l20_satellitettc.pdf
TRANSCRIPT
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Satellite Telemetry, Tracking and Control Subsystems
Col John E. Keesee
October 29, 2003 Massachusetts Institute of Technology
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Overview
• The telemetry, tracking and control subsystem provides vital communication to and from the spacecraft
• TT&C is the only way to observe and to control the spacecraft’s functions and condition from the ground
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Outline
• TT&C functions and trades• Command System functions
– Encoding/Decoding– Messages– Interfaces
• Telemetry systems– Sensors and transducers– ADC– Formats– Concerns/Design principles
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TT&C Functions
• Carrier tracking• Command reception and detection• Telemetry modulation and transmission• Ranging• Subsystem operations
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Carrier Tracking
• Two-way coherent communication– Transmitter phase-locks to the received
frequency– Transmitted frequency is a specific ratio of the
uplink frequency • Easy to find and measure the frequency
received on the ground• Doppler shift provides range rateOctober 29, 2003 Massachusetts Institute of
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Ranging
• Uplink pseudo-random code is detected and retransmitted on the downlink
• Turnaround time provides range• Ground antenna azimuth and elevation
determines satellite angular location
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Subsystem Operations
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• Receive commands from Command and Data Handling subsystem
• Provide health and status data to CD&H• Perform antenna pointing • Perform mission sequence operations per stored
software sequence• Autonomously select omni-antenna when
spacecraft attitude is lost• Autonomously detect faults and recover
communications using stored software sequence
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TT&C Trades
• Antenna size vs transmitter power• Solid state amplifiers vs traveling wave tube
amplifiers• Spacecraft complexity vs ground
complexity
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TT&C InterfacesSubsystem Requirement
Attitude Determination and Control
Antenna pointing
Command and Data Handling
Command and telemetry data ratesClock, bit sync,and timing requirementsTwo-way comm requirementsAutonomous fault detection and recoveryCommand and telemetry electrical interface
Electrical Power Subsystem Distribution requirementsThermal/Structural Heat sinks for TWTAs
Heat dissipation of all active boxesLocation of TT&C subsystem electronicsClear field of view and movement for all antennas
Payload Storing mission dataRF and EMC interface requirementsSpecial requirements for modulation and coding
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Command System
• Reconfigures satellite or subsystems in response to radio signals from the ground
• Command timing– Immediate– Delayed– Priority driven (ASAP)
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Command Functions
• Power on/off subsystems• Change subsystem operating modes• Control spacecraft guidance and attitude
control• Deploy booms, antennas, solar cell arrays,
protective covers• Upload computer programs
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Command System RF Performance
• Frequencies– S-band (1.6 – 2.2 GHz)– C-band (5.9 – 6.5 GHz)– Ku-band (14.0 – 14.5 GHz)
• BER = 10-6
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Spacecraft Command System Block Diagram
Interface circuitry
Receiver/ demodulator
Command decoder
Command logic
• Decoders reproduce command messages and produce lock/enable and clock signals
• Command logic validates the command– Default is to reject if any uncertainty of validity– Drives appropriate interface circuitry
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Complete Command System
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• GSE operator selects command mnemonic• Software creates command message in
appropriate format and encodes it• Batch commands/macros• Pulse code modulation (PCM)• Phase shift keying (PSK)• Frequency shift keying (FSK)
Ground Support Equipment
Spacecraft Command System
Radio Frequency link
Modulation
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Command Decoders
• Detects PCM encoding and outputs binary stream in non-return-to-zero format
• Outputs clock signal• Outputs lock/enable signal• Activates downstream command subsystem
components• Decentralized decoding reduces harness
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Secure Command Links
• Encryption• Authentication
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Command Message Components
• Input checkerboard bits• Synchronization (Barker word) bits• Command bits• Error detection bits
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Command Messages
• Spacecraft address• Command type
– Relay commands– Pulse commands– Level commands– Data commands
• Command select• Error detection and correction• Multiple commands
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Command Logic
• Decodes command• Validates command
– Correct address– EDAC– Valid command– Valid timing– Authenticated
• Activates circuitry
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Interface Circuitry
• Latching relays with telltales• Pulse commands• Level commands• Data commands
– Serial (enable, data and clock)– Parallel
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Telemetry Systems
• Measure physical properties from afar– Status of spacecraft resources, health, attitude, and
operation– Scientific data– Spacecraft orbit and timing data for ground navigation– Images– Tracked object location– Relayed data
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Telemetry System RF Performance
• Frequencies– S-band (2.2 – 2.3 GHz)– C-band (3.7 – 4.2 GHz)– Ku-band (11.7 – 12.2 GHz)
• BER = 10-5
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Sensors and Transducers
• Sensors change state as a function of an external event
• Transducers convert energy from one form to another
• Outputs can be– Resistance– Capacitance– Current– Voltage
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Signal Conditioning and Selection
• Conditioning ensures proper level, dynamic range, frequency response, impedance, ground reference, common mode rejection
• Commutation selects the proper sensor at a given time
• Sampling frequency determined by the Nyquist criteria
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Analog to Digital Conversion
• Converts voltages (0 – 5.1 v, or -2.56 to 2.54 v) to 2n-1 discrete values
• Quantization error decreases as n increasesType Conversion
RateWord Size Power
High Speed ADC
50*106 /sec 8 bit 2.5 W
High Resolution ADC
1*105 /sec 16 bit 1.5 W
Low Power ADC
2.5*104 /sec 8 bit 0.005 W
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Telemetry Processing
• Compression• Analysis for autonomous systems• Formatting• Storage
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Telemetry Formats
• Synchronization• Frame count• Spacecraft identification• EDAC• Frame format identification• Spacecraft time
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Multiplexing
• Frequency division multiple access• Time division multiple access• Code division multiple access
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Commutation in Data FormatsData type no. 1 bits
Type no. 2 bits Type no. 3 bits
Type no. 4 bits
Type no. 5 bits
Type no. 6 bits Type no. 7 bits
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• Commutation – sequential data time sampling– Data includes major and minor frame
identification and EDAC• Sub-commutated data – given element
represents different data in different frames• Super-commutated data – given element is
found more than once per frame
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Telemetry and Command System Block Diagram
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Command Decoder Block Diagram
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Data Handling Unit Block Diagram
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Command and Data Handling Concerns
• Interfaces to other subsystems must protect the command decoder
• No commands or transient signals may appear on command outputs during application or removal of prime power or during under/over voltage conditions
• If a commands integrity is in doubt, reject it
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Command and Data Handling Concerns (continued)
• Multiple commands are required for critical/ dangerous operations
• No single component failure can result in unintended operation
• No commands shall interrupt the uplink source to the command decoder
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References
• Pisacane, Vincent L. and Robert C. Moore, Fundamentals of Space Systems, Oxford University Press, New York, 1994
• Wertz, James R. and Wiley J. Larson, Space Mission Analysis and Design, Third edition, Microcosm Press, Torrance Ca, 1999
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