314 



GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



transit time. To determine distance, transit time can be measured by 

 resolving the magnitude of the returned deviation (2AFsin irfmr) or by the 

 relative phase lag of the returned modulation {irfmT) . In general, the greater 

 the transmitted deviation AF, the greater is the resolvability of range. 

 However, this range resolution is usually purchased at the price of in- 

 creasing the minimum bandwidth of the doppler filters. 



If linear FM is used such as is provided by a sawtooth or triangular 

 waveform, the Doppler frequency is merely shifted by the amount the 

 transmitter has deviated during the round trip transit time. The principle 

 is illustrated in Fig. 6-17. Depending upon range, modulation, and doppler 



f,-AF - 



TIME 



■Transmitted 





V^^^^ 



^Measure of Range f, 



I Doppler Frequency fj 



TIME 



Fig. 6-17 FM Range Measuring Principle. 



shift, the instantaneous received frequency will differ from the transmitted 

 reference frequency. When the two are heterodyned, a frequency modula- 

 tion is superimposed on the detected doppler signal. The magnitude of the 

 FM deviation from the doppler is a measure of range. Care must be 

 exercised in the selection of deviation values or the doppler frequency and 

 range frequency may be difficult to resolve uniquely. 



In some applications such as altimetry the range frequency greatly 

 exceeds the doppler values. For example, in doppler altimetry the electro- 

 magnetic energy is radiated normal to the direction of flight so that no 

 doppler signal results. By using triangular modulation, as in Fig. 6-17, the 

 magnitude of the resultant detected difference frequency /r is a measure of 

 altitude: i.e.. 



(6-23) 



