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Today’s
telemetry systems are built from commercial-off-the-shelf (COTS)
products. But while they all have many common elements, they are each
uniquely configured to meet specific application requirements.
A telemetry system is often viewed as two components, the Airborne System
and the Ground System. In actuality, either or both may be in the air
or on the ground.
Data acquisition begins when sensors (aka, transducers) measure the amount
of a physical attribute and transform the measurement to an engineering
unit value. Some sensors produce a voltage directly (thermocouples for
temperature or piezoelectric strain gages for acceleration), while others
require excitation (resistive strain gages, potentiometers for rotation,
etc.). Sensors attached to signal conditioners provide power for the sensors
to operate or modify signals for compatibility with the next stage of
acquisition. Since maintaining a separate path for each source is cumbersome
and costly, a multiplexer (historically known as a commutator) is employed.
It serially measures each of the analog voltages and outputs a single
stream of pulses, each with a voltage relative to the respective measured
channel. The rigorous merging of data into a single stream is called Time
Division Multiplexing or TDM.
The scheme where the pulse height of the TDM stream is proportional to
the measured value is called Pulse Amplitude Modulation (PAM). A unique
set of synchronization pulses is added to identify the original measurands
and their value. PAM has many limitations, including accuracy, constraints
on the number of measurands supported, and the poor ability to integrate
digital data.Pulse Code Modulation (PCM)
is today’s preferred telemetry format for
the same reasons that PAM is inadequate. Accuracy is high, with resolution
limited only by the analog to digital converter (ADC), and thousands of
measurands can be acquired along with digital data from multiple sources,
including the contents of the computer’s memory and data buses. In a PCM-based
system, the original PAM multiplexer’s analog output is digitized to a
parallel format. This, plus other sources of digital data, are merged
by the Output Formatter along with synchronization data for measurand
identification. The Output Formatter serializes the composite parallel
data stream to a binary string of pulses (1’s and 0’s) for transmission
on copper wire, fiber cable, or "the ether." All components
from after the sensor to the formatter comprise the encoder (see figure
below}. Other, often remote encoders are used to multiplex additional
sensor data into the main encoder’s output. Not only does this expand
the number of measurands to thousands per stream, but it also eliminates
the weight of cables required for each sensor.
The output of the main encoder
is filtered and transmitted via radio
transmitter and antenna, coax cable, telephone line, tape recorder, etc.
Filtering rounds or smoothes the square data pulses to reduce frequency
content and thus the required transmitter bandwidth. At the Ground Station,
the received data stream is amplified. Since the transmission path often
distorts the already rounded signal, a bit synchronizer reconstructs
it
to the original serial square wave train. Then, a decommutator or decom
(similar to that found in L-3’ Visual Test System or System 550) recognizes
the synchronization pattern and returns the serial digital stream to
parallel
data. The decom also separates the PCM stream into its original measurands
(also known as prime parameters) and data.
The computer (in the Visual Test System) or the telemetry front end (System
550) selects prime parameters for real-time processing; archiving to disk
or tape; display; output to strip chart recorders and annunciators; or
distribution to other computing resources according to the test plan.
L-3 Telemetry-West and its
sister divisions manufacture virtually the entire telemetry system — from
signal conditioners to antennas for the Airborne System and from antennas
to telemetry receivers for the Ground
System. The following table breaks down which L-3 divisions provide what
for today's telemetry system requirements. Just remember that when
you
need to put everything together, L-3 Telemetry-West will help
specify, integrate, and install all L-3 and third-party components for
a total telemetry solution..
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