Page 224 - Radar Technology Encyclopedia

HYBRID (junction), microwave image correction 214

Hybrid tees are used with a branch in the H-plane (from the elements of the geometrical structure of the target. This
the thin wall) and in the E-plane (from the wide wall). When provides a range image using a wideband waveform and pro-
there are identical distances from the axis of the splitter, the duces a geometric target image, its profile function (cross sec-
power arriving from the input arm is divided equally between tion vs. range) depending on the use of ultrawideband signals.
these loads. The supply arm is matched then. An E-plane The radar image may be observed visually on the display
hybrid is distinguished by the fact that the intensity vectors of screen (radar imaging) or formed and processed without visu-
the electrical field in the output sections, which are at identi- alization. The production of high-information images
cal distances from the splitting axis, are opposite. demands the use of a high-speed components and computers.
The H-branch can pass more power, while the E-branch When the aspect of the object is fixed, the received signal
changes the reactance less in the frequency band, i.e. it has a is virtually a projection of the complete image of the object,
somewhat larger passband. Hybrid tees are narrow-band formed by the methods of microwave tomography.
elements. IAM The formation of radar images is widely used in radar
Ref.: IEEE (1993), p. 610; Lavrov (1974), p. 348; Gardiol (1984), p. 282. target recognition and mapping of terrain and extraterrestrial
objects. (See also HOLOGRAM; TOMOGRAPHY, micro-
wave.) IAM
Ref.: Boerner (1985), part 1; Andreyev, G. A., Zarubezhnaya Radioelektron-
I ika, no. 6, 1989, in Russian; Rihaczek (1996); Steinberg (1991), Mensa
(1991); Trevett (1986).
IDENTIFICATION, radar (see TARGET RECOGNI- Angle-angle imaging is imaging based on the properties of
TION AND IDENTIFICATION). radar angle resolution. To form such an image a radar has to
have very good angle resolving capability, so practically only
ILLUMINATION (see APERTURE illumination).
millimeter-wave radars and especially laser radars are useful
ILLUMINATOR. An illuminator is “a system designed to to create angle-angle images. In laser radar. the receiver aper-
impose electromagnetic radiation on a designated target so ture resolving capability is used to image the target backscat-
that the reflections can be used by another sensor, typically tered illumination onto the receiver detector, where the
for purposes of homing.” It is often the tracking radar in an air optical photon energy is then converted to electrons and pro-
defense system that “illuminates” a target, thus allowing a cessed to provide an image of the target field. To optimize the
radar-guided semiactive missile to use the target-reflected image, the illumination format (pencil, floodlight, etc.) is pro-
energy as the source for terminal homing guidance. Illumina- vided by the transmitter aperture. Such images can be used in
tor radars are either narrow-beam, single-target tracking discrimination of large targets at short ranges (planes, ships,
radars, or are systems that transmit the energy and are slaved etc.). One-dimensional angle images are also used, and com-
to another radar that tracks the target. (See also RADAR, bined images in angle-velocity and angle-range coordinates.
continuous-wave; SEEKER). PCH SAL, IAM
Ref.: IEEE (1993), p. 616. Ref.: Bernard, D. S., Proc. IEEE 77, no. 5, 1989; Jelalian (1992), p. 48.
IMAGE, IMAGING, radar. A radar image is the spatial dis- Conversion of a radar image to a television image is done
tribution of the scattering sources of an object, obtained as a to raise the brightness of the image observed on the television
result of analysis of the electromagnetic field scattered by it. screen compared with the image on the cathode-ray tube of
Such characteristics may include for example the reflective the radar display. This is done through repeated registration of
capabilities in a given frequency range, the geometric shape the signal recorded in one scanning period of the radar, and
of the object, the dielectric constant of the surface material, display of the regenerated images with a high repetition fre-
and so forth. quency on television indicators. The increase in the apparent
The radar image may be constructed by various methods. average brightness is estimated by the ratio of the period of
When the dimensions of the object significantly exceed the the television frame sweep to the period of scan of the radar,
size of the resolving element of the radar, the image is formed which is on the order of 500. The conversion of radar signals
through serial or parallel viewing of resolution cells of the to television signals is done using storage cathode-ray-tubes
radar (usually parallel scanning for range and serial scanning or graphecons. IAM
for angles). The use of aperture synthesis methods (or inverse Ref.: Poole (1966), p. 223; Finkel’shteyn (1983), p. 514.
synthesis) makes it possible to significantly increase the qual-
Image correction is applied to a radar image of an surface-
ity of the image by processing the coherently summed signals
mapping radar when there are distortions in the configuration
accumulated during observation of the object moving relative
of area and elongated objects and the distances between them.
to the radar. This is usually implemented on the basis of dop-
The distortions are usually due to disturbance of scales of the
pler analysis of signals, the method of radar holograms,
image along and across the line of movement occurring
which ensure that the spatial amplitude-phase distribution of
because of instability of flight trajectory and failure to correct
the reflectivity of the target is obtained. When the signal
for the difference between the slant range and the horizontal
bandwidth is broadened to improve range resolution, the ele-
range due to the sphericity of the wave front.
ments of the radar echo from the target may be compared with