by Edward C. Whitman
|Born of a three-way marriage of early Cold War strategic necessity, World War II progress in underwater acoustics, and an extraordinary engineering effort, the Navy’s pioneering Sound Surveillance System – SOSUS – became a key, long-range early-warning asset for protecting the United States against the threat of Soviet ballistic missile submarines and in providing vital cueing information for tactical, deep-ocean, anti-submarine warfare. And although subsequent events – most notably the end of the Cold War – robbed SOSUS of much of its mission, its history remains an object lesson in how inspired, science-based engineering development can lead to extraordinary operational effectiveness.|
Indeed, rudimentary passive and active sonar techniques had already been used in World War I to search for submarines, but these earliest systems, at relatively high frequencies, achieved detection ranges of only several thousand yards under favorable conditions – and World War II sonars seldom did much better. The basic physical phenomena subsequently exploited in SOSUS to achieve longer-range submarine tracking were only discovered in the late 1930s and not adequately understood until mid-way through the 1939-1945 war.
An important early step in developing more effective sonar systems – and SOSUS in particular – was the invention of the sonic depth finder (SDF) in the early 1920s as a direct outgrowth of the rudimentary active sonars used in World War I. Not only did the SDF advance the state-of-the-art in acoustic technology, but it also facilitated detailed depth and ocean-bottom surveys with a speed and accuracy never before available using lead-line techniques. This, in turn, led to growing interest in marine geology and the adaptation of seismic methods developed for use on land to geological exploration of the sea floor. It was in this context in 1937 that Lehigh University scientist Maurice Ewing made a seminal observation while doing seismic refraction experiments in three-mile-deep water in the North Atlantic. Using underwater explosive charges as sound sources, Ewing noted that a chain of impulsive echoes – generated by repeated reflections between the ocean bottom and the sea surface – was clearly perceivable onboard his research vessel. From this result, Ewing reasoned that even allowing for a significant loss of sound intensity at each bottom and surface encounter, the sound signal of the charge – particularly at the lower frequencies – was capable of traveling great distances underwater with only limited attenuation. He further postulated that if there were horizontal sound propagation paths in the deep ocean that avoided surface and bottom reflections – a so-called “deep sound channel” – acoustic signals could travel hundreds, or even thousands, of miles and still be detectable by judiciously located hydrophones.
Almost simultaneously, another crucial element appeared with the invention
and refinement of the bathythermograph by scientists at the Massachusetts
Institute of Technology (MIT) and the Woods Hole Oceanographic Institution.
For the first time, the bathythermograph made possible the continuous
measurement of ocean temperature with depth – and most importantly,
the determination in detail
of how underwater sound speed varied with distance below the surface – the sound velocity profile (SVP). In conjunction with numerous in situ measurements of the SVP, growing theoretical understanding of how underwater sound “rays” are refracted – or bent – by vertical variations in the sound velocity provided the analytical tools to support Ewing’s hypothesis of long-range propagation paths under certain conditions of ocean temperature and depth.
In general, in warmer waters near the ocean surface, the sound speed is relatively high. At greater depths, where the water is increasingly cooler, the sound velocity decreases toward a minimum. At that point, pressure effects take over, and the sound speed begins to rise again as depth continues to increase. The deep sound channel is found at the depth where the sound velocity is a minimum. Because sound “rays” always tend to bend away from regions of higher sound velocity, a wave directed upwards from the sound channel axis will be refracted back down again – and a wave directed downwards will be bent upwards. Thus, sound paths from sources in the deep sound channel weave back and forth across the channel axis and – because they become “trapped” in a deep ocean layer away from the surface or bottom – can travel long distances with minimum attenuation. Moreover, if there exist propagation mechanisms available to bring near-surface sound down to the depth of the sound channel, those signals will also become trapped and traverse long distances with minimal loss. The sound channel axis is normally found at a depth of several thousands of feet, depending on thermal conditions, and because of the unusually warm waters of the Gulf Stream and Sargasso Sea, it lies more deeply in the Atlantic than in the Pacific.
The Deep Sound Channel – Growing ASW Potential
As part of the upsurge of ocean-acoustic research that accompanied the coming of World War II, Ewing and his colleagues performed a variety of at-sea experiments that further confirmed sound propagation in the deep sound channel, while also discovering the phenomenon of the near-surface convergence zone.1 On the basis of these experiments, Ewing proposed in 1943 that the Navy develop a system for communicating over long ranges by detonating time-coded explosive charges in the sound channel itself. Accordingly, during the spring of 1944, he supervised a major sea test in which USS Buckley (DE-51) steamed outward from a stationary receiving ship, periodically dropping small explosive charges fused for various depths. These explosions were still clearly discernable until Buckley had to break off the trial at a distance of 900 miles. By the end of the war, the Buckley experiments had led to a subsequent effort to develop an air-sea rescue system known as SOFAR – for Sound Fixing and Ranging. In the SOFAR concept, downed pilots would drop small explosive charges to the depth of the deep sound channel, where their sound output could be expected to travel for thousands of miles to deep, bottom-mounted hydrophones and triangulated to locate the survivors. At the time, however, exploiting the SOFAR channel for submarine detection at long range seems not to have been suggested, although by mid-war, the U.S. Navy was already using ray-tracing methods tactically for sonar performance prediction.
Even after World War II ended in mid-1945, the Navy continued to support a strong research program in underwater acoustics and, in particular, made enough additional progress in understanding the deep sound channel to establish – with the Army Air Force – major SOFAR networks in both the Pacific and Atlantic.2 However, with the onset of the Cold War and the growing danger of a Soviet submarine force based on the best of German World War II technology, the application of underwater sound specifically to anti-submarine warfare (ASW) became a top priority. By early 1950, the Navy had come to believe that Soviet submarines posed the greatest threat to America’s security and approached the Committee on Undersea Warfare (CUW), an academic advisory group empanelled in late 1946, for suggestions on studying the problem. The result was Project Hartwell, a series of MIT-organized technical meetings attended by top-level scientists and naval officers during the first half of the year. Not unexpectedly, long-range submarine detection was among a variety of undersea warfare topics discussed by the Hartwell participants. In this regard, physicist Frederick Hunt, former head of Harvard’s Underwater Sound Laboratory, electrified the gathering with a convincing argument that Ewing’s SOFAR channel could support long-range propagation modes sufficient for detecting submarines passively at distances of hundreds of miles. Moreover, frequencies below 500 Hz would penetrate readily to the deep sound channel from virtually any source depth. This insight – not universally accepted at the time – formed the scientific basis for SOSUS and made possible long-range undersea surveillance surprisingly early in the post-war era.
SOSUS – Early Engineering Development
As a key result of the Project Hartwell findings, the Office of Naval Research (ONR) in late 1950 funded a contract with the American Telephone and Telegraph company (AT&T) and its manufacturing arm, Western Electric, to develop an undersea surveillance system based on long-range sound propagation. Under this aegis, Bell Telephone Laboratories initiated a series of experimental trials by installing undersea listening arrays off Sandy Hook, New Jersey and Eleuthera in the Bahamas. Additionally, AT&T adapted its sound spectrograph, which had recently been invented as a tool for analyzing speech sounds, into a similar device called LOFAR – for Low Frequency Analysis and Recording – designed to analyze low-frequency underwater signals in near-real time. Both LOFAR and the spectrograph generated a frequency-versus-time representation of an incoming sound “bite” on which the time history of its spectral content was indicated by the blackening of specially-sensitized paper by an electrostatic stylus that swept repeatedly along the frequency axis. In this way, the presence of distinctive submarine sound signatures – comprising both broadband noise and discrete frequency components (“tonals”) – could be discerned against the ocean background in the composite signal picked up by an array. This body of work, largely at AT&T, was code-named Project Jezebel and placed under the direction of CAPT Joseph Kelly at the Bureau of Ships.
Meanwhile, the Navy continued to support Maurice Ewing, by then at Columbia University’s Hudson Laboratory, to study the general phenomenology of low-frequency underwater sound. This effort, augmented by additional work at Woods Hole and the Scripps Institution of Oceanography in California, was focused on establishing a solid understanding of long-range sound transmission and denoted Project Michael. When the findings of Projects Jezebel and Michael were brought together for the purpose of designing, engineering, and deploying the broad-area surveillance system envisioned by Hartwell’s Frederick Hunt, the resulting effort – with the highly classified acronym, SOSUS – was eventually given the unclassified designation, Project Caesar.
The first prototype of a full-size SOSUS installation – a 1,000-foot-long line array of 40 hydrophone elements in 240 fathoms of water – was deployed on the bottom off Eleuthera by a British cable layer in January 1952. After a series of successful detection trials with a U.S. submarine, the Navy decided by mid-year to install similar arrays along the entire U.S. East Coast – and then opted two years later to extend the system to the West Coast and Hawaii as well. These early SOSUS line arrays were positioned on the sea floor at locations that accessed the deep sound channel and oriented at right angles to the expected threat axis. Their individual hydrophone outputs were transmitted to shore processing stations called “Naval Facilities” – or NAVFACs – on multi-conductor armored cables.
At the NAVFACs, the acoustic signals were processed to create a fan of horizontal “beams,” each of which represented the composite sound signal from a small angular sector – on the order of two to five degrees wide – oriented in a particular azimuthal direction. Narrow-band time-frequency analysis in the spectral region was performed on these multiple beam outputs simultaneously using the LOFAR technique described above. The ability of narrow-band frequency analysis not only to discriminate against broadband ocean noise but also to identify characteristic frequencies associated with rotating machinery was key to detecting and classifying targets. A LOFAR analyzer was associated with each beam of each array served by a NAVFAC, and typically, the large watch floors were filled with hundreds of these “gram-writers” busily turning out LOFARgrams on “smoky paper” 24 hours a day. These records were scrutinized continually by specially-trained personnel looking for the distinctive submarine “signatures” which gave indication of a possible target along a given bearing line. Then, if simultaneous contacts were gained on multiple arrays in separated locations, the target’s position could be estimated by triangulation.