Page 58 - Radar Technology Encyclopedia

48 array, phased array, radio-optical

Phased arrays are the most advanced type of antenna tors. The larger the number of subarrays, the smaller the gain
used in modern radars. They provide the radar with flexibility of the individual oscillator. For N = 100, the parameters of a
sa
and adaptation to the assigned task: ability to change beam quasiconformal phased array can be approximated by those
position in space almost instantaneously (electronic scan- for a spherical array. AIL
ning); generation of very high powers from many sources dis- Ref.: Voskresenskiy (1981), p. 159.
tributed across the aperture; high directivity and power gain;
array radiator (see RADIATING ELEMENT).
possibility of synthesizing any desired radiation pattern
(including formation of pattern nulls in the directions of A radio-optical array is an active array that uses an active
undesired interference sources); capability of combining antenna array in which the processing of the received radio
search, track, and recognition functions when operating in signals occurs at optical frequencies. A top-level block dia-
multiple-target and severe interference environments (includ- gram of a radio-optical array is given in Fig. A87. Shown are
ing jamming); enhanced target throughput capability; and the array (1) and a spatial-temporal optical modulator (2).
compatibility with digital computers and digital signal pro- This modulator imparts a spatial-temporal modulation to the
cessing algorithms. On the other hand, they are the most com- phase of the coherent light output from the laser (3) corre-
plicated and expensive types of modern antennas. (See also sponding to the parameters of the signals received from
PATTERN, array). SAL objects imaged with the same from the output optical model
Ref.: IEEE (1993), p. 941; Johnson (1993), Chaps. 19, 20; Skolnik (1990), p. of the received radiation. This is followed by a transformation
7.1; Sazonov (1988), p. 396; Amitay (1972); Mailloux (1994). in a coherent processor (4). As a result, an optical image of
A phase-frequency array is one using frequency scanning in the received spatial-temporal signal is formed at the processor
one coordinate and phase scanning in the other. output. This information is registered on the photo detector
(5) and with the help of converter (6) is input to the computer
Ref.: Skolnik (1980), p. 303.
(7) or the analog device for final processing and decision-
A phase-phase array is one that uses phase shift to steer the
making.
beam in both coordinates. This array is typically referred to as
a phased array in the literature. Information Information
input output
Ref.: Skolnik (1980), p. 303.
Com-
A piece-wise linear array is one consisting of a combination Array puter
1 7
of flat modular subarrays in the shape of an incomplete cube,
octahedron, or other regular polyhedron. The basic difficulty Coher- Photo A/D and
Optical ent D/A
in creating these arrays lies in the electrical combination of Laser pro- detect- Con-
3 mod. 2 cessor or
the individual subarray modules without incurring excessive 4 5 verter 6
energy losses. They have found applications in receiving
antennas for the formation of spatial directivity characteris- Control
tics as a resource for coherent optics and holography. AIL
Ref.: Voskresenskiy (1986), p. 70. Figure A87 Block diagram of a radio-optical array.
A planar array has all elements located in a single plane
occupying a definite area. Planar arrays have different config- Depending on the type of light modulator used, one dis-
urations of elements: rectangular, triangular, or hexagonal, in tinguishes the following radio-optical arrays:
which the elements are positioned at the vertices of the rect- (1) Multichannel acousto-optical arrays.
angles, right triangles, regular hexagons and also at the center (2) Electro-optical modulators with electron beam ad-
of the hexagon. AIL dressing.
Ref.: Fradin (1987), p. 184; Mailloux (1994), pp. 20–27, 34, 81–87, 112– (3) Multichannel modulators addressed by an electrical
162. voltage.
Addressing is the mechanism by which the information is
A quasiconformal array is one whose radiating elements are
supplied to the light modulator.
positioned on the surface of a polyhedron having a large num-
Radio-optical arrays have a number of advantages com-
ber (up to 400) of nearly identical facets having identical flat
pared with ordinary antenna arrays. Radio-optical arrays per-
subarrays. The subarrays can be realized in the form of a belt
mit: the realization of parallel surveillance by placing receive
or as polygonal structures. The number of radiators in each
elements on flat or curved surfaces; real-time signal process-
subarray is determined by the required total quantity of radia-
ing and simultaneous panoramic surveillance in one spatial
tors in the array, the minimum allowable number of subarrays
dimension; and efficient suppression of jamming signals in
N , and the convenience of usage. Quasiconformal phased
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arrays are used to minimize the gain of the oscillators the receiving direction. These arrays do not require phase
shifters, do not have bulky beam-forming networks, and are
required in the scan region. Small oscillator gains are
wideband.
achieved by displacing the excited regions around the center
of symmetry of the array with the help of electrical commuta-