Appendix C 



PROPOSED SYSTEM FOR ANALOG PROCESSING 

 OF STEREO-SONAR SIGNALS^ 



The proposed system utilizes echo signals from a 

 pair of towed side-scan sonar fish to generate, in real 

 time, a roll of photographic film which can be 

 directly viewed in a stereoscope to produce a true 

 three-dimensional illusion of seafloor topography. 

 The parallax in the stereo-photo image pairs will be 

 optically true; that is, the image pairs will be the same 

 as those obtained with a photographic stereo-camera. 

 The system will, thus, make it possible to produce a 

 seafloor contour map by the same technique used in 

 stereo mapping of land contours. No digital computer 

 will be required as in seafloor mapping from direct 

 stereo-viewing of sonar charts. 



The proposed system is an analog computer that 

 uses a thin-film photoconductive liquid-crystal 

 sandwich to produce a series of luminous points 

 corresponding to off-track seafloor points. This series 

 of points is imaged by means of a 3-to-2-dirnensional 

 fiber-optic static-scanning system onto the stereo- 

 photo film, resulting in paris of stereoscopic traces of 

 actual topography. 



At a particular instant, the system images a pair 

 of expanding, circular arcs onto the photoconductive 

 liquid-crystal layered array. This array is capable of 

 generating light only at the intersection point of the 

 two arcs. The two circular arcs are initiated at the 

 same instant that the two sonar pulses are initiated 

 from the pair of towed fish. As the pulse from each 

 fish moves out radially into the water, its wavefront 

 has the shape of a circular arc; an optical image of 

 each wavefront is reproduced within the system such 

 that, at every instant, the radius of the optical image 

 is proportional to the radius of the acoustic wave- 

 front. Figure C-1 is a schematic of the system up to 

 the liquid-crystal sandwich. 



The system sequentially images certain inter- 

 section points of the two optically generated arcs 

 onto a stereo-photo film. This imaging of the inter- 

 section points is the analog transformation of sonar 

 signals to optical readout. This analog process 

 computes the true target coordinates from the sonar 

 parameters, Rj, R2, and B (Figure 1), just as 



Equations 1 through 6 transform sonar to optical 

 readout by means of a digital process. 



The stereoscopic imaging process is performed 

 by means of a static-scanning, fiber-optic assembly 

 which maps each intersection point in the vertical 

 plane of the sonar beam into a pair of stereo points 

 on the crosstrack line in the plane of the photo- 

 graphic film. Figure C-2 is a schematic of the 

 fiber-optics system. 



The arc-intersection points imaged onto the 

 photoconductive liquid-crystal sandwich represent 

 the beginning of shadow regions along the crosstrack 

 line. A given crosstrack line in a side-scan chart will 

 contain blank or no-signal regions because of no 

 energy being reflected from seafloor regions lying 

 behind protuberances and outcroppings. These 

 shadow regions produce negative derivatives in the 

 echo signal. A circuit between the sonar transceiver 

 and the optical-imaging system is responsive only to 

 negative derivatives. Without this feature the system 

 would image a multiplicity of arc-intersection points 

 which would not be sensible on the photographic 

 film. 



The proposed system will work for any 

 orientation of the fish-pair vector, B. If the vector, B, 

 is continually changing in magnitude and direction 

 during the tow, an additional device would be 

 required to continually adjust the relative orientation 

 of the arc-shaped slits. The successfulness of the pro- 

 posed analog system, of course, depends on 

 extremely high precision in the measurement of the 

 tow-system parameters during each line scan. 



* Invention Disclosure, Navy Case No. 57436 (patent application in preparation). The reader is 

 referred to this Disclosure for a detailed narrative/pictorial description of the proposed system. 



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