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Telemetry

Telemetry

History

System components

Telemetry is a technology of obtaining quantities or making measurements from a distant location and transmitting them to receiving equipment where they are recorded, monitored, or displayed. The word telemetry is originally derived from the two Greek roots tele for remote and metron for measure. A basic telemetry system consists of a measuring instrument or detector, a medium of transmission (sending), a receiver, and an output device that records and displays data. Today, telemetric systems are mainly used for monitoring manned and unmanned space flights (such as those operated by the National Aeronautics and Space Administration [NASA]); obtaining meteorological, oceanographic, and medical data; and for monitoring power-generating stations. It is also used in motor racing, wildlife research, and retail management, to name a few. Primarily, telemetry applies to wireless communications but can also apply to the transfer of data through wires such as with the use of telephones.

History

the word telemetry did not come into use until some medium of transmission had been invented.

Since it is defined as the communication of measurements that were taken from some distant point, the earliest telemetry system was one based on an electrical wire. Following the invention of the telegraph and later the telephone, one of the earliest known United States patents for a telemetry system was granted in 1885. These first telemetry systems were used by electric power companies to monitor the distribution and use of electricity throughout their systems. They were called supervisory systems because of their monitoring abilities. One of these original telemetry systems was installed in Chicago, Illinois, in 1912. This network used the citys telephone lines to transmit data on its several electric power generating plants to a central control station. Following World War I (19141918), this data was transmitted by the electric power lines themselves. Although electrical telemetry systems are still in use, most modern telemetry systems use radio transmissions to span substantial distances.

System components

Most of todays 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 todays 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 astronauts 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. Crewed and uncrewed space systems use radio communications. The notion of telemetry has become more familiar due to the frequent use of spacecraft exploring the solar system and, specifically, 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 sub-carrier signal that actually carries it. This radio signal may be either a single data channel or it may 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 certain set order or pattern. Time division multiplexing, which 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 assigns 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 sub-carrier has already been noted, and there are mainly two different methods available. The first is an AM (amplitude modulation) or FM (frequency modulation) 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 on 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 astronomers can obtain astronomical information from very distant planets, or biological data from within a persons body via micro-miniature transmitters.

Leonard C. Bruno

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