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Since much of their work does not call for precise positioning, they can 

 accept the ship's position - obtained by one of several electronics positioning 

 systems - as being reflective of the towed vehicle's position. Obviously 

 there are discrepancies in this procedure, but when the task at hand involves 

 surveying or mapping, for example, manganese nodules, vehicle position 

 accuracy errors of hundreds of meters or more are acceptable since the deposit 

 itself may cover hundreds of square kilometers. In shallow water investigations, 

 such as conducted by RUFAS , the error between ship and vehicle positions is 

 also acceptable owing to the relatively short length of towing cable. 



The vehicle DEEP TOW, TELEPROBE and the NRL System operate within closer 

 positioning tolerances. All of these systems are capable of positioning 

 the towed vehicle (and the support ship, in some instances) relative to a 

 bottom-mounted transponder system similar in concept to those described in 

 Section 2. 1.13. a. The DEEP TOW system reports position accuracies of 2 to 

 5m (7 to 16 ft) with a bottom-mounted system of the Marine Physics Laboratory, 

 design. The NRL System, in addition to having a bottom-oriented navigational 

 capability, also employs a surface-oriented, short baseline system for vehicle 

 tracking. 



2.3.9 Su pport Ship Requirements 



For deep towed vehicles the support ship requirements are quite rigid and, 

 similar to bottom- crawling vehicle support ship requirements, "ships of 

 opportunity" are not acceptable. Since each vehicle has its own special 

 requirements no general statement regarding support ships can be made which 

 applies to all cases. The critical components in all deep towed systems 

 are: good low speed handling characteristics, the winch, the handling system, 

 storage and deck space, and berthing accommodations. The support ship require- 

 ments for the DEEP TOW System can serve to gain an appreciation for the deep 

 towed vehicles at large. These are as follows: 



Must have installed winch or equivalent deck load carrying capability, 

 ability to install crane, space for 2m x 3m (6 ft x 10 ft) storage and 

 workshop van, at least 37 sq m (400 sq ft) of enclosed and dry lab area. 

 Ship must have good low speed propulsion (diesel electric, cycloidal 

 propulsion, variable pitch propellers, auxiliary low speed system, etc.) 

 and bow thruster with enough horsepower to turn into a reasonable wind 

 while maintaining a 4,536kg (10,000 lb) line pull at 1 to 2 knots. 

 Almost any AGOR or AGSS should be satisfactory. Offshore drilling supply 

 boats often satisfy all but the low speed capability, which could be 

 provided by a pair of Murray-Tregurtha Harbormaster units, and the 

 laboratory and living space requirements, which can be met with vans. 

 Power: 20 amps of reasonably regulated 60 Hz 110 V single phase for the 

 actual equipment. Crane has its own small diesel engine. Winch (if 

 not normally installed) requires 120 kw 440 V, 60 Hz, 3 phase power. 



All towed vehicles, but one, are launched/retrieved and towed off the stern 

 of their support ship; the exception is the NRL system. This system was 

 supported by USNS MIZAR, 81m (266 ft) long, 16m (52 ft) beam, 6m (19 ft) 

 draft and displaced 3,447t (3,800 tons). Instead of stern launch/retrieval. 



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