What are small, expendable floating hydrophone units used by antisubmarine aircraft called?

The correct answer is that small, expendable floating hydrophone units used by antisubmarine aircraft are called "sonobuoys." Sonobuoys are designed to detect underwater sounds, particularly those emitted by submarines, by deploying them from aircraft into the ocean. Once deployed, they float on the water's surface while listening for acoustic signals in the water, helping to track and locate submarines.

Sonobuoys come in different varieties, including passive and active models. Passive sonobuoys listen for sounds made by submarines, such as engine noise or cavitation from propellers, while active sonobuoys send out sound waves and listen for their echoes to detect submarines. Their expendable nature allows for widespread deployment in antisubmarine warfare operations, making them an essential tool for naval forces.

In contrast, fathometers measure water depth and provide critical information for navigation but do not serve the same purpose as sonobuoys. Depth charges are explosive devices meant to destroy submarines when dropped from ships or aircraft, rather than detecting them. Dipping hydrophones are specialized sonar devices used by helicopters to listen for submarines while in a hovering position above the water but are not the same as the floating sonobuoys used extensively for aerial surveillance.