Rangefinder

A rangefinder is a device that measures distance from the observer to a target, in a process called ranging.

Active methods use unilateral transmission and passive reflection. Active rangefinding methods include laser, radar, sonar, lidar and ultrasonic rangefinding.

Other devices measure distance using trigonometry (stadiametric rangefinders and parallax, or coincidence rangefinders). Older methodologies that use a set of known information, usually distances or target sizes, to make the measurement, have been in regular use since the 18th century.

Special ranging makes use of actively synchronized transmission and travel time measurements, hence the time difference between several received signals is used to determine exact distances. This principle is used with SatNav, the Satellite Navigation class of systems. In conjunction with a standardized model of the Globe surface a certain location on the Globe may be determined with high accuracy. Ranging methods without accurate time synchronization of the receiver are called pseudorange, used e.g. in GPS positioning.

With other systems ranging is obtained from passive radiation measurements only, hence the noise or radiation signature of the object is generating the signal that is computed for range. However such method requires multiple measurements to obtain range by taking multiple bearings instead of appropriate scaling of active pings, otherwise the system is just capable to provide a simple bearing from any sole measurement.

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