Sonobuoy
The sonobuoy , sometimes also called a sonar buoy, is a special buoy equipped with hydrophones (underwater microphones) to detect submarines or other sources of sound underwater. They are usually of maritime patrol aircraft used for submarine tracking. For this task they are thrown from the aircraft; the hydrophone is released by the impact and falls on a wire to a predetermined depth. A piece of the wire is gathered by an elastic rubber band, which decouples the movements of the hydrophone from the sea. The signal received by the buoy is sent to the aircraft in the VHF range. They are usually equipped with a flood hole closed by a salt tablet so that they sink after a predetermined time of a few hours when the tablet has dissolved.
The data sent by the buoys are evaluated in the aircraft with special analysis devices. The noises are subjected to a frequency analysis , for example . The source is deduced from the observed spectral lines and other characteristic features of the noise. If the observed noise is classified as a submarine with sufficient probability, an attempt is made to infer the position of the submarine by dropping several sonoboys in certain geometrical arrangements. Because they only listen, distances to a target can only be determined indirectly. You use the volume of the target and put it in relation to the distance from the hydrophone (loud = near, soft = far). Several buoys can be used to determine the approximate position and, after longer observation, the course and travel of the destination. The DIFAR buoys commonly used today also provide a bearing to the target.
During the Cold War, sonoboys were used for sea reconnaissance from the air, especially by the US Navy in numbers of several 100,000 per year.
Special forms of sonoboys
In addition to the standard buoys described, there are also more complex special types: DIFAR buoys ( Directional Frequency Analysis and Recording ), which enable horizontal directional determination through sound pressure gradients or through a horizontal arrangement of hydrophones. The VLAD buoys ( Vertical Line Array Directional ) buoys also dissolve vertically.
Active sono buoys work like an active sonar system and provide not only distance but also a bearing to the target. DICASS (Directional Command Activated Sonobuoy) and CAMBS (Command Activated Multibeam Sonobuoy) are common today.
To measure the environmental parameters, BT (Bathythermographic) buoys are used to measure the ambient noise and the temperature stratification.
Occasionally, the AXBTs (Airborne Expendable Bathythermograph ) are counted among the sonoboys because they are used via the same drop device on the aircraft and therefore look similar to the sonoboys. They are used to measure the speed of sound profile in the water.
Some helicopters use sonoboys as a supplement to sonar, systems that are lowered into the water with a winch on the cable and then retracted again. An active sonar is normally used for this. Helicopters generate annoying noise in the water directly below them, which makes passive localization much more difficult.
Special seawater-resistant sonar buoys in accordance with SAE AS 8045 are used on ships. The sonar buoys are attached to data protection capsules (Voyage Data Recorder VDR / S-VDR) and, in the event of activation (immersion in water), have the task of providing a location signal in the range of 25 - 50 kHz (DIN EN 61996, Maritime Shipping) for at least 30 days ), with a sound pressure of 160.5 dB.
