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Radar System Engineering

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126 LIMITATIONS OF PULSE RADAR [SEC.4.4<br />

The reader may well ask whether a phenomenon has been overlooked<br />

which could be used to distinguish some targets from others. There<br />

appears to be no possibility for such a phenomenon in the elementary<br />

process of reflection of electromagnetic waves from inhomogeneities<br />

in the medium through which they travel. A returning wave is characterized<br />

by frequency (including phase), intensity, and polarization.<br />

If two targets within the radar beam—for example, a telephone pole and<br />

a stationary man—produce echoes similar in the respects listed, they are<br />

utterly indistinguishable, as much as we might prefer to label one clutter<br />

and the other the true target. Such echoes may very well be identical<br />

in the respects listed since no significant difference exists at these frequencies<br />

between the electromagnetic properties of a man and those of a<br />

piece of wood. To put it another way, the dimension of “color” is not<br />

available because the radar cross section of most objects varies in no<br />

systematic way ,with frequency. Distinction by shape, on the other<br />

hand, is possible only when the radar beam is considerably smaller in<br />

cross section than the object viewed.

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