Telemetry covers the range of techniques for converting observations, made by sensors remote from the data collection and analysis facility, to a form that can be transmitted by an appropriate electromagnetic transmission facility.
Early telemetry from, for example, guided missiles used various modulation techniques to put a continuously varying instrument reading onto a radio frequency received by an earth station. Extremely complex analog tape recorders, with a precise time code, captured the information.
Later, as digital electronics became increasingly common, the telemetry carried information from multiple sensors, sampled repeatedly, and sent as bit streams.
Terrestrial system control
Especially when the sensors can be connected to terrestrial transmission facilities, they are often encapsulated in Internet Protocol (IP) packets and transferred via the public Internet or a virtual private network. Telemetry need not be for exotic applications such as "rocket science", but simply sending back the water level in a river or lake, or the position of an animal being tracked. System Control And Data Acquisition (SCADA) combines remote sensing and remote control.
Information security doctrine demands that while SCADA may be sent by IP, if it involves critical infrastructure, it must not travel over the Internet. It should also be encrypted.
Telemetry from moving vehicles
- See also: Foreign instrumentation signals intelligence
- See also: Telemetry intelligence
When the telemetry comes from moving platforms, such as aircraft and missiles, it is often necessary to have steerable directional antennas, sometimes very large, to stay focused on the vehicle's transmitter. To receive signals from transmitters deep in space, fully steerable parabolic antennas, 34 meters in diameter, were common.
Since size and weight are critical on missiles, and the higher the radio frequency, the smaller the antenna, there has been a trend to move to increasingly high frequencies with shorter wavelengths. By the time of Project Apollo, S-band was common, rather than the VHF of the Second World War and through the fifties.