Researchers at Stanford University have developed a new type of sonar to overcome the previously insurmountable problem of seeing underwater from the air. Sound does not travel easily between air and water: there is a 65-decibel loss, which means roughly a million-fold decrease in intensity, making it makes it virtually impossible to pick up sound reflections from the air. The new technology can map the seabed and potentially detect mines, submarines and other underwater targets from aircraft.
Currently, the only ways of using sonar from aircraft are sonar buoys (sonobuoys) dropped into the water, or dipping sonar lowered to the sea surface from a hovering helicopter. The helicopter cannot move while using dipping sonar, so it has to check one spot, raising the sonar, fly somewhere else, lowering the sonar again, and so on.
By contrast, the new Photoacoustic Airborne Sonar System or PASS, developed at Stanford with funding from the U.S. Navy, will work from a moving aircraft.
“Our vision of the proposed technology is to capture images continuously as the airborne vehicle flies over the water,” Stanford researcher Aidan Fitzpatrick told Forbes. “Similar to how synthetic aperture radar systems or in-water synthetic aperture sonar systems work today.”
https://www.forbes.com/sites/davidhambling/2021/02/04/new-sonar-sees-underwater-from-aircraft/
