Farfield calibration of large aperture arrays

Abstract

The calibration of large aperture sonar transducers in the farfield is difficult due to the problems of propagation and mechanical stability. An experiment is described in which several transducers with apertures up to 40λ were calibrated at 100 kHz, at ranges in excess of 30 m. The measurements were made in a commercial facility which consists of an instrumented barge in a flooded quarry near Cornwall, Ontario. The transducers calibrated are commercial side scan sonar tow fish. The farfield measurements were made with a hydrophone positioned from the ice surface, in late winter. A baffling arrangement arising from a composite tow vehicle configuration was demonstrated to be inappropriate. An alternative two vehicle configuration was subsequently evaluated by calibration at the same facility in summer with the reference hydrophone mounted on a guyed tower. This calibration arrangement proved less practical than the ice based scheme.