If published reports are to be believed, the US Navy neglected its anti-submarine warfare (ASW) capabilities after the conclusion of the Cold War. At that time, the only potentially hostile nation with a global submarine fleet was Russia and while Russia made great strides in quieting its later classes of attack submarines, there was no money or industrial base after the Soviet Union’s fall even to maintain its current fleet.
Because of the reduction in perceived threat, the Navy pared down its ASW patrol presence – particularly in the Atlantic – and closed key bases that supported patrol squadrons, redirecting aging patrol aircraft to other, non-ASW missions. At the same time, the Navy also reduced its ASW training budget for the surface fleet with some units never participating in coordinated and realistic ASW exercises in spite of the tiered requirements set down in the Surface Force Training Manual and as required during Inter-Deployment Training Cycles (link is to a Rand report on comparative ASW training between DDG-51 destroyer crews and their British and French counterparts).
In the meantime, the world witnessed the rapid development and proliferation of ultra-quiet diesel-electric (DE) and air-independent propulsion (AIP) submarines whose hull and plant noise signatures fell below the signal-to-noise levels detectable by the Navy’s passive detection systems, especially in noisy littoral areas. The Swedes and Australians in particular repeatedly embarassed the Navy in exercises. Their Gotland and Collins-class submarines, respectively, have been able to penetrate Navy battle groups undetected and even “sink” Los Angeles-class submarines assigned to protect the fold.
As a result, the Navy began taking steps by 2004 to recover its slipping ASW capabilities, creating an integrated ASW command structure while investing in new technologies and reconsidering long-held passive-detection doctrine, integrating into SURTASS powerful low-frequency active sonars for use in the littorals. It seems, however, that purely acoustic technologies are yielding diminishing returns, in spite of heavy investment in new ocean arrays, fiber-optic technologies and new digital signal processing suites (much of which is off-the-shelf-technology).
In spite of the Navy’s ASW lassitude, research into non-acoustic technologies has been ongoing for more than a generation, with inventors and defense researchers looking for a breakthrough technology that could change the ASW paradigm altogether. While MAD and FLIR aren’t breakthrough technologies, much research has gone into laser technology – with noticeable results.
As far back as 1966, the Office of Naval Research conducted research into the use of infrared lasers to detect surface anomalies, including the Bernoulli Hump (a surface bulge of displaced water left by shallow-running submarines) and Kelvin Waves (the characteristic V-shaped wakes left by objects moving through the water). While the results of the research appeared promising, it wasn’t until inventors began experimenting with blue-green lasers – which can penetrate ocean water – that the promise of lasers began to be realized.
In a series of US patents beginning in 1971, inventors explored practical systems to employ blue-green lasers. Let’s look briefly at some of the technologies:
US 3,604,803 (1971 – Kahn). The ‘803 Patent describes an airborne system that uses a broad and narrow beam pulsing laser to look for the “shadow” caused by a submarine in the path of the beams.
US 4,867,558 (1989 – Leonard, et al – subsequently assigned to GTE Government Systems). The ‘558 Patent’s abstract describes an airborne system that uses pulsed beams to read thermoclines:
A pulsed laser beam is directed into the water to at least the depth of the thermocline and an analysis is made of the resultant Brillouin and Rayleigh backscatter components. Wavelength shifted Brillouin scatter is mixed with the unshifted Rayleigh scatter in a self-heterodyne manner for each volume element of illuminated water, and the frequency of the heterodyne signal is measured and converted into temperature. This produces the desired temperature-depth profile of the water enabling detection of internal waves generated by submarines.
US 4,867,564 (1989 – Sweeney, et al – subsequently assigned to GTE Government Systems). The ‘564 Patent – which is related to the ‘558 Patent – describes an airborne system to do thermal-depth ocean profiles:
The subsurface temperature of a body of water such as an ocean is measured remotely by directing a laser beam deeply into the water and analyzing the resultant Brillouin and Rayleigh backscatter components. Wavelength shifted Brillouin scatter is mixed with the unshifted Rayleigh scatter in a self-heterodyne manner for each volume element of illuminated water and the frequency of the heterodyne signal is measured. This produces the desired temperature-depth profile of the water.
US 4,893,924 (1990 – Leonard, et al – assigned to GTE Government Systems). The ‘924 Patent – filed by the same inventor group – describes a submarine-based system:
Subsurface waves in an ocean are created by the turbulence in a submarine’s wake. These waves can be remotely detected by a search submarine by monitoring subsurface water temperatures using a laser. A pulsed laser beam is directed into the water to at least the depth of the thermocline and an analysis is made of the resultant Brillouin and Rayleigh backscatter components. Wavelength shifted Brillouin scatter is mixed with the unshifted Rayleigh scatter in a self-heterodyne manner for each volume element of illuminated water, and the frequency of the heterodyne signal is measured and converted into equivalent temperature values. This produces the desired temperature-depth profile of the water enabling detection of the first submarine by tracking the internal waves at or near the ocean thermocline.
US 5,270,780 (1993 -Moran, et al – owned by Science Appl. Int. Corp.). The ‘780 Patent describes an underwater LIDAR system:
A light detection and ranging (LIDAR) system uses dual detectors to provide three-dimensional imaging of underwater objects (or other objects hidden by a partially transmissive medium). One of the detectors is a low resolution, high bandwidth detector. The other is a high resolution, narrow bandwidth detector. An initiallaser pulse is transmitted to known x-y coordinates of a target area. The photo signals returned from the targetarea from this initial pulse are directed to the low resolution, high bandwidth detector, where a preliminary determination as to the location (depth, or z coordinate) of an object in the target area is made based on the time-of-receipt of the return photo signal. A second laser pulse is then transmitted to the targetarea and the return photo signals from such second laser pulse are directed to the high resolution, narrow bandwidth detector. This high resolution detector is gated on at a time so that only photo signals returned from a narrow “slice” of the target area (corresponding to the previously detected depth of the object) are received. An image of the detected object is then reconstructed from the signals generated by the high resolution detector. In a preferred embodiment, the two detectors are housed in a single digicon tube, with magnetic deflection being used to steer the beam to the appropriate detector.
US 5,504,719 (1996, Jacobs – owned by Martin-Marietta). The ‘719 Patent describes an underwater laser hydrophone array:
The present invention relates to a hydrophone and to a virtual array of hydrophones for sensing the amplitude, frequency, and in arrays, the direction of sonic waves in water. The hydrophone employs a laser beam which is focused upon a small “focal” volume of water in which natural light scattering matter is suspended and which matter vibrates in synchronism with any sonic waves present. The vibration produces a phase modulation of the scattered light which may be recovered by optical heterodyne and sensitive phase detection techniques. The sonic waves are sensed at locations displaced from the focusing lenses. Because of this remote sensing capability, the physicalhardware of an array of hydrophones may be confined to a small area comparable to the dimensions of the lenses themselves while the sensing of the sonic waves virtually occurs at widely spaced, remote focalvolumes. Thus, by combining the signals from these remote focal volumes, a virtual array of hydrophones may be formed whose dimensions are large enough in relation to the sonic wavelengths of interest to achieve high directionality but without the penalties of hydrodynamic drag usually associated with large area arrays.
US 7,283,426. (2007 – Grasso – assigned to BAE Systems). The ‘426 Patent describes an airborne pulse laser to sense water particle movement within a water column shot with the laser:
A method for detecting, tracking and locating submarines ( 24 ) utilizes pulsed coherent radiation from a laser ( 12 ) that is projected down through a water column, with particles in the water producing speckle from backscatterof the random particle distribution, with correlation of two closely time-spaced particle-based speckle patterns providing an intensity measurement indicative of the presence of a submarine. Subsurface submarine movement provides a subsurface wake which causes movement of particles such that two closely-spaced “snapshots” of the returns from particles in the same water column can detect particle movement due to the wake. The magnitude of the speckle pattern change indicates particle movement.; In one embodiment, the return signals are imaged onto an intensified CCD or APA array that capture two successive laser pulses through the utilization of dual pixel registered cameras. Note that in the subject system, phase information is converted to measurable intensity information relating to particle motion.
Clearly, work on blue-green lasers has been paying off. The Navy has begun production and deployment of the AN/AES-1 Airborne Laser Mine Detection System. Produced by Northrop-Grumman, the lightweight system is contained in a pod that can be hung off the pylon of a Sea Hawk helicopter. While I haven’t seen the system schematics, I suspect that the device incorporates one or more of the technologies just described. At the same time, DARPA is working on a powerful blue laser to provide submarine communication and detection.
So, it would appear that our “boffins” have been hard at work on non-acoustic submarine detection. And I’ll bet the systems of the future – whether airborne or submarine-based – will include a blue-green laser of considerable power.
That’s probably state of the art – for the moment.