Page 222 - Radar Technology Encyclopedia
P. 222
hologram, mosaic hologram, three-dimensional (3D) 212
This type of hologram is used for increasing the contrast aperture, the field is measured at discrete point, and the radio
and improving the quality of restored images and also to hologram is recorded through commutation or parallel pro-
increase the angle of view when inspecting optical holograms cessing of signals at the output of the receiver elements. The
transformed from radio holograms. IAM properties of quasioptical radio holograms are similar to the
Ref.: Safronov (1973), p. 204. properties of optical holograms, which possess similar ratios
of apertures to lengths of the illuminating waves.
A multiplicative hologram is registered using detection of
High-quality images of objects with a number of
the product of the object signal U and reference signal U ref
ob
waves at a high or intermediate frequency: resolved elements on the order of a hundred using an antenna
array may be obtained at ranges which in the best case
h (x,y) = Re[ U (x,y) U (x,y) ]
ob
ref
where x, y are the spatial coordinates, and Re(x) is the real amount to tens of linear dimensions of the object. For this rea-
son such radio holograms are used effectively in close-in
part of the quantity x.
radars.
The multiplicative radio hologram, like the quadratic
Coherent radio holograms using a system of antennas
hologram, can be registered in complex form. The multiplica-
separated by a distance on the order of tens or hundreds of
tive radio hologram does not have an optical analog. IAM
kilometers makes it possible to produce quality images of air-
Ref.: Tuchkov (1985), p. 131.
craft at ranges of 100 to 1,000 km, respectively. However the
A one-dimensional (1D) hologram is one in which the val- realization of such systems is associated with great technical
ues of the scattering function of an object are re-created from difficulties in supporting the coherent functioning of many
one coordinate, and from the other its integral transform. An receiver elements in large areas. IAM
image received by a radar with a linear antenna and transmit- Ref.: Safronov (1973), p. 218.
ted pulses that are short compared with the length of the tar-
A synthesized [synthetic] hologram is one formed by regis-
get can serve as an example of a one-dimensional radio
tration of a field reflected from an object at various points of
hologram. The values of the scattering function are restored
space successively in time using one receiving antenna. The
along the line of sight in the form of amplitudes of the inter-
synthesis can take place in one or several coordinates, in two
ference relief of the radio hologram.
in the picture plane (perpendicular line of sight) and a third
During restoration of images from one-dimensional radio
along the line of sight. In synthesis in the picture plane, any
holograms, special circuits are used in which an integral
relative motion of the object, the source of radiation, and the
transform is taken in accordance with one coordinate, and the
receiver in accordance with a known law may be used. The
scattering function of the object is reproduced in accordance
resolution of radio holograms in this case is determined by
with the other. IAM
the size of the synthesized aperture (length of synthesis) (see
Ref.: Safronov (1973), pp. 174, 189.
hologram). In synthesis along the line of sight, wideband fre-
A planar hologram is one in which the registered interfer- quency-modulated waveforms are used, and the resolution dl,
ence relief is an integral transform (Fourier or Fresnel) in two is determined by the width of the spectrum DF according to dl
coordinates. = pc/DF, where c is the speed of light.
Planar radio holograms are constructed on the basis of a Synthesized radio holograms are widely used in radars
real aperture in two coordinates (quasioptical hologram), on mounted on moving objects and having antennas of limited
the basis of a synthesized aperture in two coordinates (using size, on ground radars for raising the accuracy of coordinate
small antennas and a “point” aperture, or antennas with a lin- determination and receipt of target images, and also for mea-
ear aperture and range synthesis through linear frequency surement of local characteristics of scattering of radar targets
modulation of the signal), and also through a combination of in anechoic chambers. IAM
a real and synthesized aperture in different coordinates. IAM Ref.: Safronov (1973), p. 172; Tuchkov (1985), p. 130.
Ref.: Safronov (1973), p. 173.
A three-dimensional (3D) hologram is one that makes it
A quadratic hologram is registered using quadratic detec- possible to form a 3D image of an object. The three-dimen-
tion of the subject of the object U and reference waves U op sional radio hologram can be realized by three methods: (1)
ob
at a RF or intermediate frequency: through successive arrangement at specific distances from
h (x,y) = | U (x,y) + U (x,y) | 2 one another of planar hologram; (2) through formation of a
ob
op
where x, y are the coordinates of the antenna aperture. radio hologram in 3D structures which permit fixation of the
Recording of the quadratic hologram is done in quadra- changes in parameters of the electromagnetic field in the
ture form or in the form of amplitude and phase components. three dimensions; and (3) by synthesis from linear and point
Quadratic radio holograms are the analog to optical holo- elements or their combinations.
grams. IAM As a rule, radio holograms are formed in digital form
Ref.: Tuchkov (1985), p. 131. using algorithms of fast Fourier transforms. IAM
A quasioptical radio hologram is formed using antenna Ref.: Safronov (1973), p. 204.
arrays or a system of separated antennas. In contrast to syn- two-dimensional (2D) hologram (see planar hologram).
thesized holograms, a quasioptical radio hologram uses a real