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..