confined to restricted "windows," which often greatly complicates the 

 development of practical operational remote sensing systems, even 

 though the technique being employed is scientifically sound. 



C. Methods to Enhance Sensor Capabilities 



The performance of many remote sensor techniques can be improved 

 significantly if the object to be sensed can be equipped with devices 

 designed to enhance the target or supplement the signals normally used 

 by the sensor. A simple example is the familiar corner reflector de- 

 signed to concentrate and thereby enhance radar returns from a target 

 equipped with the device. Such a device can increase the effective 

 radar cross section of a target by an order of magnitude or more. Another 

 device that can significantly enhance target detectability is an active 

 beacon that responds to interrogation by transmitting energy at the 

 interrogating frequency (or some other frequency) at levels up to a few 

 orders of magnitude higher than the signal that would result from reflec- 

 tion of the incident signal alone. Active beacons of this kind are 

 available for use with most radar and sonar equipment. Many of them can 

 be coded so that the response not only aids detection but provides a 

 limited amount of identification or status information. In theory, 

 such beacons can be used with any remote sensor that operates on the 

 principle of transmitting energy to a target and performing its sensory 

 functions on the energy returned by the target. 



At the cost of somewhat more engineering complexity, transponders 

 can be built that respond to interrogation by a remote sensing device 

 by transmitting a wide variety of identification and status information. 

 The capability of such transponders is ultimately limited only by the 

 ability of the target to provide power for their operation and by the 

 ability to acquire and format target status and identification informa- 

 tion for transmission in response to an interrogation. 



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