Telemetry and command subsystem
Outline
• TT&C functions and trades
• Command System functions –
Encoding/Decoding
– Messages –
Interfaces
• Telemetry systems
– Sensors and transducers
– ADC
– Formats
– Concerns/Design principles
TT&C Functions
• Carrier tracking
• Command reception and detection
• Telemetry modulation and transmission
• Ranging
• Subsystem operations
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 rate
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
Subsystem Operations October 29, 2003 Massachusetts Institute of Technology 7
• 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
TT&C Trades
• Antenna size vs transmitter power
• Solid state amplifiers vs traveling wave tube amplifiers
• Spacecraft complexity vs ground complexity