Page 221 - Radar Technology Encyclopedia
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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