Standard-Target Calibration of Active Sonars Used to Measure Scattering: Principles and Illustrative Protocols

The standard-target sonar-calibration method was used as early as the 1940s. Its development since then is reviewed, as is the prerequisite for the method, the standard target. The theoretical basis for the method is established, with the aim of enabling sonar output signals to be expressed in absolute physical units of scattering. This is done with reference to the volume scattering coefficient s _V and physical effects intrinsic to operational sonars. A constituent of s _V , the generally bistatic acoustic sampling volume, is elaborated. Formulas are developed for the calibration constant in the energy domain and the combined transmit-receive frequency response function in the spectral domain. Practical elements of a calibration are elaborated in terms of the sonar specification, candidate calibration venue, environmental state, and calibration exercise design, including reference-beam approach. These elements are illustrated through the example of a standard-target calibration of a sonar with multiple beams at a land-based facility. The venue is specified in terms of its measurement volume, and equipment and instrumentation. Preparations for the exercise are enumerated, and reference-beam and nonreference-beam protocols are described. The intended audience for this paper is those who require a basic understanding of principles to be able to organize or undertake the standard-target calibration of new sonars on arbitrary platforms or known sonars on new platforms.