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