Original vintage 1988 Magnavox Electronic Systems Company advertisement for the company's line of anti-submarine warfare (ASW) sonobuoys - "The Navy has been dropping our sonobuoys for over 35 years.  Magnavox has been dropping the price for just as long.  Today, Magnavox sonobuoys give the U.S. Navy more than twice the capability - at half the 1953 cost.  Advances gained through technology allow us to cut cost, yet maintain the highest quality in detection capability.  Magnavox sonobuoys find the underwater threat."
 
Dimensions: Approx. 8 inches wide by 10.75 inches high.
 
A sonobuoy (a combination of sonar and buoy) is a small expendable sonar buoy dropped from aircraft or ships for ASW or underwater acoustic research.  Sonobuoys are typically around five inches in diameter and three feet long.  When floating on the water, sonobuoys have both a radio transmitter above the surface and hydrophone sensors underwater, and the devices are mission-critical platforms for enhancing Undersea Domain Awareness (UDA), providing an effective means to detect, locate, and track submarines and other underwater threats.  Playing a key role in ASW and other naval operations, they support in maintaining naval security and in protecting naval carrier strike groups.  Find an in-depth, helpfully unclassified 2013 study on the evolution of the sonobuoy here.
 
Sonobuoys are ejected from aircraft in canisters and deploy upon water impact – an inflatable surface float with a radio transmitter remains on the surface for communication with the aircraft, while one or more hydrophone sensors and stabilizing equipment descend below the surface to a selected depth that is variable, depending on environmental conditions and the search pattern.  The buoy relays acoustic information from its hydrophone(s) via UHF/VHF radio to operators on board the aircraft.
 
Modern anti-submarine warfare methods evolved from the techniques devised for the movement of convoys and battle groups through hostile waters during World War II, when it was imperative submarines be detected and neutralized long before the task group came within range of an attack.  Aircraft-based submarine detection was the obvious solution.  The maturity of radio communication and sonar technology made it possible to combine a sonar transducer, batteries, a radio transmitter and whip antenna, within a self-contained air-deployed floating (sono)buoy.
 
Early sonobuoys had limited range, limited battery life and were overwhelmed by the noise of the ocean.  They first appeared during World War II, in which they first were used in July 1942 by RAF Coastal Command under the code name “High Tea,” the first squadron to use them operationally being No. 210 Squadron RAF, operating Sunderlands.  
 
The early sonobuoys were limited by the use of human ears to discriminate man-made noises from the oceanic background.  However, the devices demonstrated the technology as viable, and with the development of better hydrophones, the transistor and miniaturization, and the realization that very low frequency sound was important, more effective acoustic sensors followed.  The sonobuoy went from being an imposing six-foot tall, two-foot diameter sensor to the compact suite of electronics it is today.
 
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 a crashed 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.
The bathythermobuoy (BT), which relays 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.
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.
The 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.
 
Information is then analyzed by computers, acoustic operators and tactical coordinators to interpret the sonobuoy information.
 
Magnavox Electronic Systems Company
Magnavox Electronic Systems Company (MESC) was the defense/industrial arm of Magnavox, a historic American electronics firm known for innovations like the first commercial TV (1920s) and video game console (Odyssey, 1972).  A specialized, defense-oriented division that branched off from the well-known consumer electronics brand, MESC specialized in military tech (radios, command systems) before being sold off, with Hughes Aircraft buying its defense segment in the mid-90s.
 
The three areas of business of the MESC operation during the late 1980s and early 1990s were C-Cubed (Command, Control, and Communication), Electronic Warfare, and sonobuoys.  When Hughes Electronics sold its aerospace and defense operations to Raytheon, the former Magnavox defense operations were transferred as well.  
 
Shortly thereafter, Raytheon spun off the sonobuoy operation to form Under Sea Systems Inc (USSI), in Columbia City, Indiana.  In 1998, Raytheon sold USSI to a British defense consortium named Ultra Electronics. The company is now a wholly owned subsidiary of Ultra, manufacturing water and acoustic sensing and communications devices for military and civil defense.
 
Active and/or passive sonobuoys may be laid in large fields or barriers for initial detection – active buoys are used for precise location, while passive buoys are 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.