4.1 INTRODUCTION

Radar systems designed to operate at high frequencies between 5- and 30-MHz are often classified as over-the-horizon (OTH) radars, because at such frequencies the signals can travel via ionospherically refracted paths, called sky waves. Such radar-to-target paths can be from 500 to 3,600 km in overall length (per hop), allowing surveillance of targets over large areas of the Earth from relatively few radar sites. Targets include low-altitude missiles and aircraft, ships under way, and ballistic missiles during their boost phase.

A second mode of HF (high frequency) radar operation is surface-wave propagation, where HF energy propagates along the Earth's curvature by diffraction. In this mode, propagation signal loss increases exponentially with range; thus, ranges against these targets are reduced to 100 300 km. An HF bistatic radar might exploit a surface-wave mode on one path while using a sky-wave mode for the other path.

With a few early exceptions, HF radars use separated transmitters and receivers to reduce the complexity and cost of the receiving array, as well as to establish receiver isolation from the direct path transmitted signal. Site separation is typically on the order of 100 km. When an HF radar uses sky-wave propagation for both paths, this separation is small compared to the target range; thus, the radar operates with single-site characteristics, for example, with small bistatic angles, and is usually called a monostatic (or near-monostatic) OTH radar.

Nearly all HF radar literature has focused on these monostatic...