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Force Projection Through the Littoral Zone: Optical Considerations

Kendall Carder*


“… Maritime supremacy is still the most effective means to project power“ (Khine Latt, personal communication, 2008). Power projection or smuggling through the littoral zone will be enhanced by stealthy transit. What defenses may be needed by 2025?

NAVIGATIONAL AIDS

Active navigational aids such as sonar in coastal waters are noisy, while quiet, optical methods may be range-limited, especially for turbid waters. Risk of visual surveillance in clearer waters may limit opposition forces and smugglers to nocturnal operations even when other considerations (e.g., tides, currents, etc.) may support daylight transits. What methods might be employed by 2025?

Recent theory and applications (e.g., Fournier 2006 and refs. cited) suggest that ultra-fast (femtosecond) near infrared (NIR) lasers can reduce exponential light attenuation to only linear attenuation for pure water. Incredibly, absorption lengths in the linear mode could be kilometers. However, uncertainties remain with regard to attenuation (absorption and scattering) of fast pulses by phytoplankton and colored dissolved organic matter, delaying practical applications of this technique until much more research takes place.

*

SRI International



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Force Projection Through the Littoral Zone: Optical Considerations Kendall Carder* “. . . Maritime supremacy is still the most effective means to project power“ (Khine Latt, personal communication, 2008). Power projection or smuggling through the littoral zone will be enhanced by stealthy transit. What defenses may be needed by 2025? NAvIGATIONAL AIDS Active navigational aids such as sonar in coastal waters are noisy, while quiet, optical methods may be range-limited, especially for turbid waters. Risk of visual surveillance in clearer waters may limit opposition forces and smugglers to nocturnal operations even when other consid- erations (e.g., tides, currents, etc.) may support daylight transits. What methods might be employed by 2025? Recent theory and applications (e.g., Fournier 2006 and refs. cited) suggest that ultra-fast (femtosecond) near infrared (NIR) lasers can reduce exponential light attenuation to only linear attenuation for pure water. Incredibly, absorption lengths in the linear mode could be kilometers. However, uncertainties remain with regard to attenuation (absorption and scattering) of fast pulses by phytoplankton and colored dissolved organic matter, delaying practical applications of this technique until much more research takes place. * SRI International 1

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1 KENDALL CARDER With a navigation or communication LIDAR always pointing below the critical angle, direct light passing through the surface for detection by aircraft would be negligible. Light reflecting off the bottom would become diffuse, reducing its perceptibility from above the surface. The marked extension of range provided by successful applications of femtosecond laser and LIDAR packages would be useful for improved navigation, mine detection, and optical communications. OPTICAL COMMUNICATIONS Even without the range extension expected using ultra-fast lasers, optics can play a role in quietly off-loading high-rate data (e.g., bot- tom imagery, mine searches) collected by AUVs. Circling around a sub- merged communication buoy, low power optical modems on the AUV can communicate with the buoy at visible wavelengths. The buoy itself can optically communicate directly with a loitering UAV using a very- thin, optical-fiber “periscope.” It can also communicate directly with UAVs from below the surface at visible wavelengths. 3D OPTICAL MODELS Entire 3D light fields have been calculated with a hybrid marine optical model (HyMOM) that calculates the response functions for vari- ous elements (e.g., water cubes, boundaries) and combines them into an environment. The environment is then modeled by inserting light at the air-sea interface and solving iteratively using a relaxation approach. The light field is calculated for all model directions at each cube face (e.g., every 25 cm). Higher resolution in directional and spatial dimensions greatly increases the memory requirements and computational speed needed for rapid solution of these problems, so simulating detection of smaller objects requires significant model enhancement in both angular and spatial resolution. Mine-detection strategies can be assisted by optical models that simu- late lighting and shadow effects. Various underwater vehicles, swim- mers, and objects of interest (e.g., mines) can be inserted into a variety of model environments for optical simulation of their ease of detection from any direction and depth. Preliminary model examinations have been made of the light structure (e.g., shadows) associated with channels and ridges, and beneath ship hulls (Reinersman and Carder, 2004), and higher-resolution models of pilings have been developed (Carder et al. 2005) as a backdrop for evaluating problems associated with the search for underwater mines. The spectral character associated with vertical and horizontal light fields about a modeled coral head (Carder et al. 2006) has

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1 OCEANOGRAPHY IN 2025 also been made to evaluate potential stress of UV-rich environments on coral and foraminifera. Stealthy force insertion or smuggling from over the horizon using effective camouflage during daylight hours may be practical by 2025. Active camouflage permits adaptation to changes in the albedo of the background scene, whether in shallow water over a bright bottom or over a dark bottom. 3D optical modeling (Figure 1) has demonstrated that sim- ple, active camouflage can reduce the contrast of an object over a bright, sand bottom by up to six-fold (Carder and Reinersman 2006). Shadows of underwater objects above bright bottoms significantly increase object detectability, suggesting how an insertion might best be detected. While it may seem counterintuitive, superior active-camouflage potential is found near the surface (Figure 2) because shadows on the bottom are much more diffuse for near surface versus near bottom transits on sunny days. Deeper transits may be appropriate on cloudy days, however. By 2025 model simulations can be carried out to select the optimum FIGURE 1 HyMOM model layout for a 2 m × 2 m × 4 m object in clear water at 1.5 m below surface over a bottom with a 30% albedo. The additional bottom Carder_Fig1.eps shadow at right is from a virtual object in a box to the right caused by model bitmap image wrapping (photons leaving one side re-enter from the opposite side to simulate an infinite ocean).

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19 KENDALL CARDER FIGURE 2 Nadir view of model results for object at 2 m and 8 m depths with Carder_Fig2.eps no lights (NL), side and top (S&T) lights, and bottom (B) lights. The numbers are the standard deviation-mean bitmap contrast. 8 m contrasts are twice 2 m ratio for image values. Adding bottom lights and extending a transparent lighted top 0.25 m and 0.5 m reduces the contrast at 8 m, but it still exceeds that at 2 m. All images are contrast-enhanced. routes and depths of transit expected for a variety of vehicles and envi- ronmental conditions. The power required to optimally camouflage any delivery package for each potential route can also be calculated to evalu- ate likelihoods for various threats. By then computer speed and memory will not be limiting for most of the problems being conceived today. Perhaps the most complex task of optical modeling in the future will be that of solving “inverse problems.” What could be making that shadow? Is that a manta ray or a mine? Is that the primary signal, or a reflec- tion, or multiple-path artifact? Are those stars being occluded randomly, or is there an intervening object? How would these problems be best addressed? Perhaps they will be addressed by decision trees designed by experts in artificial intelligence, expert systems, or machine logic. Those capabilities will, obviously, drive the need for high-speed sensors and networks as well as “reach-back” capabilities that would allow “intelli- gent” application of archived or historical data (to a real time problem).

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10 OCEANOGRAPHY IN 2025 REFERENCES Carder, K.L., P. Reinersman, D. Costello, J. Kloske, and M. Montes, 2005. Optical Modeling of Ports and Harbors in 2 and 3 Dimensions, SPIE Proceedings. 5780(49): 10 pp. Carder , K.L. and P.N. Reinersman, 2006. Optical Variability and Bottom Classification in Turbid Waters: HyMOM Predictions of the Light Field in Ports and Beneath Ship Hulls. Annual Report for N00014-02-1-0211. Office of Naval Research: Arlington, VA. 10 pp. Carder, K.L, P.N. Reinersman, and P. Hallock. 2006. A 3-D Model of Light Effects Related to the Bleaching of Corals and Foraminifera, Ocean Optics XVIII. Montreal (extended abstract on CDROM, ONR, Arlington, VA). Carder, K.L., P.N. Reinersman, and D.K. Costello, 2008. Optical Detection of Camouflaged Underwater Objects. Proceedings of Eighth International Symposium on Technology and the Mine Problem, Technologies for Mine Warfare Expeditionary Warfare and Port Security, 6 May 2008, Naval Postgraduate School, Monterey, California (CD-ROM, MINWARA). Fournier G.R. 2006. Model of the Anomalous Absorption Spectrum of Pure Water for Femto- second Laser Pulses, Ocean Optics XVIII, Victoria, B.C., Canada, October. Reinersman, P.N. and K.L. Carder. 2004. Hybrid Numerical Method for Solution of the Radiative Transfer Equation in One, Two, or Three Dimensions. Applied Optics. 43: 2734-2743.