6-3] MONOPULSE ANGLE TRACKING TECHNIQUES 301 



errors of one sign while decreasing apparent errors of the opposite sign (see 

 Paragraph 4-8). Such a phenomenon is possible because plus errors and 

 minus errors are generated alternately, not simultaneously. 



One obvious method of eliminating this source of error is to generate both 

 positive and negative errors simultaneously . A straightforward technique 

 for accomplishing this is to amplify the two signals from two overlapping 

 antenna patterns separately and compare the two amplifier outputs. This 

 technique is operable, but places severe stability requirements on the 

 amplifiers, since relative drifts in amplifier gain produce changes in indi- 

 cated correct tracking angle. 



An analogous method makes use of two spaced antennas in an inter- 

 ferometer arrangement. Signals from the two antennas are amplified 

 separately, with a common local oscillator for the two receivers, and relative 

 phase is measured at intermediate frequency. In this case the phase 

 stability requirement on the amplifiers is severe, since relative phase shifts 

 in the two channels similarly produce changes in indicated correct tracking 

 angle. 



Instability in indicated correct tracking angle may be overcome in either 

 the amplitude or the phase comparison approach by connecting the two 

 antennas in phase opposition before amplification, thus requiring only one 

 receiving channel. Direction of arrival of signals is determined as the 

 direction in which the amplifier output is near or equal to zero when increase 

 of signal is produced by misaligning the antenna pattern in either direction 

 from this so-called null point. This technique suffers from two objectionable 

 characteristics. When there is no tracking error, there also is no signal to 

 indicate presence of a target; and when there is an error signal, the sense of 

 the error is not indicated. 



Monopulse radar, as its name implies, is a tracking radar that derives all 

 its tracking error information from a single pulse and generates new and 

 independent error information with each new pulse. In a broad sense, the 

 simultaneous amplitude or phase comparison systems described above may 

 be called monopulse systems. The name monopulse, however, has become 

 restricted by common usage to still another method for generating both 

 positive and negative errors simultaneously which overcomes the principal 

 objections of the other systems. The method consists in so connecting the 

 RF circuits of two antennas that both sum and difference signals are 

 obtained simultaneously. The patterns of the two antennas overlap in the 

 conventional way for generating tracking error information, as shown in 

 Fig. 6-3. The sum signals from the two antennas merge the two patterns 

 into a single lobe pattern as shown in Fig. 6-4. The difference signals 

 produce the familiar null pattern with the sharp zero at the center, as shown 

 in Fig. d-S. The sum and difference signals are then amplified separately 

 and recombined in a product detector after amplification. 



