Introduction - Time-Frequency Transforms for Radar Imaging and Signal Analysis - page 21

fine features in addition to its position and velocity [10-12]. SAR images
reconstructed from received signals are high-resolution maps of the spatial
distribution of the reflectivity function of stationary surface targets and
terrain. High-range resolution is obtained by using wide bandwidth of the
transmitted waveform, and high cross-range resolution is achieved by coher-
ently processing returned signals from a sequence of small apertures at
different aspect angles to the radar to emulate a large aperture. If the radar
is stationary and the target is moving, the angular motion of the target with
respect to the radar can also be used to generate an image of the target. In
this case, the radar is called an inverse synthetic aperture radar (ISAR) because
it utilizes a geometrically inverse way (i.e., the radar is stationary and the
target is moving) to image the target [9, 13-16]. In ISAR, cross-range
resolution is determined by the Doppler resolution with a scaling factor.
With a high Doppler resolution, differential Doppler shifts of adjacent
scatterers on a target can be observed, and the distribution of the target's
reflectivity can be obtained through the Doppler frequency spectrum. Con-
ventional methods to obtain Doppler information are based on the Fourier
transform, called the Fourier-based image formation.
1.4.1 Range Profile
With sufficient frequency bandwidth, it is possible to generate a 1D map
of the target along the radar LOS, called the range profile or down-range
profile as mentioned in Section 1.1. Similarly, by observing a target with
relative motion with respect to the radar over a sufficient time interval, it
is possible to generate a 1D cross-range map of the target.
A range profile is a range-compressed back-scattered signal. Since time
delay is related to the distance from the radar to the target, the resulting
radar signal, as a function of time, can be interpreted as a 1D mapping of
the prominent scattering centers on the target along the radar LOS. In simple
targets, a range profile typically consists of a number of distinct peaks that
can be related spatially to the isolated scattering centers on the target.
1.4.2 Range Resolution
Radar range resolution defines the ability of resolving two point-targets
within the same antenna beam, close together in the range domain. Because
the delay-time t of a radar signal returned from a target is related to the
range R by t =2 R / c , the resolution in range is directly related to the resolution
in delay-time. The range or down-range resolution D r r is determined by
the bandwidth of the transmitted signal BW: