Module 4
Satellite Telemetry, Tracking And
Telecommand
Introduction
• Satellite communications system can be broadly
divided into two segments, ground segment and
a space segment.
• The space segment include the satellites, but
also includes the ground facilities needed to keep
the satellites operational, referred to as the
tracking, telemetry, and command (TT&C).
• Telemetry, tracking, and command functions are
complex operations which require special ground
facilities in addition to the TT&C subsystems
aboard the satellite.
Three major tasks of
TT&C
• Monitoring of the health and status of the
satellite through the collection, processing, and
transmission of data from the various spacecraft
subsystem.
• Determination of the satellite’s exact location
through the reception, processing, and
transmitting of ranging signals.
• Proper control of satellite through the reception,
processing, and implementation of commands
transmitted from the ground.
Satellite Control System (TT&C)
Telemetry
• The telemetry, or telemetering, is measurement at a
distance.
• Telemetry signals include attitude information, obtained
from sun and earth sensors; environmental information
such as the magnetic field intensity & direction and the
frequency of meteorite impact.
• Spacecraft information such as temperatures, power
supply, voltages, and stored-fuel pressure.
• Certain frequencies have been designated by
international agreement for satellite telemetry
transmissions.
• The telemetry subsystem transmits information about the
satellite to the earth station.
Data Collection
• Measurements and status of health
• Power functions
• Telemetry functions
• Telecommand functions
• Attitude control functions
• Propulsion functions
• Structure functions
• Antenna functions
• Tracking functions
• Payload functions
• Miscellaneous functions
• Measurements:
– Acceleration, velocity, displacement
– Angular rate, angular position
– Pressure
– Temperature
– Density
– Resistance
– Voltage, current
– Intensity
– Electric field, magnetic field
Tracking
• Tracking of the satellite is accomplished by the satellite
beacon signals which are received at the TT&C earth
stations.
• Tracking is important during the transfer and drift orbital
phases of the satellite launch.
• Once it is on station, the position of a satellite will tend to
be shifted as a result of various disturbing forces.
• Therefore, it is necessary to track the satellite’s movement
and send correction signals as required.
• Tracking beacons are transmitted in the telemetry channel,
or by pilot carriers at frequencies in one of the main
communications channels, or by special tracking antennas.
• Satellite range from the ground station is required from
time to time. This can be determined by measurement of
the propagation delay of signals.
Doppler Effect for tracking a Satellite
Command
• The command subsystem receives command signals
from the earth station, often in response to
telemetered information.
• The command subsystem demodulates and decodes
the command signals and routes these to the
appropriate equipment needed to execute the
necessary action.
• Attitude changes, communication transponders
switched in and out of circuits, antennas redirected,
and station keeping maneuvers carried out based on
command signals.
• To prevent unauthorized commands from being
received and decoded, the command signals are often
encrypted.
• 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
• GSE operator selects command mnemonic
• Software creates command message in appropriate
format and encodes it.
• Pulse code modulation (PCM)
• Phase shift keying (PSK)
• Frequency shift keying (FSK)
Transducers
• Transducers convert energy from one form to
another
• Outputs can be
• –Resistance
• –Capacitance
• –Current
• –Voltage
Signal Conditioning and Selection
• Signal conditioner: may be passive or active
• Conditioning ensures Amplification, Noise
filtering, proper level, dynamic range, frequency
response, impedance, ground reference, common
mode rejection and automatic gain control.
• Commutation selects the proper sensor at a given
time
• Sampling frequency determined by the Nyquist
criteria.
Multiplexing
• When a series of input signals from different sources have to be
transmitted along the same physical channel, multiplexing is used
to allow several communication signals to be transmitted over a
single medium.
• Frequency division multiplexing (FDM)
– FDM places multiple incoming signals on different frequencies. Then
are they are all transmitted at the same time.
– The receiving FDM splits the frequencies into multiple signals again
• Time division multiplexing (TDM)
– TDM slices multiple incoming signals into small time intervals.
– Multiple incoming lines are merged into time slices that are
transmitted via satellite
– The receiving TDM splits the time slices back into separate signals
FDM
TDM
Base Band Telemetry
Command handling
Data Handling
C&DH
C&DH Issues
• 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.
• 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.
