Satellite Telemetry, Tracking and Control Subsystems Col John E. Keesee October 29, 2003 Massachusetts Institute of Technology 1 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 October 29, 2003 Massachusetts Institute of Technology 2 Outline ? TT&C functions and trades ? Command System functions – Encoding/Decoding – Messages – Interfaces ? Telemetry systems – Sensors and transducers – ADC –Formats – Concerns/Design principles October 29, 2003 Massachusetts Institute of Technology 3 TT&C Functions ? Carrier tracking ? Command reception and detection ? Telemetry modulation and transmission ? Ranging ? Subsystem operations October 29, 2003 Massachusetts Institute of Technology 4 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 October 29, 2003 Massachusetts Institute of Technology 5 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 October 29, 2003 Massachusetts Institute of Technology 6 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 October 29, 2003 Massachusetts Institute of Technology 8 TT&C Interfaces Subsystem Requirement Attitude Determination and Control Antenna pointing Command and Data Handling Command and telemetry data rates Clock, bit sync,and timing requirements Two-way comm requirements Autonomous fault detection and recovery Command and telemetry electrical interface Electrical Power Subsystem Distribution requirements Thermal/Structural Heat sinks for TWTAs Heat dissipation of all active boxes Location of TT&C subsystem electronics Clear field of view and movement for all antennas Payload Storing mission data RF and EMC interface requirements Special requirements for modulation and coding October 29, 2003 Massachusetts Institute of Technology 9 Command System ? Reconfigures satellite or subsystems in response to radio signals from the ground ? Command timing – Immediate – Delayed – Priority driven (ASAP) October 29, 2003 Massachusetts Institute of Technology 10 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 October 29, 2003 Massachusetts Institute of Technology 11 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 October 29, 2003 Massachusetts Institute of Technology 12 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 October 29, 2003 Massachusetts Institute of Technology 13 Complete Command System October 29, 2003 Massachusetts Institute of Technology 14 ? 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 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 mass October 29, 2003 Massachusetts Institute of Technology 15 Secure Command Links ? Encryption ? Authentication October 29, 2003 Massachusetts Institute of Technology 16 Command Message Components ? Input checkerboard bits ? Synchronization (Barker word) bits ? Command bits ? Error detection bits October 29, 2003 Massachusetts Institute of Technology 17 Command Messages ? Spacecraft address ? Command type – Relay commands – Pulse commands – Level commands – Data commands ? Command select ? Error detection and correction ? Multiple commands October 29, 2003 Massachusetts Institute of Technology 18 Command Logic ? Decodes command ? Validates command – Correct address –EDAC – Valid command – Valid timing – Authenticated ? Activates circuitry October 29, 2003 Massachusetts Institute of Technology 19 Interface Circuitry ? Latching relays with telltales ? Pulse commands ? Level commands ? Data commands – Serial (enable, data and clock) – Parallel October 29, 2003 Massachusetts Institute of Technology 20 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 October 29, 2003 Massachusetts Institute of Technology 21 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 October 29, 2003 Massachusetts Institute of Technology 22 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 October 29, 2003 Massachusetts Institute of Technology 23 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 October 29, 2003 Massachusetts Institute of Technology 24 Analog to Digital Conversion ? Converts voltages (0 – 5.1 v, or -2.56 to 2.54 v) to 2 n -1 discrete values ? Quantization error decreases as n increases Type Conversion Rate Word Size Power High Speed ADC 50*10 6 /sec 8 bit 2.5 W High Resolution ADC 1*10 5 /sec 16 bit 1.5 W Low Power ADC 2.5*10 4 /sec 8 bit 0.005 W October 29, 2003 Massachusetts Institute of Technology 25 Telemetry Processing ? Compression ? Analysis for autonomous systems ? Formatting ?Storage October 29, 2003 Massachusetts Institute of Technology 26 Telemetry Formats ? Synchronization ? Frame count ? Spacecraft identification ?EDAC ? Frame format identification ? Spacecraft time October 29, 2003 Massachusetts Institute of Technology 27 Multiplexing ? Frequency division multiple access ? Time division multiple access ? Code division multiple access October 29, 2003 Massachusetts Institute of Technology 28 Commutation in Data Formats Data 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 October 29, 2003 Massachusetts Institute of Technology 29 ? 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 Telemetry and Command System Block Diagram October 29, 2003 Massachusetts Institute of Technology 30 Command Decoder Block Diagram October 29, 2003 Massachusetts Institute of Technology 31 Data Handling Unit Block Diagram October 29, 2003 Massachusetts Institute of Technology 32 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 October 29, 2003 Massachusetts Institute of Technology 33 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 October 29, 2003 Massachusetts Institute of Technology 34 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 October 29, 2003 Massachusetts Institute of Technology 35