Page 77 - Fundamentals of Radar Signal Processing

systems, they have two powerful advantages. First, they can image a scene

through clouds and inclement weather due to the superior propagation of RF
wavelengths. Second, they can image equally well 24 hours a day since they do
not rely on the sun for illumination; they provide their own “light” via the
transmitted pulse. If the example of Fig. 1.21 were repeated in the middle of a
rainy night, the SAR image on the left would not be affected in any noticeable
way, but the optical image on the right would disappear entirely.

To obtain fine-resolution imagery, radars use a combination of high-
bandwidth waveforms to obtain good resolution in the range dimension and the
synthetic aperture radar technique to obtain good resolution in the cross-range
dimension. The desired range resolution is obtained while maintaining adequate
signal energy by using pulse compression waveforms, usually linear FM. A long
pulse that is swept over a large enough bandwidth β and processed using a
matched filter can provide very good range resolution according to Eq. (1.35).

For example, range resolution of 1 m can be obtained with a waveform swept
over 150 MHz. Depending on their applications, modern imaging radars usually
have range resolution of 30 m or better; many systems have 10 m or better
resolution, and some advanced systems have resolution under 1 m.
For a conventional nonimaging radar, referred to as a real aperture radar,
the resolution in cross-range is determined by the width of the antenna beam at

the range of interest and is given by Rθ as shown in Eq. (1.26). Realistic
3
antenna beamwidths for narrow-beam antennas are typically 1° to 3°, or about
17 to 52 mrad. Even at a relatively short imaging range of 10 km, the cross-
range resolution that results would be 170 to 520 m, much worse than typical
range resolutions and too coarse to produce useful imagery. This poor cross-
range resolution is overcome by using SAR techniques.
The synthetic aperture technique refers to the concept of synthesizing the

effect of a very large antenna by having the actual physical radar antenna move
in relation to the area being imaged. Thus, SAR is most commonly associated
with moving airborne or space-based radars, rather than with fixed ground-
based radars. Figure 1.23 illustrates the concept. By transmitting pulses at each
indicated location, collecting the range data for each pulse, and properly

processing it together, a SAR system creates the effect of a much larger phased
array antenna being flown along the aircraft flight path. As suggested by Eq.
(1.9) (though some details differ in the SAR case), a very large aperture size
produces a very narrowly focused effective antenna beam, thus making possible
very fine cross-range resolution. The SAR concept is explained more fully in
Chap. 8. A more modern and robust viewpoint based on integrating over a range
of angles is also given there.