differences are revealed in the image, vessel wakes 

 are frequently detectable for several miles astern, 

 due to the mixing of power plant cooling water and 

 the frictional heat of the vessel's passage through 

 the water. 



Special note should be taken here of systems that operate 

 in the visible but that utilize several narrow spectral channels (as 

 opposed to the much broader "channels" of color images) to emphasize 

 small differences in object reflectance and thereby discriminate between 

 objects of materials of special interest (such as pollutants, or differ- 

 ent types of fish). Such sensors, referred to as multispectral scanners 

 (MSB) or multichannel ocean color sensors (MOCS), are usually electro- 

 optical in nature, providing a signal similar to color TV, although 

 direct recording of the images on film is possible by the use of narrow- 

 band filters. 



No discussion of optical sensors is complete without some 

 consideration of the processing and handling of the raw data output. 

 Photographic film requires chemical development, usually effec'ted at 

 the end of a reconnaissance mission (e.g., when the aircraft lands), 

 although rapid-processing systems are in existence that can provide on- 

 board output for examination or data transmission within seconds or 

 minutes of the actual exposures. Typically, short-range (i.e., close- 

 up) photography will allow for immediate interpretation, while long-range, 

 wide-area surveillance images (from high-altitude aircraft or satellites) 

 require more extensive and detailed examination, often requiring several 

 hours or even days by expert photo-interpreters before useful, specific 

 data are developed. 



On the other hand, electrooptical systems provide real- 

 time outputs capable of immediate display and examination and (since the 

 output is in the form of electrical signals) are readily amenable to 

 radio transmission to a remote (shore-based) facility. To immediate 

 advantages accure: 



70 



