OCEAN ELECTRONIC NAVIGATIONAL AIDS 



47 



ANTENNA 

 ASSEMBLY 



XECTRONIC 

 SWITCH -^ 



INDICATOR 



RECEIVER 



TRANSMITTER 



MODULATOR 



Block Diagram of Radar 



PiGDEE 4-5. — Block diagram of Radar. 



In the receiver, which employs the superheterodyne principle, the radio 

 energy reflected back from the target is converted into a form that may be 

 presented visually on an indicator or scope. Since a very small amount of 

 power is reflected by an object the receiver must amplify it many times. 

 Because the same antenna is used for outgoing and incoming signals a method 

 of disconnecting the receiver from the antenna is needed during Intervals 

 when the transmitter is operating. Due to the rapid switching that is neces- 

 sary, an electronic switch is used. 



It is the indicator of a Radar that presents the information collected in the 

 form best adapted to efiBcient use of the equipment. The indicator commonly 

 used in navigation is the plan-position-indicator, commonly abbreviated PPI, 

 which presents on the scope a continuous polar picttu-e of the surrounding 

 area. 



RADAR FREQUENCY BANDS 



The following bands are now provided in the United States for merchant 

 marine Radar and associated beacon: 



Radar Beacon 



10-centimeter 3000-3246 MC. 3256 MC. 



5-centimeter 5460-5650 MC. 5450 MC. 



3-centimeter 9320-9500 MC. 9310 MC. 



