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The matter of distance in telemetry is relative, however, because
such systems may also be employed for obtaining data from sites that
are near to the receiving instruments but that are difficult, impossible, or
dangerous for human observers to encounter. For example, biological

sensors of various kinds may be used within the human body to transmit
information on medical conditions to detectors placed outside the body.
Other examples include the use of telemetry for running tests of engines,

for detecting flaws or changing conditions in industrial systems, or for
obtaining data from dangerously radioactive sites.
Meteorologists make use of a wide range of telemetric devices to
obtain information from the upper atmosphere for use in making their

weather forecasts. Such meteorological uses were, in fact, the first to
which the techniques of radio telemetry were applied.
In any telemetric system, the equipment used must be able to make

a measurement of a physical quantity, produce a signal that can be
modified in some way to carry the measured data, and relay this encoded
signal over some form of transmission link. The receiving equipment

must then be able to decode the signal and to display it in some format
for analysis and, probably, for recording. Usually more than one signal
must be sent over the transmission link at any one time, in which case

some form of multiplexing must be used. This can be done by
employing different frequency bands for the measurement of different
quantities or by splitting up the signal into discrete time intervals to
which the quantities to be measured are assigned.

The coding techniques used are commonly digital; the use of
pulse-code modulation, by which continuous waves are transformed into
a binary-code signal, has been enhanced in recent decades by the

advances made in the digital computer field and in microelectronics.

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