Telemetry is a system of sending data, usually measurements, over a distance. The term comes from the Greek telistos ("remote") and metros ("measure"). Telemetered data may be physical, environmental or biological. Telemetry is typically used to gather data from distant, inaccessible locations, or when data collection would be difficult or dangerous for a variety of reasons. In telemetry, specialized instruments carry out measurements of physical quantities, and store or transmit the resulting signal, often after some initial signal processing or conversion.
Telemetry is frequently used on space missions, both manned and unmanned. It informs ground control about the condition of the crew and of various critical parts and functions of the spacecraft, as well as observations made by the spacecraft of its surroundings. Telemetry is also used in many other applications, such as conveying information from the bottom of a oil drilling rig to the surface or tracking the location of a vehicle on the ground by GPS (global positioning system).