Page 45 - Fundamentals of Radar Signal Processing

For an unmodulated pulse, the bandwidth is inversely proportional to its
duration. To increase waveform bandwidth for a given pulse length without

sacrificing energy, many radars routinely use phase or frequency modulation of
the pulse.
Desirable values of range resolution vary from a few kilometers in long-
range surveillance systems, which tend to operate at lower RFs, to a meter or
less in very fine-resolution imaging systems, which tend to operate at high RFs.

Corresponding waveform bandwidths are on the order of 100 kHz to 1 GHz, and
are typically 1 percent or less of the RF. Few radars achieve 10 percent
bandwidth. Thus, most radar waveforms can be considered narrowband,
bandpass functions.

1.3.2 Antennas
The antenna plays a major role in determining the sensitivity and angular
resolution of the radar. A wide variety of antenna types are used in radar
systems. Some of the more common types are parabolic reflector antennas,

scanning feed antennas, lens antennas, and phased array antennas.
From a signal processing perspective, the most important properties of an
antenna are its gain, beamwidth, and sidelobe levels. Each of these follows
from consideration of the antenna power pattern. The power pattern P(θ, ϕ)
describes the radiation intensity during transmission in the direction (θ, ϕ)
relative to the antenna boresight. Aside from scale factors, which are

unimportant for normalized patterns, it is related to the radiated electric field
intensity E(θ, ϕ), known as the antenna voltage pattern, according to

(1.3)

For a rectangular aperture with an illumination function that is separable in the
two aperture dimensions, P(θ, ϕ) can be factored as the product of separate one-
dimensional patterns (Stutzman and Thiele, 1998):

(1.4)

For most radar scenarios, only the far-field (also called Fraunhofer)
power pattern is of interest. The far-field is conventionally defined to begin at a
2
range of D /λ or 2D /λ for an antenna of aperture size D. Consider the azimuth
2
(θ) pattern of the one-dimensional linear aperture geometry shown in Fig. 1.5.
From a signal processing viewpoint, an important property of aperture antennas
(such as flat plate arrays and parabolic reflectors) is that the electric field
intensity as a function of azimuth E(θ) in the far field is just the inverse Fourier
transform of the distribution A(y) of current across the aperture in the azimuth