by sky wave effects, and the survey activities are generally restricted to peri- 

 ods between sunrise and sunset. Lane ambiguity problems are also troublesome 

 with phase comparison systems and require that the navigator know his initial 

 checkpoint and keep an accurate track record. For most systems autoinatic 

 track -plotting and recording equipment is available for this purpose. 



The Decca Two -Range and the Raydist DM hydrographic survey sys- 

 tems use circular plot ranging methods which require two shore stations . Both 

 systems use phase comparison techniques for establishing ranges from the re- 

 ceiver to each shore station. The Electronic Position Indicator (EPI) system is 

 a pulsed -phase measuring system which sequentially measures the range from 

 the survey vessel to each shore station. It was designed to work at ranges as 

 great as 400 miles, while Decca and Raydist are limited to ranges of the order 

 of 150-200 miles. 



The Lorac (Long -Range -Accuracy) system was developed for oil 

 exploration activities . It uses a cw phase comparison method which requires 

 three shore stations . This system is used by both commercial and Government 

 survey groups for near-shore control work. Working ranges are 100-150 

 nautical miles . Ranging accuracies of 1:50, 000 are reported as being readily 

 obtained with this system . 



C. SHORT-RANGE NAVIGATION SYSTEMS 



Short-range navigation systems (Table II-3) are defined here as being 

 capable of achieving a very high order of positional accuracy at ranges from 

 line of sight up to about 50 miles . Both pulse -time and phase comparison 

 methods are used, at frequencies generally in the 3-10 cm band. Because of 

 the high frequency and small size of the equipment in this class, most of the 

 shore stations are small and highly portable . Where the highest accuracy is 

 required, appropriate atmospheric measurements are made, and corrections 

 for variations in the propagation velocity are calculated. 



The Shoran system is a pulse -time ship -positioning system operating 

 on radar principles . It has been in use for over 15 years and is a standard 

 method of in-shore survey control. Transponders to shape and reinforce the 

 return signal are placed at each of the two required shore stations and give the 

 system a useful range of over 50 miles . 



The Alpine -429 Precision Radar Ranging Unit was developed to be 

 operated in conjunction with standard radar transmitting equipment . The unit 

 is capable of a fixed operational accuracy of ± 80 feet at ranges to about 50 

 nautical miles . A remote Electrical Track Plotter increases the over-all ac- 

 curacy and utility of the system . 



arthitt: ZD.HittlcJnr. 



