OVERVIEW

Array technology for passive direction finding has been developing as an outgrowth of two factors: first, the need to respond retrodirectively (transmit in the same direction as received) to a threat for jamming purposes; second, the need to measure a signal to a high degree of accuracy for targeting or aiming purposes. One of the main concepts of the field of electronic support measures (ESM) is the use of the electronic receiving systems as a means to detect signals for threat determination optimally and as a method of improving or multiplying the effectivity of jamming or electronic countermeasures (ECM). These functions are a subset of the concept of electronic warfare. With the advent of phased arrays, passive direction finding has again stolen a page out of the handbook of radar design.

Early radar systems first utilized scan then monopulse passive lobing in the receiver to illuminate and detect targets. As the number of targets (and radars) in a given environment increased, it became obvious that improved resolution in all areas would be needed. Doppler, moving-target indicators, and range-gating techniques were honed to a high degree of perfection to improve range resolution, and monopulse improvements normalized the radar signal returns, reducing false clutter and glint variations. Despite these changes, it became obvious in the early 1950s that fundamental changes would be needed in the antenna systems to improve angular resolution in both azimuth and elevation. Additionally, the requirement for improved security for countermeasures immunity and the ability to be able...