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height-finder, squinted-beam                                   HOLOGRAM, HOLOGRAPHY, radar         210



           A squinted-beam height finder uses a modified monopulse  A V-beam height finder, depicted in Fig. H3, uses two broad
           beam pair, pointed far enough above the horizon to minimize  radar beams from a single radar scan in azimuth. One of the
           multipath reflection errors. The technique is similar to using  beams is tilted in the elevation plane so as to produce an out-
           off-axis  monopulse tracking and offers reduction but not  put whose delay, in the azimuth scan cycle, is proportional to
           elimination of the troublesome multipath errors on low-eleva-  elevation angle.
           tion  targets. Figure  H2 compares conventional monopulse
           and squinted-beam antenna patterns. DKB














                                                                  Figure H3  V-beam geometry (from Fink,  1989, p. 25.49,
                                                                  reprinted by permission of McGraw-Hill).
                                                                    The elevation error is proportional to the difference
                                                                between  the  two  azimuth  readings multiplied by the  cotan-
                                                                gent of the tilt angle. (See also PATTERN, V-beam).PCH
                                                                Ref.: Fink (1989), p. 25.49.
                                                                HETERODYNING is the process of the conversion of mod-
                                                                ulated radio-frequency  oscillations into modulated oscilla-
                                                                tions of a lower intermediate frequency to make subsequent
                                                                amplification more efficient. SAL

             Figure H2 Multipath effects on monopulse and squinted-beam  Ref.: Terman (1955), p. 568; Popov (1980), p. 84.
             height finders: (a) conventional S,D monopulse, (b) squinted-
                                                                A HIT is “a target echo from one single pulse.”
             beam low-angle  technique  (from  Skolnik, 1990, Fig. 20.12,
                                                                Ref.: IEEE (1990), p. 18.
             p. 20.37, reprinted by permission of McGraw-Hill).
                                                                HOLE, radar. A radar hole is region with reduced coverage,
           Time-difference height finding uses the multipath propaga-
                                                                caused by the extension of the radar range in other directions
           tion phenomenon to obtain additional target data needed for
                                                                due to the effect of ducting. For example, air targets above a
           estimation of altitude. The time delay between the direct path
                                                                surface duct, within which the radar range against surface tar-
           and the  specularly  reflected multipath, over  a flat earth, is
                                                                gets is enhanced, might be missed, though they would be nor-
           given by
                                                                mally detected when there is no ducting. SAL
                                cos q     2h h                  Ref.: Skolnik (1980), p. 451.
                                    t
                                            r t
                          d =  R ------------- –  1 » -------------
                           0    cos y      R                    HOLOGRAM, HOLOGRAPHY,  microwave.  Radio-fre-
           where  R is target range, q  is elevation  angle,  y is grazing  quency  (RF) holography  is a method of restoration  of the
                                 t
           angle at the surface, h  is radar antenna altitude, and h  is tar-  wave front of a radio wave in which the indicator, the radio
                                                       t
                             r
           get altitude. If the range, radar altitude, and one-way (receiv-  hologram, registers information both about the amplitude and
           ing) multipath delay  d /c are known, target altitude can be  about the phase of the field dispersed by the object. As a
                              0
           calculated. The calculation is more complicated over a spher-  result of subsequent illumination of the radio hologram, the
           ical earth, but it can  be performed easily  with modern  restoring wave forms an image of the object. The processes of
           computers.                                           formation and restoration of images constitute virtual forward
               The multipath delay is very small for surface-based  and reverse integral Fourier and Fresnel transforms depend-
           radars, so the technique is restricted in practice to airborne  ing on the curvature of the front of the object and reference
           radars and those on high mountain sites, where  the delay  waves.
           commonly exceeds the processed pulse width. When the sur-  An RF  hologram is  a fixed picture of interference
           face is too rough to support specular reflection, a spread mul-  between the field scattered by the object (object wave) and a
           tipath return is received, but the delay to its leading edge is  coherent  reference wave. From  the picture received, the
           still useful for target height measurement. DKB      image of the object is restored by the methods of RF hologra-
           Ref.: Long (1992), p. 349.                           phy.  RF  holograms are formed by two basic methods. The
                                                                first is associated with the  creation  of a real  aperture
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