45 



from this motion, must "park" inside a garage undergoing rapid vertical excursions. 

 In some instances it has been necessary to place the launcher on the bottom 

 to conduct the docking procedure- 



2.1.13 Sub-Surface Navigation 



There are several navigational techniques available to the ROV operator: 

 visual sighting, magnetic and gyrocompass, and acoustic locating techniques. 

 The magnetic compass and gyrocompass are well known dead reckoning navigation 

 techniques which do not require a detailed discussion herein. 



Visual sighting entails simply knowing the vehicle's location by identifying 

 (on the CCTV monitor) a known object or feature whose position coordinates 

 have been previously located. The object or feature may be a coded structural 

 node, a pipeline field joint or lines painted on the structure specifically 

 for providing navigational assistance. The technique is as old as human eyesight 

 and is as accurate as the drawings from which the operator is working. This 

 procedure, obviously, requires that some degree of underwater visibility is 

 present and that recognizable features of known position are available. 



Acoustic navigational techniques used by ROVs are essentially the same systems 

 developed for manned submersibles . For convenience, they are herein categorized 

 as bottom-oriented and surface-oriented systems. There are many manufacturers 

 of sub-surface navigation systems, the systems described herein, however, are 

 only those which are actually being used within the ROV community. 



2. 1.13. a Bottom-Oriented Systems 



This category consists of systems that employ bottom-mounted reference points 

 (i.e., acoustic transponders) which synoptically provide three or more ranges 

 to the vehicle which are used to triangulate its position relative to the 

 references. Four bottom-mounted systems are reportedly in use: ATNAV (AMF 

 Electrical Products Development Division, Herndon, Virginia) ; AUTRANAV MK 3 

 (Polytechnic Marine Ltd. , Daventry, Northants) ; the Mesotech system (Mesotech 

 Ltd., North Vancouver, British Colombia); and the ELA system (ELA, Montrouge , 

 France) . 



The same fundamentals control the use of all the systems, and except for frequency 

 variations, depth capability, cost, and position accuracies, the systems are 

 quite similar in operation. 



ATNAV System - This system is used by Sub Sea Surveys Ltd. , and the Super- 

 vis or~of~Salvage7 U.S.N. The ATNAV (Acoustic Transponder Navigation) system 

 hardware is comprised of four underwater transponders: (three are bottom- mounted, 

 one is on the vehicle) and surface support craft equipment consisting of a 

 Command/Interrogator and Ranging Receiver, a mini- computer , a teletype, an 

 x-y plotter and interface electronics. With these components, a real-time, 

 continuous x-y plot and x-y-z printout of the surface craft and the vehicle 

 relative to the three bottom- mounted transponders is provided. Coverage within 

 an area of ISOkm^ (50 miles^) is possible. The vehicle's position (relative 

 to the transponders) and the surface craft is attainable to ±0.9m (3 ft) 

 depending upon the accuracy of water sound speed measurements and surface ship 

 velocity data. 



