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279   MOVING-TARGET DETECTOR (MTD)                                MOVING TARGET INDICATION (MTI)



               The MTI precanceler eliminates most of the fixed clutter,  signals of the next interval. The result is cancellation of fixed
           reducing the dynamic range required in subsequent process-  targets and passage of targets with pulse-to-pulse change in
           ing.  The doppler filter bank is typically based  on the FFT  detected amplitudes.
           algorithm, providing the following advantages over a delay-  The resultant video signal is
           line canceler:  (1)  the  signal-to-noise  ratio is improved by
                                                                                vt () ksin=  ( 2p f t –  f )
                                                                                                0
                                                                                            d
           coherent integration within each of the n filters, whose band-
                                                                and that delayed from the previous transmission is
           width will be 1/n that of the canceler; (2) doppler frequency
           measurement is available, based on the filter number in which    vt –  T ) ksin=  [ 2pf t –  T ) –  f ]
                                                                             (
                                                                                            (
                                                                                           d        0
           detection occurs; (3) the filter bandwidth can be adjusted by
                                                                The resulting canceler output is
           amplitude or frequency weighting (windowing), giving better
                                                                                           æ
           range sidelobe reduction; (4) adaptive thresholding can be        Dv =  A cos  2pf t –  T ö  f
                                                                                              ---  –
                                                                                   D      d è  2 ø  0
           applied to each filter, permitting rejection of moving clutter
           (e.g., weather clutter).                             where k is the amplitude of the video signal, f  is the doppler
                                                                                                     d
               The use of burst-to-burst PRF diversity is necessary to  frequency, f  is the initial phase shift, and T is the pulse repe-
                                                                          0
           fill blind speeds that fall within the target velocity region, and  tition interval. The amplitude of the canceler output is
           it provides a means of rejecting multiple-time-around (range-          A  =  2ksin ( pf T )
                                                                                   D         d
           ambiguous) target echoes. The adaptive thresholding applies  and it is a periodic function of T and f , called the MTI (fre-
                                                                                               d
           separate thresholds to each filter: the nonzero velocity chan-  quency) response (Fig. M28). This is the classical response of
           nels use a  range-cell-averaging CFAR to adapt to moving
           clouds  of precipitation, while the  zero-velocity  channel
           threshold is generated by the clutter map, which applies a sep-  2
           arate threshold for each range cell. In the zero-velocity chan-
           nel, which bypasses the MTI precanceler, targets at zero   Amplitude
           radial velocity and at the blind speeds can be detected if they  1
           exceed by a sufficient margin the clutter stored in the map.
               The MTD is essentially a low-PRF pulsed doppler pro-
                                                                        0  1   0     1     2     3     4     5
           cessor, and was originally developed in the mid-1970s at the           Frequency (units of 1/T)
           MIT Lincoln Laboratory. The initial design used a three-pulse
           precanceler and an eight-pulse doppler filter bank. DKB, SAL  Figure M28 MTI frequency response.
           Ref.: Schleher (1991), pp. 37–52.
                                                                a single-delay canceler. It shows that cancellation will occur
           MOVING-TARGET INDICATION (MTI). MTI is the pro-
                                                                not only for zero-velocity targets but for those with f  = i/T.
           cess of rejecting fixed or slowly moving clutter while passing                                  di
                                                                The speeds  v  =  lf /2 at which this undesired cancellation
           echoes from targets moving at significant velocities. In most   i    di
                                                                occurs are termed blind speeds.
           cases the MTI is sensitive only to radial components of veloc-
                                                                    To improve this simple response, more complex cancel-
           ity, but area MTI techniques can provide sensitivity to angular
                                                                ers can be used: those of the three-pulse, or occasionally the
           components as well.
                                                                four-pulse type; cancelers with  feedback (infinite-impulse-
               In conventional MTI,  discrimination between echoes
                                                                response filters); or more complex filters operating on range-
           from fixed and moving targets is based on the doppler effect.
                                                                gated receiver outputs. The most common MTI systems pass
           The filter technique can be realized either in the time or fre-
                                                                video (baseband) pulses through the cancelers, using parallel
           quency domain. Implementation in the time domain is based
                                                                channels for in-phase (I) and quadrature (Q) phase-detected
           on the fact that the phase of the fixed target echo does not
                                                                components.  Other approaches pass intermediate  frequency
           change from pulse to pulse, while that of the moving target
                                                                (IF) signals through a vector canceler.
           changes at a rate corresponding to the doppler frequency. This
                                                                    The voltage response of an m-pulse canceler to a target
           leads to the delay-line canceler implementation (Fig. M27), in
                                                                with velocity v is
           which the phase-detected (bipolar)  video  signals  (whose
                                                                                                  m
                                                                                  v =
           amplitudes depend on their phase angles) are fed into a delay      H () [   2 sin ( pvv ¤  b  )]
                                                                                m
           equal to the pulse repetition interval and subtracted from the
                                                                shown in Fig. M29 for  m = 1 and  2. Failure to center the
                                                                rejection notch on clutter moving with velocity Dv leads to a
                     Bipolar video                  Unipolar video
                                                                clutter response  H (Dv), degrading the performance of the
                                                     to indicator              m
                           Delay line
                  Receiver           Subtractor  Full-wave      system.
                            T = 1/prf  circuit  rectifier
                                                                    The wide rejection notch formed for m > 1 causes serious
                                                                loss in target detection (see LOSS, velocity response) unless
             Figure M27 MTI receiver  with  delay-line canceler (after  staggered PRF or PRF diversity is used. A typical staggered
             Skolnik, 1980, Fig. 4.4, p. 104).                  PRF velocity response is shown in Fig. M30.
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