TABLE OF CONTENTS In a low-prf system (range unambiguous), reflections from ranges beyond that of the target add to the total received. The relative amount of clutter power returned from distant ranges depends critically on the propagation conditions. Commonly encountered evaporation ducts, for instance, enhance clutter returns from ranges well beyond the "standard atmosphere" horizon [583].
In a high-prf system (Doppler unambiguous), the interpulse period may be shorter than the round-trip delay to the maximum required target-detection range, and clutter returned from ranges shorter than that of the target may fold onto the target's reflection. Any pulse repetition interval shorter than the round-trip delay to the target is subject to short-range clutter interference. In such cases, antenna beam shape effects can have a major influence on the degree of clutter interference, and propagation effects are less important.
The effects of antenna design on short-range surface clutter have been analyzed in [584]. Some highlights of that study are mentioned here. In accordance with practice in many modern systems, the horizon search function is assumed to be separate from a volume search function. A single row of horizon search beams has been assumed.
Received surface clutter is given by an integral along a constant range contour (Fig. 7.19). The amount of clutter power from a given clutter element varies with the corresponding antenna coordinates in accordance with
| (7.16) |  |
| where P c = | received clutter power |
| ? and ? = | azimuth and elevation angles with respect to antenna boresight |
