738 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



required for high-altitude operation. Contrary to CW systems, all pulse 

 and FM systems are plagued by so-called "altitude hole" problems which 

 are discussed in later sections. 



Coherency. In order for a doppler system to function, it is necessary 

 for the received signal and a reference signal to be made phase-coherent 

 with one another. This can be accomplished in a number of ways. How- 

 ever, there are primarily three methods which are commonly used. 



In one method the transmitter is itself an incoherent pulsed transmitter 

 and the fore and aft returns of the system are used to beat against one 

 another, since they are themselves always of the same phase. This method 

 has been called external or self -coherence. With this type of coherency, it is 

 frequently necessary to employ physically stabilized antennas in order to 

 assure range overlap at all times. 



Continuous wave (CW) systems are coherent by the nature of their trans- 

 mission; i.e. the transmitted, received, and reference signals are continuous- 

 ly present at all times. 



Coherent pulse doppler systems can be made coherent by using a 

 continuously running oscillator as driver (as well as a source of reference 

 signal), feeding its output to an amplifier, and pulsing or chopping the 

 transmitted signal in the transmitter output circuit. This can be accom- 

 plished in a gridded klystron tube, traveling wave tube, and /or a crystal 

 modulation circuit, the latter having exhibited considerable advantage over 

 the gridded klystron, primarily because of the state of the art of these tubes. 

 A radar transmitter of this type is called a coherent radar transmitter and has 

 recently found application in a number of other radar systems. 



Frequency-modulation— continuous-wave (FM-CW) systems are coherent 

 in much the same way as pure CW systems; i.e., the transmitted, received, 

 and reference signals are continuously present at all times. However, in 

 many FM-CW systems the fore and aft return spectra are mixed in a 

 manner similar to that used in the incoherent pulse systems in order to 

 eliminate the undesirable effects of incidental frequency modulation in the 

 transmitter. 



Frequency of Transmission. Doppler radar systems have been 

 designed at X band (8800-10,000 Mc) and at Kg band (13,500 Mc). While 

 the latter frequency band has certain advantages over the former from the 

 viewpoint of gain and over-water operation for the same antenna aperture 

 size, it brings with it slight degradations as regards operation in weather, 

 particularly rain. The importance of these weather effects has yet to be 

 determined and may well not be significant. 



Polarization. The types of polarizations presently used for doppler 

 radars are linear and circular-odd (opposite rotation received), although 



