Method for Large Sonar Calibration and Backscattering Strength Estimation

This paper presents the technological implementation of method for calibration and backscattering strength estimation for sonar systems. In this method, the electrical and acoustic quantities of the sonar are measured separately rather than using the standards target method. Also outlined are the advantages and disadvantages of this method in comparison to the standard target method. The presented method is based on the acoustical calibration of the receiving and transmitting arrays and comprises the precise electrical calibration of the receiving and transmitting electronics. The acoustic calibration of the transmitter and receiver is based on measuring certain characteristics for each individual channel during the manufacturing process. Transmit Voltage Response (TVR), Open Circuit Response (OCR) and array receive and transmit directional responses (beam patterns) are measured in both vertical and horizontal planes. The transmit and receive responses relate the acoustic pressure to the projector driving voltage or to the voltage generated by the hydrophone. The electrical calibration addresses the issue of the linearity of the system with controlled Time Varied Gain (TVG) and accounts for a difference in the gain and phase for each channel in the data acquisition device. TVG is an analog amplifier in which gain is controlled by a Digital Signal Processor (DSP). The characteristic of this amplifier can be nonlinear or can depart from the requested shape. That discrepancy needs to be addressed and compensated for to achieve precision calibration. The basic backscattering strength estimation is done in real-time, utilizing both calibration data and sonar acquired data. The compensation and stabilization for pitch and roll for both receiving and transmitting arrays are additionally incorporated into the real-time beamforming. The transmitting pitch and receiving roll stabilization are conducted using state-of-the-art FPGA architecture followed by the pitch and roll compensation of the Tx-Rx beam envelopes executed during the signal processing procedure. Extensive sound bottom coverage, which constitutes the yaw compensation, is accomplished using the multi-ping technique over the frequency range. As an additional advantage to this method, the echo level processing for each receiving beam is controlled by the adaptive bottom-detection algorithm. The advantages of this method are demonstrated using a large, five ton, hull mounted sonar system where the standard target method can not be successfully conducted