Figure 14 

 Over-the-Horizon Radar 



F-Layer 



Surface of the Earth 



Minimum Range 

 to Shoreline 



Transmitter 



and Receiver 



Location 



Source: Stanford Research Institute 



nature of most of the military work in the field 

 and the high cost of OTHR, use of this system 

 will be contingent upon close cooperation be- 

 tween the Department of Defense (DOD) and 

 the Coast Guard. 



Microwave Radiometry'*^ 



Microwave radiometers operating alone 

 offer very little promise as a means of iden- 

 tifying fishing vessels or their catch. However, 

 if combined with transponders onboard ship, 

 they are a promising system which would 

 locate, identify, and classify ships in almost 

 any weather, day or night and provide other 

 data on sea state, sea ice, and rainfall rates as 

 well. 



A radiometer is merely a sensitive detector 

 which receives and measures the brightness 

 temperature of microwave energy naturally 

 emitted and reflected by surfaces. Detection of 

 a ship is possible because the microwave 

 energy thus reflected by a ship is different 

 than that of the surrounding ocean. A wooden 

 ship appears radiometrically "warmer" and a 

 steel ship "cooler" than the ocean. It is an en- 

 tirely passive system, as opposed to active 

 techniques which measure the reflection of 

 signals which have been transmitted by radar. 

 One of the advantages of the passive system is 

 that it allows surveillance without radiation, 

 therefore, the target does not know it is being 

 observed. 



Microwave radiometers have been used 

 routinely in satellites to measure whether con- 



53 



