Standards Development

Scope

The methodology for the standard-target calibration of active sonar systems used for imaging and for measuring scattering will be elaborated for eventual standardization in measurement protocols for individual sonars. This applies to echo sounders, multibeam sonars, sidescan sonars, parametric sonars, and other active sonar systems. The nominal span of sonar operating frequencies is from 1 kHz to several megahertz, including both narrowband and broadband signals. Excluded from the standard are active sonars whose output cannot be calibrated because of uncontrolled processes.

Purpose

Active sonar is widely used for purposes of underwater imaging and measurement. Exemplary applications include, among others, quantification and tracking of aquatic organisms, imaging of the bottom habitat, measurement of suspended particulates, e.g., sand and mud, inspection of ship hulls and underwater structures, e.g., offshore oil platforms and pipelines, search and discovery by law enforcement agencies, mine detection and localization, as in mine countermeasures (MCM), underwater archaeology.

While many of these applications are conducted with uncalibrated sonars, all would benefit from calibration, or measurement of the performance characteristics of the sonar. This can enable quantitative uses, detection of performance problems, including changes over time, verification of manufacturer or user specifications, among other things.

Calibration is conducted for various sonars, depending on the application and user. However, this can be quite cumbersome and time-consuming, while subject to error propagation if the traditional analytical approach is followed. According to such an approach, the sonar is viewed as an assembly of pieces, each of which can be calibrated separately before combination of the part-results in an overall transfer function or characteristic, or in a sensitivity factor.

The standard-target method of calibrating sonar, the subject of the Technical Specification being proposed in this NWIP, is radically different. A single external element, the standard target, or a compact object with known acoustic backscattering properties, is suspended at a known position in the transmit and receive beams of the sonar transducer(s), enabling the overall transfer characteristic to be determined by a single measurement. The method can be executed as the sonar is configured for use. It is simple and rapid, hence cost-effective, while achieving unprecedented precision and accuracy. Guidelines have been published on the standard-target method, especially for application to scientific echo sounders and certain multibeam sonars. None of these works has been published by an international standards body. In addition, the method is more broadly applicable to any sonar that maintains control of the signal in internal processing. The operating frequency of candidate sonars is from about 1 kHz to several megahertz, including both narrowband and broadband signals.

The diversity of sonar types, manufacturer-specific features including evolving models, transducer shapes and sizes, and available venues that are suitable for calibration is enormous. far exceeding what could be expressed in a single set of specific measurement protocols. At the same time, present knowledge is sufficient to elaborate principles that can be used to guide development of specific calibration protocols for the arbitrary active sonar. This is the basis for preparing a new work item to formulate a Technical Specification.