Bearing Deviation Indicator for Sonar

This paper describes the bearing deviation indicator (BDI), a war development that greatly increased the effectiveness of sonar equipment in antisubmarine warfare. Limitations of sonar equipment without BDI are explained, the value of crossed-lobe techniques for improved bearing accuracy is discussed, and the value of simultaneous lobe comparison in underwater sound work is pointed out. Methods of meeting the over-all requirements are described, and examples are given of service equipment. Certain auxiliary features of these equipments-time-varied gain, reverberation-controlled gain, and own-Doppler nullification-are discussed, and mention is made of specialized test equipment.