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211   HOLOGRAM, HOLOGRAPHY, radar                                                      hologram, mosaic



           (quasioptical RF hologram), while the second presupposes  Ref.: Safronov (1973), pp. 45, 166; Bakhrakh (1980); Tuchkov (1985),
           the use of a synthesized aperture (synthesized RF hologram).  p. 130.
           In both cases, the reference wave is created artificially. It is  A complex hologram is a form of recording of a radio holo-
           introduced to the receive  circuit from  the generator of the  gram when it is represented by a complex function of field
           transmitted signal. Combined methods are also used. Thus in  coordinates. The complex form is convenient in an analytical
           holographic earth-scanning radars they use a combination of  description of the process of image reconstruction.  Single-
           synthesis along the track line and a real aperture (in the form  and  two-dimensional practical registration circuits corre-
           of a one-dimensional antenna array) in the transverse direc-  spond to  a complex radio hologram. In the first  case, the
           tion. The hologram is represented in the form of the sum of  amplitude and phase of the result field are registered; in the
           the data components and the background. The data compo-  second, the quadratic components of the complex radio holo-
           nents are the product of the integral  Fourier transform or  gram. IAM
           Fresnel transform of the scattering function of the object and  Ref.: Tuchkov (1985), p. 132.
           the function describing  the  field created  by the reference
                                                                A  Fourier(-transform) hologram is one that registers the
           source.
                                                                interference picture formed during  interaction of  an object
               The structure of the interference picture depends on the
                                                                wave with a spherical front in the plan of the hologram with a
           curvature of the leading edge of the object and reference
                                                                spherical reference wave having a radius of curvature equal to
           waves and is described in the form of a Fourier, Fresnel, or
                                                                the mean radius of curvature of the object wave.
           Fraunhofer radio hologram. Based on the type of registration,
                                                                    Fourier radio holograms are the sum of the products of
           one distinguishes between quadratic and multiplicative radio
                                                                complexly linked Fourier transforms of the scattering func-
           Algerians obtained using a quadratic detector or a decoder-
                                                                tion of an object and reference waves. A reverse  Fourier
           multiplier, respectively.
                                                                transform of the function describing the radio hologram is
               The basic characteristic of the RF hologram is its resolu-
                                                                used for reconstruction of the image.
           tion, which  is defined, for wavelength  l, distance  r  to the
                                                                    The Fourier radio hologram in particular is implemented
           object, and size d of the hologram, in a plane parallel to the
                                                                in synthesized radio holograms by turning of the object about
           hologram as
                                                                the center of mass with a fixed transmitting and receiving sys-
                                     lr
                                 d l ³ -----                    tem. IAM
                                  p  d
                                                                Ref.: Safronov (1973), p. 42; Tuchkov (1985), p. 133.
           and in the transverse plane as
                                                                A Fraunhofer (diffraction) hologram registers the interfer-
                                       2
                                    2lr
                                d l ³ -----------               ence picture from the interaction of planar or spherical refer-
                                 t    2
                                     d                          ence waves with planar waves from the object. With a planar
               A single image is restored from a multiplicative RF holo-  reference wave, a Fraunhofer radio hologram has the same
           gram, while when a quadratic RF hologram is used, an inter-  mathematical structure as the Fourier hologram. In the case of
           fering  background  also arises,  in addition to the image.  a spherical reference wave, the Fraunhofer  hologram  is the
           Restoration of images is possible optically or digitally. In the  sum of products of the complexly-linked Fourier transforms
           first case, the same circuits are used for imaging in visible  of the scattering function of the object and the Fresnel trans-
           light as are used in optical holography. Digital processing  form of the reference wave. IAM
           makes it possible to raise the speed of measurement and the  Ref.: Safronov (1973), p. 41.
           dynamic range of the measurement equipment, and to opti-
                                                                A Fresnel(-diffraction) hologram registers the interference
           mize the parameters of image reconstruction. In the optical
                                                                picture formed during interaction of planar or spherical refer-
           method of restoration, a visible image of the radar object is
                                                                ence waves  with an object  wave having  a spherical  phase
           obtained. Due to the difference in the frequency of light and
                                                                front. The Fresnel radio hologram is the sum of complexly
           the radio frequency used in forming the radio hologram, there
                                                                linked products of the Fresnel transform of a scattering func-
           is a change in the dimensions of the image compared with the
                                                                tion of an object and a Fourier transform (Fresnel) of a planar
           size of the object. The scale of the image depends on the type
                                                                (spherical) reference wave.
           of hologram (Fourier, Fresnel, or Fraunhofer radio hologram)
                                                                    In practice, Fresnel radio holograms formed using refer-
           and the curvature of the front of the restoring wave and may
                                                                ence oscillations with planar  phase  fronts are most widely
           be increased or reduced. When restoring the images obtained,
                                                                used. Restoration of images by such radio hologram is done
           for example, using antenna arrays, a large number of images
                                                                by a wave with a planar phase front, parallel to the plane of
           arise. Exceptions to the overlay of images are achieved
                                                                the radio hologram. IAM
           through selection of a specific period of quantization.
                                                                Ref.: Safronov (1973), p. 56.
               Radio holographic methods  are widely used in radar
                                                                A mosaic hologram is obtained through synthesis of individ-
           imaging, terrain mapping, discrimination and  identification,
                                                                ual radio holograms formed under various conditions at vari-
           and radio astronomy for precise determination of the coordi-
                                                                ous moments of time in the same or different points in space.
           nates of celestial objects. IAM
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