Command and Data Handling(C&DH)

AERSP 401A

C&DH

• Principal Function– Processes and distributes commands; processes,

stores, and formats data

• Other Names– Spacecraft Computer System– Spacecraft Processor

• References– 10.4.4, 11.3, Chapter 16

Sizing the C&DH SubsystemStep What’s Involved Reference

Prepare Command List

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

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

10.4, 11.3

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

10.3, Chapter 16

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

13.3

Identify Processing Requirements

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

11.3

Identify Storage Requirements

Compare data rates of payload and spacecraft to the communications subsystem’s ability.

11.3

Select Equipment Configure the subsystem and select components to meet requirements

11.3

Typical Characteristics of Basic Components for C&DH

Component Mass (kg) Power (W) Comments

Command Unit 5.0 5.4 standby

1.4 operating

Redundant unit, 9 user addresses capacity, 18,892 commands

Pulse Code Modulation Encoder

5.5 5.5 Redundant unit, 250 or 1,000 bits/sec

64 word, 8 bit frame

5 subcommunicated channels

Space Computer System Key Concerns

• More on-board processing is desirable and inevitable– Can increase functionality with major savings in weight, power, and cost– Can significantly reduce operations and, therefore, lifecycle costs– Can dramatically increase system flexibility – software is, but far, the least expensive “on-

orbit replacement unit”

• Formally correct approach is to decide on the appropriate software and then size the computer to meet the need

– Not a realistic alternative in today’s environment for most functions– Net result is a major performance, cost, and schedule risk

• Key computer system trades– On-board vs. ground processing– What to automate– Selection and sizing of hardware– Language selection– Software selection or development

Computer Systems Development Process

Define Requirements

Develop System Baseline

Expand Baseline Concepts

Evaluate System Effectiveness

• Evaluate Mission Objectives

• Perform Functional Partioning

• Evaluate Candidate Architectures

• Perform Functional Flow Analysis

• Establish Block Diagram for System

• Evaluate and Select Hardware Instruction Set Architecture

• Evaluate and Select Software Language

• Define Processing Tasks

• Establish Computer Size and Throughput Estimates

• Verify Requirements Traceability

• Evaluate Baseline Against Design Drivers

• Assess Development Issues for System Baseline

• Evaluate System Testability

Candidate Architectures

Centralized Architecture

Earth Sensor Thrusters

GPSCentral

Processor

Star Tracker

GyrosWheels

Accelerometer

Candidate Architectures

Ring Architecture

Earth Sensor

Thrusters

GPS

Central Processor

Star Tracker

Gyros

Wheels

Accelerometer

Candidate Architectures

Earth Sensor

Thrusters GPS

Central Processor

Star Tracker

Gyros

Wheels

Accelerometer

BUS Architecture

Architecture Trades• All architectures must meet mission requirements to be considered in

architecture trades

• Typically, architecture trades are somewhat subjective, although standard discriminators exist:

Physical Characteristics Economic Characteristics• Size• Weight• Power• Design for Instability

• Complexity• Risk/Maturity• Cost/Schedule• Maintainability

Reliability Others• Fault Tolerance• Reconfiguration

• Use of “enabling technologies”• Proof-of-concept

Final processing architecture is frequently a hybrid – Final processing architecture is frequently a hybrid – optimizing all aspects of the designoptimizing all aspects of the design

Architecture Selection

• If mission requirements strongly emphasize weight and power and those budgets are fixed, some reliability and fault tolerance must be sacrificed to meet needs.

• If mission life is critical and payload operation is the highest requirement, some additional weight and power may be needed to enhance reliability.

Summary• Software offers major advantages in performance, recurring cost,

“lifecycle”, flexibility, and reliability• High autonomy offset by significant disadvantages

– High non-recurring cost– Development risk is large – only way to know the requirements is to

write the software in advance– Difficult process to manage and measure progress

• In the near term, computers will become most important in:– LightSats where needed performance cannot be achieved without them– Constellations or recurring applications, where non-recurring cost can

be amortized

Command Decoder Block Diagram

Uplink

Onboard Computer

Hardline Test

Command Source Arbitration

Command Message Validation

Command Message Decoding

Over/Under Voltage Detect

Serial Digital

Low-Level Discrete

High-Level Discrete

Command Decoder

Prime Power

Command Outputs

Pulse

Pulse

Data

Clock

Enable

Data Handling Unit Block Diagram

High-Level Analog

Low-Level Analog

Passive Analog

Serial Digital

Analog to Digital Converter

Data Formatter and Control Logic

Downlink

Onboard Computer

Hardline Test

Bi-Level

Multiplexed Signal Inputs