Synthetic aperture sonar calibration using active transponders

Abstract

There is an emerging desire within the synthetic aperture sonar community to establish the theory and practical means for calibrating imagery, such that each pixel value represents an actual backscatter coefficient. Image calibration has long been standard practice for remote sensing synthetic aperture radar. However, there are a number of challenges that make calibration of sonar imagery more difficult. Among these is the fact that good inexpensive passive targets (like corner reflectors used in radar) do not have an exact counterpart in underwater acoustics. Passive calibration targets for imaging sonar, such as solid spheres or specially-designed focusing spheres, are used in practice, but active transponders offer far more capability and flexibility. The advantages and applications of active transponders are discussed, along with the current development status of a project at GTRI to design and build an affordable and recoverable calibration device.