External Markings Each sonobuoy has marked on the sonobuoy case the following information: nomenclature or type, serial number, manufacturer’s code number, RF channel number, contract lot number, weight, and prelaunch setting. Sonobuoy type and RF channel number are also stamped on each end of the buoy. Sonobuoys with EFS will have no RF channel number markings because the channel will be selected by the operator. Deployment The sonobuoy is aircraft deployable by any of four methods: spring, pneumatic, free-fall, or cartridge. Because descent velocities can exceed 120 feet per second, a descent-retarding device is used to increase aerodynamic stability and to reduce water-entry shock. A parachute or a rotating-blade assembly (rotochute) is used as the descent-retarding device. Because of the different descent characteristics of the parachute and rotochute, do not intermix the two. With intermixed sonobuoys, the spacing of the tactical pattern submarines might be missed. Water Entry and Activation The force of water impact, will not be right and or battery activation, initiates the deployment or jettison of-the various sonobuoy components. Jettisoning of the bottom plate allows the hydrophone and other internal components to descend to the preselected depth. Upon the release of the parachute or rotochute, the antenna is erected. In some sonobuoys, a seawater- activated battery fires a squib, which deploys a float containing the antenna. A termination mass and/or drogue stabilizes the hydrophone at the selected depth, while the buoyant sonobuoy section or float follows the motion of the waves. A section of elastic suspension cable isolates the hydrophone from the wave action on the buoyant section. Most of the sonobuoys in the fleet today are equipped with seawater-activated batteries, which provide the power required for the sonobuoy electronics. Data transmission from the buoys usually begins within 3 minutes after the buoy enters the water. In cold water and/or water with low salinity, the activation time might be increased. Some sonobuoys now have nonwater-activated lithium batteries. Sonobuoy Operating Life At the end of the preselected time, the sonobuoy transmitter is deactivated. The sonobuoy has either an electronic RF OFF timer, or, as is most common, the transmitter is deactivated when the buoy is scuttled. At the end of the sonobuoy life, or for some types of sonobuoys upon RF command, a mechanism allows seawater to flood the flotation section in the buoy. In some cases, the flotation balloon is deflated to scuttle the unit. Either way, the unit fills with seawater and sinks. SONOBUOY CLASSIFICATION Sonobuoys are grouped into three categories: passive, active, and special purpose. Passive sonobuoys are used in LOFAR and DIFAR systems. Active sonobuoys are used in CASS and DICASS systems. Special-purpose sonobuoys are used in missions other than ASW. These sonobuoys and acronyms, along with their meanings and relation- ships to each other, are discussed below. Passive Sonobuoy The passive sonobuoy is a listen-only buoy. The basic acoustic sensing system that uses the passive sonobuoy for detection and classification is known as the low-frequency analysis and recording (LOFAR) system. LOFAR SYSTEM.— With this system, sounds emitted by the submarine are detected by a hydrophone that has been lowered from a passive omnidirectional sonobuoy. Data regarding the frequency and amplitude of these sounds are then transmitted by the sonobuoy antenna to the receiving station. At this station, normally on the aircraft, the sound data is analyzed, processed, displayed, and recorded. The basic LOFAR display plots the frequency of the sound waves against the intensity of their acoustic energy, and against the duration of the sound emission. This data can be displayed on a video screen and printed out. The data is also recorded on magnetic tape for storage and retrieval when desired. DIFAR SYSTEM.— The directional low- frequency analysis and recording system (DIFAR) is an improved passive acoustic sensing system. Using 4-25