radar, but the cost would be high and proba- 

 bly would have to be shared by several agen- 

 cies. 



Over-the-Horizon Radar** 



Use of over-the-horizon radar (OTHR) 

 techniques would allow detection of fishing 

 boats at much greater distances and would 

 allow coverage of much larger areas than 

 those covered by microwave radar. 



This is because remote sensing using signals 

 in the microwave and other very high fre- 

 quency ranges is constrained by the essen- 

 tially line-of-sight nature of the signal. For all 

 practical purposes, this means that the sensors 

 must be elevated in order to operate over sig- 

 nificant distances. 



The use of over-the-horizon radar reduces 

 this constraint by making use of signals in the 

 high frequency range in which energy waves 

 are refracted by the atmosphere or ionosphere 

 to follow the curvature of the earth. 



High frequency energy has been used for 

 communications since the earliest days of 

 radio. The technology for generation, 

 transmission, and reception of high frequency 

 energy is well developed and the effects of the 

 atmosphere and ionosphere on the signals are 

 well understood. However, some aspects of 

 using high frequency signals are not so well 

 understood. Among these are the reflection 

 characteristics of material objects at high fre- 

 quency. Means of concentrating and coding 

 high frequency transmissions to enhance 

 radar operation and the processing of radar 



returns in order to extract more information 

 about the object detected also are still being 

 developed. 



OTHR has been developed primarily for 

 military use and several experimental 

 systems, capable of performing a number of 

 useful functions, have been built by the Naval 

 Research Laboratory, the National Oceanic 

 and Atmospheric Administration, and other 

 groups. 



Two types of OTHR might be useful in fish- 

 eries enforcement, a skywave mode and a 

 groundwave mode: 



Skywave OTHR takes advantage of the 

 refractive property of the ionosphere, which 

 causes the radar to curve back to earth at dis- 

 tances ranging from 500 to 2,000 nmi (see 

 figure 14). Thus wide area coverage is possible 

 from a single site. For instance, a single 

 skywave OTHR located in Utah could provide 

 surveillance coverage over the entire Pacific 

 Coast (see figure 15). 



Groundwave OTHR, in which radio energy 

 travels along the curved earth surface, pro- 

 vides much more limited coverage, but may be 

 useful in specific regions. Groundwave OTHR 

 has an operational radius of a few hundred 

 miles. Thus, while ships out to and beyond the 

 200-mile zone could be detected from a shore 

 station, many stations would be required to 

 cover the entire coast. 



Both systems can provide continuous sur- 

 veillance of very large areas so that the general 

 location of all fishing boats of at least a certain 

 minimum size can be monitored on a full-time 

 basis. If transponders are installed on the 

 boats, detection can be enhanced and other 

 useful information can be obtained. 



Because of their capability to cover greater 

 distances and larger areas, OTHR techniques 

 have good potential for use in fisheries en- 

 forcement. However, due to both the classified 



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