unit

0 0

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