764 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



14-11 DESIGNING AIRBORNE RADAR SYSTEMS 

 EXPLICITLY FOR WEATHER MAPPING 



The designer of airborne weather radar systems must be fully cognizant 

 of a very important principle. With the one exception of purely weather 

 reconnaissance radar, the airborne system is a highly valuable but not 

 indispensable tool to the aircraft operator. Unless the added weight, space, 

 and power consumption taken up by the system can be justified through a 

 realization of an improvement in flight safety, flight time, passenger 

 comfort, or the accomplishment of an otherwise doubtful mission, the 

 economics of flight make its inclusion prohibitive. This implies a system 

 which is versatile, accurate, reliable, and capable of providing the pilot with 

 sufficient usable weather information, while at the same time eliminating 

 all unnecessary frills or refinements. 



This principle militates against the design of a limited system which 

 might be only an impending weather warning device, or a very limited- 

 range system. To be of value, the radar must have a range commensurate 

 with the speed of the user aircraft. Information must be made available in 

 sufficient time to allow the pilot to make well-considered decisions and take 

 corrective actions. This may require a range of only 30 miles, for aircraft 

 with cruising speeds of the order of 120 mph, or a range of 150 miles for 

 speeds approaching Mach 1. 



Generally, airborne weather radars require reasonably good definition, 

 accurate range calibration, simplicity of operation, and inherent high 

 reliability under adverse service conditions. Also highly desirable is 

 antenna stabilization against pitch and roll, climb, and bank. In order to 

 increase its versatility and utilization, considerable attention has been 

 directed toward means of adapting the radar to ground mapping, terrain 

 avoidance, drift angle determination, and radar beaconry. 



Among the refinements normally given up in the interest of simplicity 

 and small size are special scope displays, variable scan rates, variable pulse 

 lengths, and the high-peak transmitter power levels often associated with 

 ground weather radars. Operating frequency must also be compromised in 

 the interest of size, weight, and power conservation. In order to obtain a 

 better understanding of the interrelation of the many factors involved in 

 optimizing a system design, it is well to review some of the basic radar 

 equations. 



14-12 MODIFYING THE RADAR RANGE EQUATION FOR 

 THE WEATHER PROBLEM 



The form of the radar equation most generally used (see Paragraph 3-2) 

 equates the power reflected to the receiver to the balance of the radar 

 parameters: 



