A calibration system for large-aperture sonar arrays

Acoustic calibration of a large-aperture, low-frequency sonar array presented several problems not normally encountered in conventional sonar array calibration:
\bullet
Far-field measurements were not possible due to sound field refraction from warm water near the surface.
\bullet
Pulse separation of surface reflections couldn't be used due to the free-time/pulse-length ratio.
\bullet
Rotator shaft indexing for beam patterns was not feasible due to barge motion and extreme hydrophone separation. A calibration system was designed and built which utilized quasi-far-field measurements with a directional' transducer and an auxiliary acoustic angular position reference system. The system was installed and used to obtain transmit and receive beam pattern and response data with calibration accuracy close to conventional techniques.