Introduction - Time-Frequency Transforms for Radar Imaging and Signal Analysis - page 4

Radar range profiles can provide target information about target length
and positions of strong scatterers, such as a radar dish, engine intakes, and
other scattering centers as shown in Figure 1.3. The dimension transverse
to the radar LOS is called the cross range. Because the Doppler shift of a
scatterer on a target is proportional to the cross range of the scatterer by a
scaling factor, the projection of the target reflectivity distribution on the
cross-range dimension can be obtained from the distribution of Doppler
shifts, called the Doppler profile. With high-resolution Doppler profiles, the
locations of strong scatterers and the target's extents in the Doppler dimension
can also be obtained as illustrated in Figure 1.4. By combining range profiles
and Doppler profiles, a two-dimensional (2D) radar image may be generated
[9-12]. A radar image is a spatial distribution of the target's reflectivity
mapped onto a range and Doppler plane. A range-Doppler image can be
converted to a range and cross-range image if we have accurate knowledge
of the scaling factor, which is determined by the rotation rate and the
wavelength of the transmitted signal [11, 12].
An important factor of the image quality is its resolution (i.e., the
ability to separate closely spaced scatterers in range and in cross range). The
resolution along the radar LOS to the target is called the range (or down-
range) resolution. The resolution transverse to the radar LOS is called the
cross-range resolution. The minimum distance in the range D r r , and in the
cross range D r cr , by which two point-scatterers can be separated, is the
resolution of the image. A rectangle with sides D r r and D r cr is called a
resolution cell. Range resolution is determined by the frequency bandwidth
of the transmitter and the receiver. For an X-band radar operating at
10,000 MHz frequency, a bandwidth of 5% of the radar operation frequency
(i.e., 500 MHz) can yield 1-ft range resolution. To obtain high cross-range

Figure 1.3 Radar range profile of an aircraft.