radar operation and the processing of radar returns to extract more 

 ittformation about objects detected. 



Development of OTHR, primarily for military use, has 

 been underway for several years and systems capable of performing a 

 number of useful functions have been built. Two particular experimental 

 OTHR that have been instrumental in exploring the capabilities and limi- 

 tations of the technique are the SEA ECHO OTHR developed by the National 

 Ocean and Atmospheric Agency (NOAA) and the Naval Research Laboratory 

 (NRL), and the Wide Aperture Research Facility (WARF) developed for the 

 Defense Advanced Research Projects Agency (ARPA) and the Office of Naval 

 Research (ONR) by Stanford Research Institute (SRI). 



b. Performance Potential 



Use of OTH radar techniques would allow detection of 

 fishing boats to much greater ranges and would allow coverage of much 

 larger areas compared to those covered by microwave radar. For instance, 

 a single skywave OTH radar located in Utah could provide surveillance 

 coverage over the entire Pacific Coast of CONUS. While such a configura- 

 tion may not be the most suitable, it demonstrates the wide area coverage 

 that can be provided from a single site. Skywave OTHR takes advantage 

 of the refractive property of the ionosphere, which causes the radar 

 beam to curve back to earth at distances ranging from 500 to 2000 nmi. 



Another OTHR technique, which uses frequencies in the 

 2- to 6-MHz region, employs surface-wave propagation in which radio 

 energy travels along the curved earth surface. It is fortuitous that 

 the attenuation of surface waves is much lower over water than it is 

 over land, thus making possible the detection of ships at sea at dis- 

 tances beyond 200 nmi. Since the coverage of a surface-wave radar is 

 limited to a region with a radius of a few hundred miles around the 

 radar, the surveillance coverage is much less than for a skywave radar. 



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