Uncertainty in bearing estimation with the Volume Search Sonar

Abstract

The Volume Search Sonar uses a cylindrical receiver array consisting of 40 staves of 9 elements each mounted parallel with the axis of the cylinder, itself aligned with the fore-aft axis of the towbody on which it is installed. 27 fore-aft beam pairs are formed with 27 sub-arrays of 16 contiguous staves. Previous work [Brogan et al., J. Acoust. Soc. Am., 115(5-2), 2547, 2004] has shown that a combination of monopulse and conjugate product processing applied to beam pairs yielded the time and angles of arrival necessary to obtain bathymetry and seafloor acoustic backscatter imagery from seafloor echoes received at the VSS array. Here we analyze the estimation uncertainties for these angles of arrival. In monopulse processing applied to fore-aft beams with common phase centers, a signal-to-noise ratio (SNR) in excess of 15 dB is necessary for the standard deviation of the angle estimate to remain below 0.5° in long-range mode (±4° about broadside), whereas an SNR in excess of 20 dB is required to obtain the same angular uncertainty in short-range mode (±8° about broadside). For the conjugate product processing applied to pairs of adjacent across-track beams with distinct phase centers, an SNR in excess of 24 dB is required to maintain the standard deviation of angle estimates below 1°, highlighting the need to improve such estimates by filtering the real and imaginary parts of the conjugate product output.