Special-Purpose Sonobuoys

the passive directional sonobuoy (fig. 4-31), DIFAR operates by detecting directional information, and then frequency multiplexing the information to the acoustic data. This signal is then transmitted to the aircraft where it is processed and the bearing is computed. Subsequent bearing information from the buoy can be used to pinpoint, by triangulation, the location of the sound or signal source. Active Sonobuoy The active sonobuoy is either self-timed (the sonar pulse is generated by the buoy at a fixed pulse length and interval) or command actuated. The command activated buoy is controlled by a UHF command signal from the aircraft. An active sonobuoy uses a transducer to radiate a sonar pulse that is reflected back from the target. The time interval between the ping (sound pulse) and the echo return to the sonobuoy is measured. Taking the Doppler effect on the pulse frequency into con- sideration, this time-measurement data is used to calculate both range and speed of the submarine relative to the sonobuoy. RO SONOBUOYS.— Self-timed active sonobuoys, known as range-only (RO) sonobuoys, are set to ping for a limited period, starting from the time they are deployed. These buoys will provide information on range of targets only. CASS SONOBUOYS.— The command activated sonobuoy system (CASS) allows the aircraft to deploy the sonobuoy, but the buoy will remain passive until commanded to ping. This allows the aircraft to surprise the submarine. DICASS SONOBUOY.— The addition of a directional hydrophone turns the CASS sonobuoy into a DICASS buoy. A DICASS sonobuoy allows the aircraft acoustic analysis equipment to determine the range and bearing to the target with a single sonobuoy. DICASS sonobuoys are replacing the RO and CASS sonobuoys. Special-Purpose Sonobuoys There are three types of special-purpose sonobuoys in use today. These are the BT, SAR, and the ATAC sonobuoys. These sonobuoys are not designed for use in submarine detection or localization. Figure 4-31.-Block diagram of the DIFAR sonobuoy. BATHYTHERMOBUOY.— The bathythermo- buoy (BT) is used to measure water temperature versus depth. The water depth is determined by timing the descent of a temperature probe. Once the BT buoy enters the water, the probe descends automatically at a constant 5 feet per second. The probe uses a thermistor, a temperature- dependent electronic component, to measure the temperature. The electrical output of the probe is applied to a voltage-controlled oscillator. The oscillator’s output signal frequency modulates the sonobuoy transmitter. The frequency of the transmitted signal is linearly proportional to the water temperature. The water temperature and depth are recorded on graph paper that is visible to the ASW operator. The sonobuoy signal is processed by the acoustic 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 of an aircraft crash site, a sunken ship, or survivors at sea. The buoy can be launched from aircraft equipped to launch sonobuoys or deployed over the side by hand. Nominal RF output is 1 watt for 60 hours on sonobuoy channel 15 (172.75 MHz). A floating microphone is provided for one-way voice communi- cation. The RF beacon radiates automatically and continuously, regardless of whether the microphone is used. A flashing light and dye marker are incorporated in the buoy. The buoy also has an 8-foot tether line for attaching the buoy to a life raft or a person. 4-26