Radar and lidar (LIDAR) are one of the most efficient tools for surface survey and mapping. They can be equipped on airplanes and satellites to provide accurate data through clouds and even forests. But they can’t do much to survey under the sea, because the sea absorbs a lot of signals. At present, the most effective way to map the seabed is still sonar, but because sonar signals need to be sent underwater, many oceans have not been mapped. < / P > < p > if the sonar signal is sent directly from the air to the underwater, more than 99.9% of the energy will be lost in the conversion process. That’s why the outside world becomes so quiet when you dive to the bottom of the pool. The remaining 0.1% of the energy can only produce a sonar signal, but when it returns to the air from the water, it loses 99.9% of the energy. < / P > < p > military forces around the world mainly use sonar to detect submarines, and mainly rely on equipment at the bottom of the ship. At present, the closest to the airborne sonar system is Thales’s flash low-frequency broadband system. The cable hanging at the bottom of the helicopter hangs down from the cable and submerges into the sea like a tea bag. But this method is low efficiency, high cost and not suitable for large-scale survey. < / P > < p > now, a team at Stanford University has developed a new type of UAV underwater survey technology, which has found the technology to send and receive sonar signals from airborne platforms without touching water. This development will eventually make it easier to map the seabed and survey marine life. < / P > < p > although sound waves and radar can not effectively transmit information when facing the air-water barrier, the research team found that photoacoustic method can obtain signals through the barrier in at least one direction. Photoacoustic airborne sonar system (pass) emits laser to the water surface, and its intensity pulse reaches the required audio frequency. With the absorption of laser energy, it generates ultrasonic wave in the water, which can be used as an effective sonar wave to bounce underwater objects before jumping back to the water surface. < / P > < p > the study’s first author, graduate student Aidan Fitzpatrick, said: “if we can use light in air with good light transmission and use light in water with good sound transmission, then we can do both.” visit. < / P > < p > it’s true that almost all the energy of sound waves will be consumed when propagating from water to the air, but this one-way loss still leaves enough signals to be “heard” by the sensors on the airborne equipment. “We have developed a system that is sensitive enough to compensate for this order of magnitude of loss, and still can detect and image the signal,” said Amin arbabian, who led the study After the signal is recorded, it is analyzed by software to create a 3D image of the submerged object. The software can correct the refraction of sound waves when they enter the air from the water. < / P > < p > and then the deployment tool itself. “Our vision for this technology is to be in a helicopter or a drone,” Fitzpatrick said. We hope that the system can fly at an altitude of tens of meters above the water. ” Helicopters and drones both push a lot of air and make a lot of noise, and this (not to mention the noise of the environment itself) may pose a greater challenge to ultra sensitive transducers as they try to pick up tiny pieces of debris. Global Tech