6-5] FM/CW RADAR SYSTEMS 315 



where Tr = modulation period in seconds 



AF = peak transmitted deviation in cps 



h = altitude in feet 



c = velocity of propagation (984 X 10^ ft /sec). 



For more conventional radar applications using sinusoidal FM for 

 ranging, the peak frequency deviation/^ on the doppler signal is a measure 

 of range (see Equation 6-22); i.e., 



fr = 2AFs\mr/mT (6-24) 



or = 2Ai^7r/„/^^ 



where R = range in feet 



AF = peak transmitted deviation in cps 



fm = deviation rate in cps. 



Use can be sometimes made of the fact that the doppler frequency is 

 precisely proportional to range rate. Integration of range rate can produce 

 an actual measure of range provided the constant of integration can be 

 determined. 



Stability Requirements. The principal theoretical advantage of CW 

 radar systems derives from their ability to distinguish moving targets in the 

 presence of clutter. The maintenance of this advantage in a practical design 

 places strict requirements on the short-term frequency stability of the 

 transmitted energy; i.e., the coherence that exists between the transmitted 

 and received signals. Long-term frequency drift is generally not a problem 

 provided that short-term coherency exists. 



Any modulation that tends to broaden the spectrum of a wanted signal 

 may destroy the signal-to-noise ratio in the received gate. Any modulation 

 of interference signals may produce sidebands which interfere with the 

 resolution of desired signals. When interference signals, such as feedthrough 

 or clutter, are very large, short-term stabilities in excess of 10~"^ may be 

 required. 



The process of coherent detection of a time-delayed signal alters the 

 angular modulation index as has been indicated by Equation 6-22. 



In a sfalic situation the RF reference phase can be adjusted to minimize 

 sensitivity to angular modulations; this is impossible for a gimbaled or 

 moving radar. Alternatively the radar detection reference could be time- 

 delayed an amount equal to the round trip to target delay. However, this 

 is usually impractical because of the continually changing delays of one or 

 more targets. 



