In-situ acoustic calibration technique for range and bearing arrays

Achieving maximum angle measurement accuracy from a Short Baseline (SBL) or Super Short Baseline (SSBL) array requires precise knowledge of the acoustic centers of the multiple receiving elements. Individual SSBL arrays, in which all receiving elements are part of the same structure and have baselines generally under 20 centimeters, are usually calibrated in an acoustic test tank. However, when an SBL array, where the individual receiving elements are separate structures and have baselines up to a few meters, is mounted on a vessel then the calibration must be performed in-situ. One method of calibrating an SBL array on a ship in dry dock would be to perform a laser survey of the installation mounting for each of the SBL elements. On a ship of opportunity the user could use the foundation mounting locations obtained from the ship´s drawing to determine these locations. Still, the location of the acoustic center of each SBL element relative to the mounting is unknown. Now, this error may be minor depending on the physical size of the SBL array and on the accuracy requirements imposed on the array, but a dry dock installation requires a large amount of time and is therefore not very cost effective or even possible when using a ship of opportunity. When the accuracy requirements for an installed array require precise knowledge of the locations of the acoustic centers of the receive elements in-situ calibration methods must be used. This also allows for a diver installation of the array instead of a dry dock installation. One method of determining the unknowns of the installed array is to deploy transponders and collect acoustic data as the ship maneuvers within the field of transponders. Required measurements would include acoustic times of arrival and time-differences-of-arrival, as well as the ship´s speed, heading, attitude and geodetic position as a function of time. The purpose of this paper is to discuss the number of transponders required, the required ship´s track within the field of transponders and the requirements imposed upon the ship´s equipment to calibrate an installed array. Results of simulations will also be presented.