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Definition:
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A device that measures the distance to a target, such as the glacier surface, by timing the return of echoes from acoustic (typically ultrasonic) pulses emitted by the device itself. A suitably mounted sonic ranger can yield a continuous record of relative change of surface height and, given information on density changes, can contribute to monitoring of accumulation or ablation. Knowledge is required of the speed of sound in the medium traversed by the pulse and its echo, which may in turn require knowledge of the temperature of the medium. The medium is usually air.
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