Telemetry, from the Greek tele (far) and metron (measure), is the collection of data using automated sensors that transmit their results to a central monitoring point. A telemetric sensor may be stationary (e.g., fixed on the sea floor) or aboard a mobile platform (e.g., airplane, spacecraft, missile, submarine). The quantities sensed are usually simple variables that can be reported at regular intervals, such as temperature, pressure, humidity, altitude, fuel level, battery voltage, salinity, vibrational intensity, alarm status, or the like. Complex, high-speed signals such as video are usually not termed telemetry, even when they are collected remotely by unattended devices.
The raw output of a remote sensor is often an analog signal, that is, a voltage or current that varies smoothly with time. Before transmission, such a signal is usually converted to digital form by the process of analog-to-digital conversion or sampling. In sampling, an analog signal is examined at evenly-spaced moments and a binary number assigned to its magnitude; the larger the sensor output, the larger the binary number. The raw bitstream produced by sampling is organized by the telemetry device into standard-length frames containing added information specifying data type, time of acquisition, and so forth. If the transmission channel is noisy, the signal may also be subjected to error-correction coding to allow recovery of data from errors. The signal may also, in some military applications, be encrypted before transmission. The final telemetry signal is sent from the data-collection point using radio, sonar, coaxial cable, or some other medium to a receiving station, where it is recorded and monitored by computers or human operators.
Telemetry is employed for many purposes throughout the commercial, scientific, and military sectors. For example, controllers of missiles, torpedoes, spacecraft, or remotely piloted aircraft such as the Predator require access to numerical information of many sorts in order to monitor and adjust the performance of these complex machines. Telemetric data may also be used for surveillance purposes, as when deep-sea acoustic sensors are used to track submarine movements, and is essential to the control of spacecraft, whether crewed or robotic.
█ FURTHER READING:
Wilson, Elizabeth. Introduction to AMMOS Telemetry Processing. Jet Propulsion Laboratory, NASA. October 18, 2001. <http://tel.jpl.nasa.gov/~betsy/mm/intro.htm> (Nov. 14, 2002).
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