This Sonar System Could Reveal the Secrets of the Ocean’s Floors

A team of engineers at Stanford University have created an airborne sonar system that “bends” light and sound to map the ocean floors.

Advaith Malka
Published in
4 min readDec 29, 2020


The PASS uses both light and sound to map the ocean floor (Credit: ArbabianLabVideos)

Five percent. Only five percent of our home planet’s oceans have been explored. This is because it is far easier to send a human to space than it is to send one to the depths of the ocean. Traditional SONAR devices have attempted to solve this problem. Using sound waves to determine the proximity of an object, SONAR has, without a doubt, revolutionized the industry. Unfortunately, SONAR does have some limitations. For example, SONAR waves will lose almost 99.99% of their energy when traveling from air to water and vise versa. This makes it almost impossible to use SONAR airborne for large-scale ocean mapping projects. Fortunately, to combat this issue, researchers from Stanford University have developed the Photoacoustic Airborne Sonar System (PASS), which uses both light and sound to generate high-quality images of the ocean floor from the air.

What Is the PASS?

The function of the PASS is best summarized by Aidan Fitzpatrick, the author of the study:

“If we can use light in the air, where light travels well, and sound in the water, where sound travels well, we can get the best of both worlds.”

An illustration of how the PASS works (Credit: Aidan Fitzpatrick)

Simply put, Aidan Fitzpatrick and his team have found a way to combine both light and sound to generate high-quality images of the ocean floor.

How Does the PASS Work?

A picture showing the photoacoustic effect being used to detect hemoglobin in red blood cells (Credit: Science in School)

The PASS utilizes a principle called the photoacoustic effect to function. The photoacoustic effect, which was first discovered by Alexander Graham Bell in the late 1800s, occurs when light is absorbed by an object and causes thermal expansion. As a result, the object emits sound waves, which can then be detected by an ultrasound sensor.

In this case, Fitzpatrick and his team have used the photoacoustic effect on water.

When the PASS fires a special laser, modulated at the desired acoustic frequency, onto the surface of the water, thermal expansion occurs on the surface of the ocean, which causes sound waves to be emitted from the water surface. Once underwater, these acoustic waves behave similar to SONAR waves, reflecting off objects and sending signals back to the UAV. Once the UAV receives these waves, the data is processed to form human-recognizable images.

What Is in Store for the PASS?

A reconstructed image of an “S” by the PASS (Credit: Aidan Fitzpatrick)

While the PASS is not ready for production yet, Fitzpatrick thinks it should be fairly straightforward to get the machine ready for real-world testing. Given 6–12 months, he estimates that the PASS will be set for testing on real UAVs, out in the real-world ocean.

Once ready for wide-scale mass production, the PASS could change the world as we see it. Large-scale ocean mapping projects could take place, uncovering new biomes and potentially finding new species. The PASS may also be of use for the military, where it could detect underwater threats or submarines from the air. Not only that, but Fitzpatrick also envisions this system to be used for finding lost airplanes or sunken ships. The PASS may allow us to uncover the mystery of Malaysia Airlines Flight 370, which disappeared somewhere in the South Indian Ocean.

(Circled above) Sound waves still need to pass through the air-water barrier, causing them to lose some of their energy.

Unfortunately, a small problem (still) persists: the electromagnetic waves from the PASS still need to pass the air-water barrier to be picked up by the UAV. As a result, the electromagnetic waves sent out by the PASS still lose about 90% of their energy. This is more efficient than traditional SONAR, which loses 99.99% of its energy, but improvements can still be made. The researchers might be able to solve this efficiency problem if they find a way to mount a smaller sensor that could sense the incoming sound waves underwater. Nevertheless, the PASS is a great idea that could change our perception of the ocean and change the world for years to come.