propulsion (by means of mechanical strength), provide electrical power (via metallic conduc- 

 tors), and provide communications (via coaxial transmission lines). Decisions, data interpreta- 

 tions, and command control are accomplished aboard ship; the vehicle is positioned by 

 maneuvering the surface vessel and towing the package into place. Advantages include sizable 

 payload weight, high speed, and long endurance. Disadvantages are low maneuverability, lack 

 of "stop-and-look" capabiUty, the need for a sizable surface support vessel, large turnaround 

 space and time requirements for deep tow depths, and restrictions on direction of tow be- 

 cause of winds and currents. 



A second approach, the tethered vehicle (fig IB), attempts to overcome some of the 

 disadvantages inherent in the towed system. In the tethered vehicle, propulsion is added to 

 the sensor package so that the strength of the cable no longer serves in that role. Onboard 

 propulsion gives the vehicle "stop-and-look" capability, which makes it highly maneuverable 

 and reduces the required size of the support ship. The most critical function of the cable is 

 now the electrical power transfer required to supply power for the vehicle's thrusters and 

 sensors. It is desirable that such tethered cables be neutrally buoyant; hence exotic cabling 

 materials such as copperclad aluminum, aramid fiber, and thermoplastic rubber are preferred 

 in such applications. The tethered vehicle, having the ability to stop and "station keep" in 

 the water, has the important advantage of becoming a remote work platform with a high 

 degree of maneuverabihty. The price paid for maneuverability, however, is a reduction in 

 speed and a "temperamental" cable arrangement which may become entangled or broken. 

 The support ship also must have good stationkeeping ability to stay within "watch circle" 

 of the submerged vehicle. In addition, substantial deck handling systems are still required for 

 vehicle and cable. Moreover, the stage is set for the "vicious circle" between cable diameter 

 and system size ; as the system is required to go deeper and move faster, the cable must 

 become larger. 



Suppose that a tethered vehicle could be constructed which would carry an onboard 

 power source for propulsion. The only remaining function of the cable now would be com- 

 munications to and from the support vessel, and therefore the tether cable itself could be 

 quite small in diameter. Such an approach is depicted in figure IC. We have chosen to call 

 this realization a "supervisory-controlled" vehicle. An example of this approach is the wire- 

 guided torpedo. Such a system does not require great size to operate at great depths because 

 it does not drag its tether behind; the tether is paid off or deployed from small storage canis- 

 ters aboard the vehicle and tends to be nearly motionless in the water column even if the 

 vehicle moves with great speed. Note that, since such a deployed link is expendable, entan- 

 glement is not a problem. 



Unfortunately, metalHc wire guides are unsuitable for inspection missions that require 

 such wide-bandwidth sensors as television and high-resolution sonar because of bandwidth 

 Umitations (a few kilohertz over kilometer lengths, at best), but this shortcoming is relatively 

 easy to overcome by using the wide available bandwidth capability of optical fiber technol- 

 ogy. In addition, the high in-water weight of a metaUic wire guide results in an unavoidable 

 vehicle weight change caused by deployment of the heavy cable during the mission. For 

 example, a weight savings of 15 to 1 was calculated for a 0.035-inch-diameter tether cable. 

 This imphes a differential weight change of approximately 100 pounds when lengths of 10 

 kilometers are to be deployed. The nearly neutrally buoyant fiber-optic cable, on the other 

 hand, causes a much smaller vehicle trim problem under the same deployment conditions. 

 Moreover, three recent technology breakthroughs have occurred within the last 5 to 10 years 



