Phase calibration of sonar systems using standard targets and dual-frequency transmission pulses

Abstract

The phase angle component of the complex frequency response of a sonar system operating near transducer resonance is usually distorted. Interpretation and classification of the received sonar signal benefits from the preservation of waveform fidelity over the full bandwidth. A calibration process that measures the phase response in addition to the amplitude response is thus required. This paper describes an extension to the standard-target calibration method to include phase angle, without affecting the experimental apparatus, by using dual-frequency transmission pulses and frequency-domain data processing. This approach reduces the impact of unknown range and sound speed parameters upon phase calibration accuracy, as target phase is determined from the relationship of the two frequency components instead of relying on a local phase reference. Tungsten carbide spheres of various sizes were used to simultaneously calibrate the amplitude and phase response of an active sonar system in a laboratory tank. Experimental measurements of target phase spectra are in good agreement with values predicted from a theoretical model based upon full-wave analysis, over an operating frequency of 50-125 kHz.

A primary method for the complex calibration of a hydrophone from 1 Hz to 2 kHz

“…Driven by the needs of the ultrasound community, calibrations can now include phase measurement using non-linear acoustic wave propagation [5,6], time-delay spectrometry [7,8], and pulse excitation [9,10] methods. Lower frequency, underwater calibrations are also using phase [11].…”

“…The type H48 reference is used well below it first resonance, but the low end of the frequency range can be affected by the low-frequency cutoff, problematic when secondary references are needed to measure frequencies below 5 Hz. Extending the coupler reciprocity calibration measurement to include phase is important to provide traceability for secondary references and to reduce Driven by the needs of the ultrasound community, calibrations can now include phase measurement using non-linear acoustic wave propagation [5,6], time-delay spectrometry [7,8], and pulse excitation [9,10] methods. Lower frequency, underwater calibrations are also using phase [11].…”