
Sensor Systems

Sensors are either "active" or "passive." When you use an active sensor, 
the enemy can detect your presence from the sensor emission. Passive 
sensors allow you to "run silent," since the enemy cannot detect your 
presence from sensor emission. Transmitting sensors require that an energy
be used to send a signal out to bounce off the target. The energy may be 
radio frequency (RF), audio or light. 

RF energy is used in radar and radios. Radar originated in England in the 
1920s, and the word "radar" is an acronym, from "radio direction and 
ranging." At the time, radar was a novel approach for using the wireless 
to find targets. Today's radar systems are sophisticated tools designed to
furnish specific information such as elevation, speed, direction in 
azimuth and number of targets. Some radar systems can even distinguish 
rough outlines of the returning echo to give a shape that approximates the
actual vessel detected.

