of the surrrounding ocean, (A wooden ship appears radiometrically 

 warmer and a steel ship cooler than the ocean,) 



An example of a simple airborne microwave radiometer 

 detection system is shown in Figure 5. The system consists of an antenna 

 capable of scanning the sea surface, a total power receiver, and a 

 display device capable of showing the "sensed" radiometric map of the 

 sea below. 



(1) Apparent Microwave Brightness Temperature 

 of Fishing Vessels and the Sea 



The apparent temperature of the ocean at microwave 

 frequencies depends on factors such as geographic location, salinity, 

 sea state, angle of incidence, polarization, and the observing microwave 

 frequency. An example of the variation of sea brightness temperature 

 with some of these factors is shown in Figure 6. Brightness temperatures 

 at other microwave frequencies in the 10- to 90-GHz range show generally 

 similar behavior although the temperature ranges from approximately 

 100° K at about 100 GHz to slightly over 200° K at 90 GHz^ for nadir view- 

 ing. 



Fishing vessels are visible to a microwave radiometer 

 because they have a different apparent microwave brightness temperature 

 than the surrounding sea. Fishing vessels constructed of good conductors 

 such as steel will be good reflectors and tend to reflect the very cold 

 (at microwave frequencies) sky into the radiometer. Curve A in Figure 6 

 is an estimate of the apparent brightness temperature of a perfectly 

 reflecting ship. Wooden fishing vessels will have a low reflectivity 

 (i.e., good emissivity) and display a microwave temperature close to 



* 

 Microwave brightness temperature is primarily a measure of RF emissivity 



rather than actual thermal temperature. 



51 



