Page 226 - Radar Technology Encyclopedia

imaging, range-doppler impedance, intrinsic, of the medium 216

tude or RCS in range and doppler coordinates. Waveforms with a graphecon also requires a wide band. For this reason,
with thumbtack ambiguity functions (phase-coded wave- transmission methods often use digital encoding with com-
forms or pulse trains) are typically used to produce such pression, taking advantage of the inherent redundancy to fit
images. The latter have an advantage associated with the the information within a narrowband channel.
absence of side lobes with respect to velocity near the central Transmission of the radar image is used when automated
peak. collection and imaging of data received by separate radars are
To increase the detail of the image, doppler range-finding required, at a central control point, for example at the air-traf-
processing is used. This provides high range resolution fic control center of an airport. IAM
through pulse compression and coherent summing of signals Ref.: Ridenour (1947), Ch. 17; Finkel’shteyn (1983), p. 525.
accumulated in the process of observing a moving target (the
Two-dimensional imaging shows the distribution of scatter-
principle of reverse synthesis of the antenna aperture). The
ing sources of a target on a two-coordinate plane. Two-
algorithm for obtaining the image boils down here to a two-
dimensional images include images in two angular coordi-
dimensional Fourier transform of a fixed echo frequency.
nates and in range and doppler velocity coordinates. A two-
IAM
dimensional image is virtually a projection of the basic three-
Ref.: Mensa (1981); Munson, D.C., Proc. IEEE 71, no. 8, 1983.
dimensional image of a solid target and therefore has a signif-
A synthesized image can be either icantly smaller volume of information about the target in
(1) The simultaneous display on the plan position indica- comparison with a three-dimensional image.
tor (cathode-ray tube) of signals received in the active chan- A two-dimensional image depends on the target aspect,
nel and target returns discriminated during several preceding which usually requires obtaining many images in the interest
rotations of the antenna. The latter returns are generated by a of target discrimination, measurement of their scattering char-
digital computer which stores the data at a rate no less than acteristics, and restoration of the three-dimensional image.
20 Hz to exclude the flicker effect. In observation of surface-distributed targets, for example
(2) An image of an object obtained by synthetic aperture in radar scanning of the earth, two-dimensional images pro-
method when there is relative movement of the object and the vide significant information about the distribution of charac-
radar. The image is formed as a result of a Fourier transform teristics over the surface. IAM
of stored radar data using a computer. IAM Ref.: Kuchkov (1985), p. 126; Wehner (1987).
Ref.: Finkel'shteyn (1983), p. 520.
IMPEDANCE. Impedance is “the ratio of the phasor equiva-
Three-dimensional imaging presents the distribution of scat- lent to a steady-state sine-wave voltage or voltage-like quan-
tering sources of a target depending on the values in three tity (driving force) to the phasor equivalent of a steady-state
coordinates. Images in coordinates of range and two angles, sine-wave current or current-like quantity (response).” In a
in the coordinates of range, doppler velocity, and angle are two-conductor transmission line it is: the ratio of the complex
three-dimensional images. voltage between the conductors to the complex current on one
The three-dimensional image may be formed from the conductor in the same transverse plane; while in a waveguide
total of two-dimensional images by the methods of tomogra- it is “a nonuniquely defined complex ratio of voltage and cur-
phy. The production of three-dimensional geometric images rent at a given transverse plane in the waveguide, which
of small targets (with dimensions close to the wavelength) depends on the choice of representation of the characteristic
using ultrawideband waveforms is promising. Such methods impedance.”
are based on Lewis-Boyars equations, which make it possible IEEE (1993), p. 620.
to relate a three-dimensional geometric image and its scatter-
Impedance matching is “the control of impedance for the
ing characteristics. IAM
purpose of obtaining maximum power transfer or minimum
Ref.: Wehner (1987), Ch. 8; Astanin (1989), p. 177; Reedy, E. K. Proc. IEEE
reflection.” The maximum possible power from the source is
Nat. Radar Conf., Los Angeles, CA, Mar. 12–13, 1986, pp. 7–11.
received when a load impedance connected to a source is
Image transmission is the transmission of radar information adjusted to be equal to the complex conjugate of the imped-
in the form of signals forming radar images. Surveillance ance of the source. Impedance matching for minimal reflec-
radar image transmissions consist of range data (video or syn- tion is typically used in transmission lines with the aid of
thetic video) and antenna angular data, along with synchroni- various impedance-matching circuits. At microwaves the
zation signals. Antenna angular position is usually main impedance-matching circuits are matching stubs and
represented by a binary code, in the interval between the transformers. SAL
video from maximum range and the next transmission. Ref.: Johnson (1984), pp. 43.1–43.27.
In the simplest method of direct transmission of data,
The intrinsic impedance of the medium is the ratio of the
various carriers are used with frequency division multiplex-
effective or root-mean-square values of the electric E- and
ing, either by RF cable or radio channel. A drawback of the
magnetic H-field intensities. The usual notation is Z = E/H.
0
method of direct transmission is the complexity and wide
SAL
bandwidth of the multichannel communications line. Trans-
mission of the radar image converted into a television image Ref.: Fink (1975), p. 1.43.