1. Command and Data Handling (C&DH)
AERSP 401A

2. C&DH Principal Function Other Names References
Processes and distributes commands; processes, stores, and formats data
Other Names
Spacecraft Computer System
Spacecraft Processor
References
10.4.4, 11.3, Chapter 16

3. Sizing the C&DH Subsystem
Step
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
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.
Select Equipment
Configure the subsystem and select components to meet requirements

4. Typical Characteristics of Basic Components for C&DH
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
Redundant unit, 250 or 1,000 bits/sec
64 word, 8 bit frame
5 subcommunicated channels

5. 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

6. 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

7. Candidate Architectures
Centralized Architecture
Earth Sensor
Thrusters
GPS
Central Processor
Star Tracker
Gyros
Wheels
Accelerometer

8. Candidate Architectures
Ring Architecture
Earth Sensor
Thrusters
GPS
Central Processor
Star Tracker
Gyros
Wheels
Accelerometer

9. Candidate Architectures
BUS Architecture
Earth Sensor
Central Processor
Accelerometer
Gyros
Thrusters
Wheels
Star Tracker
GPS

10. Physical Characteristics Economic Characteristics
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 – optimizing all aspects of the design

11. 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.

12. 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

13. Command Decoder Block Diagram
Command Outputs
Pulse
Data
Clock
Enable
High-Level Discrete
Uplink
Command Source Arbitration
Command Message Validation
Command Message Decoding
Onboard Computer
Low-Level Discrete
Hardline Test
Serial Digital
Over/Under Voltage Detect
Command Decoder
Prime Power

14. Data Handling Unit Block Diagram
Multiplexed Signal Inputs
High-Level Analog
Analog to Digital Converter
Data Formatter and Control Logic
Downlink
Low-Level Analog
Onboard Computer
Passive Analog
Hardline Test
Bi-Level
Serial Digital