P-2 Neptune, S-2 Tracker, S-3B Viking and P-3 Orion for anti-submarine warfare. Operation Sonobuoys are classified into three categories: active, passive and special purpose. Active sonobuoys emit sound energy (pings) into the water and listen for the returning echo before transmitting information—usually range and bearing—via UHF/VHF radio to a receiving ship or aircraft. The original active sonobuoys pinged continuously after deployment for a predetermined period of time. Later, Command Activated Sonobuoy System (CASS) sonobuoys allowed the aircraft to trigger pings (or buoy scuttling) via a radio link. This evolved into DICASS (Directional CASS) in which the return echo contained bearing as well as range data. Passive sonobuoys emit nothing into the water, but rather listen, waiting for sound waves (for instance, power plant, propeller or door-closing and other noises) from ships or submarines, or other acoustic signals of interest such as an aircraft's black box pinger, to reach the hydrophone. The sound is then transmitted via UHF/VHF radio to a receiving ship or aircraft. Special purpose sonobuoys relay various types of oceanographic data to a ship, aircraft, or satellite. There are three types of special-purpose sonobuoys in use today. These sonobuoys are not designed for use in submarine detection or localization. BT—The bathythermobuoy (BT) relay bathythermographic and/or salinity readings at various depths. Laying a pattern of sonobuoys is often preceded by laying one or more bathythermobouys to detect density/temperature strata. Such strata can act as sonar reflectors or, conversely, as waveguides. SAR—The search and rescue (SAR) buoy is designed to operate as a floating radio frequency beacon. As such, it is used to assist in marking the location of an aircraft crash site, a sunken ship, or survivors at sea. ATAC/DLC—Air transportable communication (ATAC) and down-link communication (DLC) buoys, such as the UQC, or "gertrude", are intended for use as a means of communication between an aircraft and a submarine, or between a ship and a submarine. This information is analyzed by computers, acoustic operators and tactical coordinators to interpret the sonobuoy information. Active and/or passive sonobuoys may be laid in large fields or barriers for initial detection. Active buoys may then be used for precise location. Passive buoys may also be deployed on the surface in patterns to allow relatively precise location by triangulation. Multiple aircraft or ships monitor the pattern either passively listening or actively transmitting to drive the submarine into the sonar net. Sometimes the pattern takes