but it does not appear adequate to support classification of vessels. 

 In addition, the several valuable remote-sensing capabilities of micro- 

 wave radiometers suggest that microwave radiometers could provide, in 

 addition to the ocean surveillance, information on sea state, ice packs, 

 rainfall rate, and possibly oil spills. 



b. Performance Analysis 



In this section we investigate the performance that can 

 be expected from a microwave radiometer as a detector of fishing vessels. 

 Specifically we are interested in the maximum detection range, since this 

 determines cost, and in the range of environmental conditions under which 

 detection will be possible. Two key constraints do much to determine the 

 characteristics of a passive microwave sensor. The requirements of near 

 all-weather capability constrains the frequency from going too high 

 where attenuation from weather is excessive and the need to maximize 

 angular resolution and sensitivity preclude use of the lower microwave 

 frequencies because of the large beamwidths associated with antennas of 

 practical size. 



(1) Environmental Attenuation and Choice of Frequency 



The tropospheric gases absorb and radiate microwaves 



primarily because of the presence of oxygen and water vapor. Inspection 



of an atmospheric transmission curve such as that computed by Blake''' 



and shown in Figure 7 indicates that there are three windows in which 



microwave transmission is possible. These windows are summarized in 

 Table 10. 



In addition to the attenuation and radiation from 

 the tropospheric gases that are always present, there will be attenua- 

 tion and radiation from any rain droplets and fog. Attenuation from 

 rain and fog increases as the square of the operating frequency as shown 



58 



