Page 233 - Radar Technology Encyclopedia

223 integration time INTERPOLATION

quent noncoherent integration is possible over the time-on- wave by a phase comparison of the signals received at sepa-
target. PCH rate antennas or separate points on the same antenna.” The
Ref.: Barton (1988), pp. 17, 24. resolution is inversely proportional to the baseline d of the
interferometer (the distance between antennas) and can be
video integrator (see noncoherent integration).
estimated by the width of the main interference lobe l/d. In
INTELLIGENCE, radar. There are two different interpreta- terms of the method of practical realization, interferometers
tions of the term “radar intelligence”: (1) the process through are subdivided into additive, which perform coherent addition
which information about a particular “target” radar is of signals and detection of resultant signal, and multiplicative,
obtained and (2) use of radar in the collection of intelligence which multiply the signals and perform integration. The
information concerning the operations, technical capabilities, drawback of additive interferometers is the presence of a con-
and intent of the target subject. stant component of the output signal, which is determined by
The first definition includes radiation intelligence (RAD- the power of the received signals. This degrades the detection
INT), and other means, such as visual observation and pho- characteristics against a background of interference.
tography, that may reveal significant information about the The basic difficulty in realization of radar interferome-
subject radar or radars. Passive reception of an operating ters is elimination in the ambiguity of angular coordinates,
radar’s RF emission can yield data such as radar frequency, caused by the multilobe structure of the dependence of the
effective radiated power (ERP), azimuth and elevation beam- output signal on the angle (interference pattern). To eliminate
widths, antenna scan rate and time-on-target, volumetric cov- this ambiguity, wideband waveforms are often used, reducing
erage, antenna sidelobe pattern, and transmitted waveform the lobes other than the main lobe in the interference pattern.
characteristics. From these, other radar characteristics inde- Additional antennas, spaced within the bounds of the base are
pendently obtained through visual means, and some assump- also used.
tions concerning technology-driven nonobservables such as Interferometers are used in radar surveillance of the
internal radar losses and receiver noise figure, it is possible to earth, in radiometry of the earth’s surface, and in radio astron-
establish the role and mission of the radar, to estimate its omy.
nominal detection range, to determine whether the radar was A synthetic aperture interferometer uses the principle of
designed to detect targets in land and weather clutter, and to a synthetic aperture to receive the signals. In contrast to a
provide some insight into the potential vulnerability of the conventional interferometer, it permits measuring the range of
radar to ECM. the targets, along with the angles, if the range is approxi-
Radar intelligence in the alternative sense includes the mately of the same order as the interferometer base. When the
use of space or airborne radar to observe areas of the earth’s base is oriented along the target velocity vector and the beam-
surface for evidence of operational military activity, ground width of the interferometer beams q is comparatively small,
mapping to establish industrial capability, the amount and the resolution along the path of target direction dL is
location of natural resources, the environmental health of cer-
4l R 0
tain regions, and the monitoring of fishing activity, as well as d L = ------ ------
×
2
b
support to the interdiction of illegal drug activity, and so q
forth. PCH and range resolution
INTERFERENCE. Electromagnetic interference is the l R 0
×
d R = ------ ------
reception of waves other than produced by the transmitter or 2q b
target of interest. The main sources of interference are clutter, where R is the target range.
0
noise, jamming, and other factors discussed under ELEC- These values differ from corresponding resolution
TROMAGNETIC COMPATIBILITY. Interference typi- parameters for active synthetic aperture radars by factor of
cally has a random nature and it is the main factor limiting the R /b specific for interferometers only and indicate that in con-
0
detection performance of a radar. SAL trast to SAR, the resolution in this case is higher for range
Wave interference occurs when two or more waves from dif- than along the target motion direction. Synthetic aperture
ferent sources are superimposed, resulting in intensification interferometers are used in terrain observation radars and
or weakening of the resultant electromagnetic field intensity, radar astronomy where because of large bases and motion due
depending on the relative phases of the waves. In radar appli- to the earth’s rotation, it is possible to determine the angular
cations, wave interference manifests itself in distortion of the location of a sky object with high accuracy. IAM
coverage pattern due to multipath propagation. (See PROPA- Ref.: Barton (1964), p. 513; Reutov (1970), p. 83; Dulevich (1978), p. 551;
Mel'nikov (1980), p. 116.
GATION.) An application of the interference phenomenon is
the interferometer used in precision phase-based angular INTERPOLATION is the process by which target position
measurement. (See INTERFEROMETER’). SAL is measured within a fraction of the radar resolution cell
Ref.: Mayzel’s (1972), p. 9; Meeks (1982), p. 43. width. In angle, the process is also known as beam-splitting.
INTERFEROMETER, radar. A radar interferometer is “a Interpolation of position within the resolution cell is funda-
receiving system that determines the angle of arrival of a mental to the process of measurement of target coordinates.