3.2 MISSION SCENARIO IMPACT AND TETHER DYNAMICS 



While the fabrication process influences the selection of cable design and the binding 

 force of the adhesive, the mission scenario, which includes such factors as current profile and 

 standoff considerations, also affects the selection process. The pullout force required will be 

 set by the binding agent or its equivalent (note that the final design option may call for a 

 mechanical pullout-limiting device and the use of a minimal-binding-force adhesive). Different 

 binding forces on the reel could produce minimum pullout tensions ranging from nearly zero 

 up to the breaking point of the tether. (Such wide ranges are easily produced through the 

 use of cyanoacrylate adhesives.) 



Payout reels are envisioned on both the support vessel and the tethered underwater 

 vehicle. The canister on the support vessel would pay out tether to compensate for heave 

 and other ship movements. The reel on the tethered vehicle would pay out cable as the 

 vehicle moves in the water and as the drag pulls it out to form a catenary. Differential pull- 

 out tensions would affect the scope in such a catenary. A too-large binding force and the 

 resulting pullout tension would tend to inhibit catenary formation. Conversely, a too-small 

 binding force would tend to permit the excessive deflection of tether by current in the water 

 column. 



The choice of a tether design and the magnitude of the binding force required as a 

 result is considered important to the overall design for a specific application. The tension on 

 the tether and the amount of catenary to be formed during deployment could be determined 

 by extensive laboratory and sea tests, but the cost of such tests would be prohibitive. It was 

 decided instead to construct a mathematical model by means of finite element analysis to 

 predict the behavior of the fiber-optic tether during deployment. The calculations necessary 

 for the model are performed on a computer, and the results assist the mission planner in 

 choosing the proper cable type and profile for a given scenario (see appendix). 



3.3 MODULATION AND MULTIPLEXING TECHNIQUES 



Using a fiber-optic data channel to link an undersea vehicle with its support platform 

 allows high-bandwidth, low-noise, real-time communications vital to many missions. Typically, 

 such a link is required to transmit one or more television channels, sonar video, and command/ 

 control information. The optimum fiber-optic transmission system for such applications must 

 be capable of repeaterless operation despite high optical cable loss, must satisfy realistic 

 optoelectronic and fiber bandwidth constraints, and must have low signal distortion, minimal 

 power consumption, a straightforward hardware realization, and low cost. Traditional digital 

 and linear analog fiber-optic modulation techniques, when applied to video-bandwidth signals, 

 fall short of satisfying these requirements. 



3.3.1 THEORETICAL DISCUSSION 



A nonlinear analog modulation scheme, pulse frequency modulation (PPM), has been 

 shown to be suitable for the transmission of video-type signal information (ref 8). Investiga- 

 tions at NOSC have involved characterization of the processing gain obtainable with this 

 technique, and resulted in a set of design equations that predict system performance and 

 allow the designer to optimize parameters to suit his particular requirements (ref 9 and 10). 

 In conjunction with the theoretical analysis, the Center has developed prototype fiber-optic 



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