Missile Attack Warning

Abstract

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. A second mode of HF (high frequency) radar operation is surface-wave propagation, where HF energy propagates along the Earth's curvature by diffraction. 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. When a 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 systems. Recently, however, considerable information on bistatic HF radars has become available, which describes their military applications, specifically for ballistic missile launch warning. The purpose of this chapter is to document this new information, prefaced by a review of relationships between HF and VHF/ UHF (very-high frequency/ultra-high frequency) bistatic radar operation.