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System Components

Most of today's telemetry systems consist of an input device known as a transducer, the radio wave medium of transmission, an instrument to receive and process the signal, and some type of recording or display instrumentation. The transducer obtains whatever it is that is being measured or monitored, like temperature or pressure, and converts that value into an electrical impulse. Transducers can have their own power source or they can be externally powered. Some examples of today's transducers are those used in weather balloons to obtain and convert measurements of temperature, barometric pressure, and humidity. Transducers are also used in manned space flights to measure an astronaut's heartbeat, blood pressure, and temperature as well as other biomedical indices. Transducers can also be employed for such mundane tasks as measuring the flow rate in a pipe.

Once something is measured and converted into an electrical signal by the transducer, this data must then be transmitted. Simple forms of telemetry, like a remote metering system, use wire links to a central control room. While radio is the preferred medium for use over long distances, other more specialized alternates are available, such as beams of light or sonic signals. Manned and unmanned space systems use radio communications, and we have become more familiar with the notion of telemetry as it applies to a distant, unmanned spacecraft taking measurements as it approaches another planet and sending them back to Earth. These radio telemetry systems use what is called a modulated signal since the data is varied by a radio link. This means that the data is varied or modulated by a subcarrier signal that actually carries it. This radio signal may be either a single data channel or it may be carry several types of information. Called multiplexing, this system combines several types of information into a single signal and is used for reasons of cost and efficiency. To be effective, various forms of separating out the data from the single signal are employed. One method is called the time division multiplexing system in which data is sent and received in a certain, set order or pattern. Time division multiplexing that involves a sequential action are common and very efficient. An alternative to time division in multiplexing is a system called frequency division. Where time division combines channels sequentially, frequency division system assign each channel its own frequency band. Although the frequency bands are individually allocated, they are still combined for simultaneous transmission.

Another communication link is called the address-replay system. This special program sends data only after receiving a command signal. Modulation by a subcarrier has already been noted, and there are mainly two different methods available. The first is an AM or FM system similar to commercial radio. The other is one of several types of pulse-based methods in which data is coded digitally into pulse groups and then transmitted. User needs and preferences usually depend which system is chosen. At the receiving end of the entire telemetry system, signals are separated and can be displayed in real time and/or stored by computers. Telemetry technology is being extended to the point where we can obtain astronomical information from the farthest planet, or biological data from within a person's body via micro-miniature transmitters.

Leonard C. Bruno

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Science EncyclopediaScience & Philosophy: Swim bladder (air bladder) to ThalliumTelemetry - History, System Components