It would be reasonable to assume, therefore, that a construction 

 vessel would very seldom be further than 200 miles from land during a 

 typical operation. Consequently, it can be assumed that there will be little 

 difficulty in positioning a vessel within 50 feet of the chosen location. 



For operation on seas from 5 to 300 nautical miles from land, there 

 are three electromagnetic positioning systems which possess the best 

 combinations of range, cost, and accuracy of operation: 



1. British Hyperbolic Navigation System. ^^ The British Hyperbolic 

 Navigation System is built by Decca and has a range of 



425 nautical miles. The accuracy is within 25 to 250 feet. 

 High frequency signals from land-based transmitters are used 

 in the system. Only one vessel can use the system at any one 

 time, although a time sharing scheme can be implemented to 

 permit more than one ship to receive signals. 



2. Raydist System. ^^ The Raydist System has an operating range 

 of about 200 miles and an accuracy of 12 to 100 feet. The 

 range is reduced to approximately 100 miles at night or in bad 

 weather. Two land-based transmitters are used in this system. 

 There are some operational difficulties,if, for any reason, power 

 is lost at either of the transmitters. 



3. LORAC B.2^ LORAC B has an operating range of 300 miles 

 and an accuracy of 1 5 to 400 feet. There are four transmitters 

 in this system. 



For any operation in excess of 300 nautical miles from land, the best 

 that can be hoped for is an accuracy of ± 1/2 nautical mile. 



It has been stated that positioning to within ±30 to 40 feet would be 

 optimal for deep ocean construction operations (Reference 28, p. 2.6). If 

 this range is assumed to be acceptable, then it is safe to conclude that posi- 

 tioning will not be a serious problem for any surface supported lift system 

 operating in a depth range of to 6,000 feet. This assumption appears even 

 more reasonable when it is realized that navigational satellites are presently in 

 orbit which permit position determination of nearly the above tolerance 

 regardless of the distance from land. In short, practically any required 

 accuracy for positioning is either within or nearly within the state-of-the-art. 



101 



