11.2 DYNAMIC TARGET IMAGING

We noted in Chapter 10 that to create imagery we rely on the sorting of scattering centers in both range and cross-range according to phase shifts in the signal recorded from one frequency to the next, or from one aspect angle to the next. It is implicit in the generation of imagery that the relative phase angles of the data so recorded be due only to target orientation, not downrange, uprange, or cross-range target translation. But since both the location and the orientation of the target change from one instant to the next during dynamic testing, phase changes due to this motion must either be accommodated or accounted for. As we shall see, several effects come into play during the measurement, none of which seems minor.

The chirped waveform of fig. 2.18 is the one preferred in the dynamic test community in lieu of the frequency-stepped waveform, probably because it is cheaper and easier to form, and because, at least as judged from Brookner's list of the parameters of operational tactical radars, one sweep of the waveform can be completed in a few dozen microseconds [2]. In the pulse compression form of reception, a dispersive medium is used to reduce the pulse width to a few microseconds or less [3], during which time the change in the phase of the received signal is reduced to a fraction of a radian over the duration of the pulse. Regardless of the phase change experienced over the pulse...