their true physical temperature (Curve B in Figure 6). In general, 

 the apparent microwave temperature of an object depends more on its 

 emissivity than on its actual physical temperature. 



Most fishing vessels over 100 ft will be steel with 

 some portions that are wood so that they will appear at microwave fre- 

 quencies to be cooler than the ocean but warmer than the ideal perfect 

 reflector (Curve A in Figure 6). Measurements at 90 GHz of the bright- 

 ness temperatures of ocean water and ships at sea and at dockside re- 



Q 



ported by King et al., are summarized in Table 8. It can be seen that 

 the ships are cooler than the ocean. King et al. were also able to 

 detect the wake of ships as a region of warmer water. Presumably, this 

 results from the foam produced which is a better microwave emitter than 

 the sea (higher emissivity) resulting in a higher apparent microwave 

 brightness temperature. A similar mechanism is operative when the wind 

 whips the sea and the effect is used to infer wind speed from the en- 

 hancement in apparent microwave brightness temperature resulting from 

 the foam. 



Table 8 



ANTENNA TEMPERATURES OF SELECTED 

 TARGETS AT 90 GHz 



Four ships, 425-ft to 708-ft long. 

 Five ships, 250-ft to 750-ft long. 

 Source: Reference 9 



54 



