Multibeam echosounder calibration with split-aperture transmission

Results are described for a multibeam echosounder calibration experiment in which the elevation angle of a standard spherical target was estimated using a split aperture geometry on the transmit array. This is an extension of the two transmitters - one receiver concept described by Messer et al. (IEEE TAES 32-3, 1158-1164, 1996). Here, three transmitters and one receiver were used to obtain elevation angle estimates from the difference in time delays or phase delays across each baseline. The tests were conducted from a barge at dockside in about 10 m of water depth. A 500-kHz multibeam echosounder was mounted on a pan and tilt unit and deployed at the end of a pole. The tests were done using a standard tungsten-carbide sphere, 38.1 mm in diameter, suspended on a monofilament nylon line, in the far field of the transducer arrays vertically below the sonar head. Quasi-orthogonal linearly frequency modulated pulses with about 96 kHz bandwidth were transmitted in pairs (downchirp then upchirp) from each baseline. When the target remains stationary relative to the sonar for the duration of a ping pair cycle (<;0.1 s) elevation angles estimates were self-consistent within ±0.2° for the time delay method as well as for the phase delay method. Results of the two methods agreed to within 0.2° -0.3°.