Imaging Through Obscuring, Discrete-Scatterer Media With Chirped Trains of Wide-Band Pulses

We describe an approach to construct a pulsed signal which, when propagating in an obscuring, discrete-scatterer medium (such as clouds, fog, dust or other aerosols) experiences reduced attenuation. The approach can thus find possible applications in communication or high resolution target detection. In the proposed scheme, the transmitted signal consists of a coherent train of short wide-band pulses emitted at chirped (linearly varying) time intervals. As the energy of a single pulse which has traveled a large distance in the medium is almost entirely concentrated in the precursor-type structures associated with its leading and trailing edges, the increase of the signal total energy is achieved by using trains with a large number of pulses. The (down-)range resolution is controlled by the chirp bandwidth characterizing the distribution of individual pulses in the train (that bandwidth may be sufficiently large to achieve a resolution typical of millimeter-wave radar), while the high cross-range resolution can be attained in analogy to the usual synthetic-aperture imaging. Processing of the received signal involves filtering associated with the chirp characteristics, analogous to that in the conventional matched-filter techniques. It is also shown that the proposed approach provides flexibility in choosing the chirped-train center frequency, e.g., to maximize the amount of energy passing through a selected atmospheric absorption window or to maximize the signal energy carried to a desired penetration depth.