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monopulse, phase-phase                                      MOVING-TARGET DETECTOR (MTD)           278



               The phase difference is                          the receiving  channel output have equal  amplitude  and  the
                                                                difference signal equals zero.
                         Df = 2pDR/l = 2pL/l sinq,
                                                                    The difference signal is used to control the beam position
           where  l is  the wavelength. This  provides the  capability to  during the tracking process. The sum signal is used both as a
           define angle  q  relative  to the  measured magnitude of the  reference signal and for target acquisition and measurement
           phase shifts of signals reflected from a target.     of its range and velocity. The direction finding characteristic
               The direction-finding characteristic is          is
                                                                                               –
                                                                                                 2
                                                                                              1
                                  2    æ 2pL   ö                              S q() K mq cos=  pd  ( f f )
                            =
                        S q() K V    sin  ---------- sin q
                                pd lim  è  l   ø
                                                                where K  is the phase detector transmission factor, m is the
                                                                       pd
                                                                slope of the difference pattern over its operating region, and
                                                                f , f are the phase shifts in the channels. An advantage of
                                                                    2
                                                                 1
                                                                such  a direction finding system is the lax requirement for
                                                                match of receiving channel responses, which explains its
                                                                wide use in modern monopulse systems. AIL
                                                                Ref.: Sherman 91984), Ch. 5; Leonov (1986), p. 78.
















             Figure M24 Phase-phase  monopulse system: (a) elementary
             patterns, (b) signal paths, and (c) block diagram (after Leonov,
                                                                  Figure M25 Sum-and-difference monopulse system: (a) block
             1986, Figs. 1.1 and 4.2, p. 68).
                                                                  diagram, (b) elementary antenna patterns, (c) sum and differ-
                                                                  ence patterns (after Leonov, 1986, Fig. 4.3, p. 70).
           where   K  is the phase detector transmission factor, and V lim
                   pd
           is  the  limiting threshold  relative to amplitude of  signals  at
                                                                MOVING-TARGET DETECTOR (MTD). The MTD is an
           phase detector input. A shortcoming of a system with a phase
                                                                enhanced configuration of moving-target indicator (MTI) that
           angular discriminator is the great dependence of direction
                                                                combines a series of features to improve clutter rejection and
           finding  accuracy on  the degree  of identity of  the phase
                                                                target detection. The main features  in an MTD are (a) the
           response of the receiving channels and their stability. AIL
                                                                MTI precanceler, (b) the doppler filter bank, (c) use of burst-
           Ref.:Sherman (1984), Ch. 5; Leonov (1984), p. 67.
                                                                to-burst PRF diversity, (d) adaptive thresholding, and (e) the
           Sum-and-difference monopulse is a method of determina-  clutter map (Fig. M26).
           tion  of angular coordinates  in monopulse systems using
           amplitude or phase direction-finding and a sum-and-differ-
           ence angle  measurement method  (sum-and-difference dis-
           criminator).  Most widely used are  amplitude sum-and-
           difference monopulse systems, a block diagram of which for
           target direction finding in one plane is depicted in Fig. M25a.
           Figures M25b  and M25c depict  the elementary  patterns,
           F (q) and F (q), the sum pattern S(q), and difference pattern
                     2
            1
                                        s
                                         i
           D(q), of such a system, with + and - gns designating the rel-
           ative phase. It is evident from this figure that the phase of the
           difference signal changes depending on target direction rela-
           tive to the equisignal direction and may be either at 0 or 180°
           relative to the phase of the sum signal. When the direction to  Figure M26 Block diagram of typical moving target detector
           the target coincides with the equisignal direction, signals in  (from Schleher, 1991, Fig. 1.29, p. 39).
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