Aviation Electronics Technician 1 - Historic Naval Ships Association

External MarkingsEach sonobuoy has marked on the sonobuoy casethe following information: nomenclature or type,serial number, manufacturer’s code number, RFchannel number, contract lot number, weight, andprelaunch setting. Sonobuoy type and RF channelnumber are also stamped on each end of the buoy.Sonobuoys with EFS will have no RF channel numbermarkings because the channel will be selected by theoperator.DeploymentThe sonobuoy is aircraft deployable by any offour methods: spring, pneumatic, free-fall, orcartridge. Because descent velocities can exceed 120feet per second, a descent-retarding device is used toincrease aerodynamic stability and to reducewater-entry shock. A parachute or a rotating-bladeassembly (rotochute) is used as the descent-retardingdevice. Because of the different descentcharacteristics of the parachute and rotochute, do notintermix the two. With intermixed sonobuoys, thespacing of the tactical pattern will not be right andsubmarines might be missed.Water Entry and ActivationThe force of water impact, or battery activation,initiates the deployment or jettison of-the varioussonobuoy components. Jettisoning of the bottomplate allows the hydrophone and other internalcomponents to descend to the preselected depth.Upon the release of the parachute or rotochute, theantenna is erected. In some sonobuoys, a seawateractivatedbattery fires a squib, which deploys a floatcontaining the antenna. A termination mass and/ordrogue stabilizes the hydrophone at the selecteddepth, while the buoyant sonobuoy section or floatfollows the motion of the waves. A section of elasticsuspension cable isolates the hydrophone from thewave action on the buoyant section. Most of thesonobuoys in the fleet today are equipped withseawater-activated batteries, which provide the powerrequired for the sonobuoy electronics. Datatransmission from the buoys usually begins within 3minutes after the buoy enters the water. In cold waterand/or water with low salinity, the activation timemight be increased. Some sonobuoys now havenonwater-activated lithium batteries.Sonobuoy Operating LifeAt the end of the preselected time, the sonobuoytransmitter is deactivated. The sonobuoy has eitheran electronic RF OFF timer, or, as is most common,the transmitter is deactivated when the buoy isscuttled. At the end of the sonobuoy life, or for sometypes of sonobuoys upon RF command, a mechanismallows seawater to flood the flotation section in thebuoy. In some cases, the flotation balloon is deflatedto scuttle the unit. Either way, the unit fills withseawater and sinks.SONOBUOY CLASSIFICATIONSonobuoys are grouped into three categories:passive, active, and special purpose. Passivesonobuoys are used in LOFAR and DIFAR systems.Active sonobuoys are used in CASS and DICASSsystems. Special-purpose sonobuoys are used inmissions other than ASW. These sonobuoys andacronyms, along with their meanings and relationshipsto each other, are discussed below.Passive SonobuoyThe passive sonobuoy is a listen-only buoy. Thebasic acoustic sensing system that uses the passivesonobuoy for detection and classification is known asthe low-frequency analysis and recording (LOFAR)system.LOFAR SYSTEM.— With this system, soundsemitted by the submarine are detected by ahydrophone that has been lowered from a passiveomnidirectional sonobuoy. Data regarding thefrequency and amplitude of these sounds are thentransmitted by the sonobuoy antenna to the receivingstation. At this station, normally on the aircraft, thesound data is analyzed, processed, displayed, andrecorded. The basic LOFAR display plots thefrequency of the sound waves against the intensity oftheir acoustic energy, and against the duration of thesound emission. This data can be displayed on avideo screen and printed out. The data is alsorecorded on magnetic tape for storage and retrievalwhen desired.DIFAR SYSTEM.— The directional lowfrequencyanalysis and recording system (DIFAR) isan improved passive acoustic sensing system. Using4-25