The History of Sonobuoys The term sonobuoy is a contraction of the words sonar and buoy. During the waning days of World War One, the British Admiralty invented an effective Sound Navigation and Ranging (SONAR) system for its warships to detect subsurface obstacles. However, it wasn’t until the Second World War that the ravaging wolf‐packs of German U‐Boats convinced the allies that a remote device was needed to detect the submarines as they approached from astern. The proposed solution was to drop buoys equipped with hydrophones from convoy ships. These special "radio sonobuoys" fitted with HF transmitters operated on one of six frequencies to relay detection of a submarine to the convoy, alerting them of an imminent attack. As a result of the joint war effort between the UK and USA, early sonobuoy development activities took place at the War Research Laboratory of Columbia University, the Sonar Research Unit of Portland and the Royal Aircraft Establishment at Farnborough. By March 1942, airborne tests using Blimp K‐5 and the US Navy submarine S‐20 validated the overall concept of using sonobuoys for detecting submarines and resulted in the first sonobuoy system becoming operational in early 1943. These early trials demonstrated an acoustic detection range of 3 miles with an RF range of 5 miles. The first successful launch of a sonobuoy from an aircraft was made from a US Army B‐18 bomber in July 1942. One month later, buoys were dropped against a German U‐boat. Early in 1943, the US Navy deployed the system in B‐24 Liberator's flying from Argentia, Newfoundland, and from Dunkeswell, Devon. Nonetheless, it was a Canadian Hudson, operating out of Iceland, which later that year claimed the first submarine 'kill' for an aircraft using sonobuoys. Hudson Antisubmarine Patrol Aircraft Proving its merit, the US Navy ordered 59,700 CRT‐1A sonobuoys, which were used in antisubmarine action around the world during the Second World War. Although somewhat unsophisticated, these B‐
sized (6.875” x 60”) omni‐directional sonobuoys played an essential part in the birth of airborne ASW. By 1950, Ultra was producing sonobuoys for the UK MoD using a Ministry of Aviation design. The SSQ‐
2, an A‐size (4.75” x 36”) Low Frequency Analysis and Recording (LOFAR) sonobuoy, replaced the CRT‐1A and entered production in 1953. The CRT‐1A was followed by the development of the CRT‐4 directional sonobuoy in early 1943, which was finally accepted into inventory in 1955 as the SSQ‐1. Operationally, these sonobuoys were laid in patterns of five buoys and the operator estimated the signal strength of the target on each of the buoys in contact. The navigator then used concentric circle overlays in an attempt to localize the submarine using relative signal strength. As technology was added to the ASW aircraft, the use of Doppler was use to ascertain whether the submarine was travelling towards or away from a particular sonobuoy as well as its speed of advance. In the late 1950s, Canada, the UK and the USA entered into a tripartite agreement to standardize the principal features of sonobuoys. This arrangement was eventually extended to NATO and today the NATO subgroup 41 meets annually to discuss interoperability. CRT‐1A Sonobuoy In 1960, the SSQ‐28 commenced production and was specifically designed to be deployed from the US Navy’s P‐2V Neptune aircraft. With its 95 foot sensor depth and seawater battery power, the SSQ‐28 is viewed as the first true LOFAR sonobuoy and was able to detect underwater sounds between 10 and 2,500 hertz. In 1961, the production version of the first active sonobuoy, the B‐size SSQ‐15, entered operational service. Although weighing more than 60 pounds, the SSQ‐15 was reliably able to provide the airborne ASW operator with active detection ranges of more than 2,500 yards. With the emergence of the large, four‐engine P‐3 Orion ASW aircraft in 1962, sonobuoys began to take on more importance as the Cold War simultaneously began to heat up. Accurate sonar predictions were required in order to tactical employ the new generation of air deployed sonobuoys and, as a consequence, the SSQ‐36 bathythermograph buoy was developed. With the ability to map sea temperature as a function of depth, the sound velocity profile could be calculated and detection ranges and probable submarine operating depths determined. SSQ‐36 Bathybuoy The SSQ‐41 LOFAR emerged in the mid 1960s as the standard omni directional (LOFAR) sonobuoy and was manufactured by Ultra, Magnavox, and Sparton. Canada and the UK developed their own versions of these sonobuoys (SSQ‐527 and SSQ‐906 respectively) and literally millions of these sonobuoys were manufactured with production ending in the 1980s. Although the LOFAR sonobuoy offered relatively long range passive detection of submerged submarines, localization and tracking was difficult and highly dependent on Doppler analysis. The SQ‐53 series of Directional Frequency Analysis and Recording (DIFAR) sonobuoys was developed in order to overcome this issue. The first DIFAR sonobuoy entered production in the late 1960s and has been the mainstay of passive ASW ever since. The latest versions include Canada’s SSQ‐53D(3), the US SSQ‐53F, and Britain’s SSQ‐955. Although serving essentially the same purpose, each of these DIFAR sonobuoys are unique in the way in which they function. The SSQ‐
955 is a G‐size (4.75” x 16.5”) gravity launch buoy that employs a digital uplink to the aircraft, the SSQ‐
53F contains not only a DIFAR sensor, but also a calibrated omni sensor, as well as, an uncalibrated omni sensor near the surface. Lastly, the SSQ‐53D(3) was designed to offer superior performance in the low frequency regime, where much of a submarine’s propulsion plant sound is found. Similarly, the range only SSQ‐15 active sonobuoy evolved into the SSQ‐47 and then, ultimately the SSQ‐62 Directional Command Active Sonobuoy System (DICASS). The latest variant being the SSQ‐62E, which offers the selection of four different acoustic ‘ping’ frequencies and the ability to command the sonobuoy to different operating depths after it has been deployed. Today quieter submarines operating in littoral waters have once again tilted the antisubmarine war in favour of the submarine. However, a counter volley, in the form of multi‐statics, is being launched by the airborne ASW community. Using high‐power, low frequency sources coupled with specially designed passive receivers, the maritime patrol community hopes to recoup some, if not all, of the lost ground. Ultra is deeply involved in all aspects of multi‐static development in the UK, Canada, and US through the on‐going development and manufacture of the SSQ‐926, SSQ‐565, and SSQ‐125 multi‐static active sources and SSQ‐981, SSQ‐573, and SSQ‐101 multi‐static receivers. Ultra’s Portfolio of Sonobuoys