Aviation Electronics Technician 1 - Historic Naval Ships Association

the passive directional sonobuoy (fig. 4-31), DIFARoperates by detecting directional information, andthen frequency multiplexing the information to theacoustic data. This signal is then transmitted to theaircraft where it is processed and the bearing iscomputed. Subsequent bearing information from thebuoy can be used to pinpoint, by triangulation, thelocation of the sound or signal source.Active SonobuoyThe active sonobuoy is either self-timed (thesonar pulse is generated by the buoy at a fixed pulselength and interval) or command actuated. Thecommand activated buoy is controlled by a UHFcommand signal from the aircraft. An activesonobuoy uses a transducer to radiate a sonar pulsethat is reflected back from the target. The timeinterval between the ping (sound pulse) and the echoreturn to the sonobuoy is measured. Taking theDoppler effect on the pulse frequency into consideration,this time-measurement data is used tocalculate both range and speed of the submarinerelative to the sonobuoy.RO SONOBUOYS.— Self-timed active sonobuoys,known as range-only (RO) sonobuoys, are set to pingfor a limited period, starting from the time they aredeployed. These buoys will provide information onrange of targets only.CASS SONOBUOYS.— The command activatedsonobuoy system (CASS) allows the aircraft todeploy the sonobuoy, but the buoy will remain passiveuntil commanded to ping. This allows the aircraft tosurprise the submarine.DICASS SONOBUOY.— The addition of adirectional hydrophone turns the CASS sonobuoy intoa DICASS buoy. A DICASS sonobuoy allows theaircraft acoustic analysis equipment to determine therange and bearing to the target with a singlesonobuoy. DICASS sonobuoys are replacing the ROand CASS sonobuoys.Special-Purpose SonobuoysThere are three types of special-purposesonobuoys in use today. These are the BT, SAR, andthe ATAC sonobuoys. These sonobuoys are notdesigned for use in submarine detection orlocalization.Figure 4-31.-Block diagram of the DIFAR sonobuoy.BATHYTHERMOBUOY.— The bathythermobuoy(BT) is used to measure water temperatureversus depth. The water depth is determined bytiming the descent of a temperature probe. Once theBT buoy enters the water, the probe descendsautomatically at a constant 5 feet per second.The probe uses a thermistor, a temperaturedependentelectronic component, to measure thetemperature. The electrical output of the probe isapplied to a voltage-controlled oscillator. Theoscillator’s output signal frequency modulates thesonobuoy transmitter. The frequency of thetransmitted signal is linearly proportional to the watertemperature. The water temperature and depth arerecorded on graph paper that is visible to the ASWoperator. The sonobuoy signal is processed by theacoustic equipment on board the aircraft.SAR BUOY.— The search and rescue (SAR)buoy is designed to operate as a floating RF beacon.As such, it is used to assist in marking the location ofan aircraft crash site, a sunken ship, or survivors atsea. The buoy can be launched from aircraft equippedto launch sonobuoys or deployed over the side byhand. Nominal RF output is 1 watt for 60 hours onsonobuoy channel 15 (172.75 MHz). A floatingmicrophone is provided for one-way voice communication.The RF beacon radiates automatically andcontinuously, regardless of whether the microphone isused. A flashing light and dye marker areincorporated in the buoy. The buoy also has an 8-foottether line for attaching the buoy to a life raft or aperson.4-26