GR-3

Sound Ranging Set GR·3·C is designed to detect the sound of artillery fire or shell detonation and record the difference in arrival time of the sound wave at a maximum of eight microphone locations. The microphones may be placed in various geometrical configurations (line, arc, triangle:, space array) at intervals up to a maximum of 2,000 yards. Either one of two outposts are located forward of the array of microphones at a distance sufficient to interrcept the sound wave and start operations of the recording equipment before the sound reaches the microphones. The signal detected by each microphone is loca1ly amplified and transmitted over wire lines to a central station where it is recorded photoghaphically by an oscillograph. The record exhibits identifying characteristics of the sound waves and is used for reading the differences in times of arrival of the same sound at the different microphones. This is explained in detail in FM 6-120, Field Artillery Field Manual, The Observation Battalion.