The concept of airborne early warning (AEW) is straightforward. AEW is a method that force commanders can use to extend their 'eyes' to gain a more complete and extended picture of the theater of battle. The majority of sensors employed by force commanders to assess the overall situation are 'line-of-sight' type sensors. This means that the sensors will see in approximately straight lines (with a slight bending due to atmospheric refraction) out to their maximum effective range. The curvature of the earth presents a significant limitation to ground-based, line-of-sight sensors, as shown in Figure 2.1. Also depicted in Figure 2.1 is an AEW platform that extends the available line-of-sight sensor coverage for the detection of threats that would otherwise be masked by the curvature of the earth. Table 1.1 (Chapter 1) includes numerical examples of line-of-sight radar range versus the radar and target heights. The advantages of increased platform height for extending the maximum detection range, especially for low-altitude targets, are evident.

In its simplest terms, a radar is a device for detecting and locating objects (targets) of interest. The basic principle of operation involves the transmission of electromagnetic waves and, some time later, the reception of the waves reflected from the target (the echo). The term 'target' generally designates an object of interest to the radar user in a given situation. In addition to the inescapable 'noise' present in all radio systems, 'clutter' is used to denote the radar reflections received from area-or volume-distributed physical entities that interfere with detection of the desired targets. Two of the most common forms of clutter are ground clutter and sea clutter, which result from the generally rough nature of the surface of the earth, whether land or sea.

The purpose of this paper is to address the capabilities and needs of airborne early warning (AEW) radar. Radar-earth-target geometry, with its associated problems of atmospheric refraction, attenuation, and multipath effects . Relative aircraft-earth-target velocities, with the associated Doppler-processing challenges . Restrictions on frontal area and weight that limit antennas to relatively small sizes, which thereby contribute to poor angular resolution and increased Doppler spreading, complex aircraft/antenna geometry, which contributes to larger antenna sidelobes and distortion of the main lobe . Jamming and other electromagnetic interference. AEW radars use two basic modes of operation: (1) the Doppler mode to detect moving aircraft over land and sea, and (2) the ordinary pulse or non-Doppler mode for detecting stationary or slow-moving targets such as boats or ships. The longrange Doppler detection mode, with its requirements made stringent by operating from a moving platform, distinguishes the AEW radar from others.

This chapter addresses effects of aircraft structure on antenna patterns, aperture efficiency of low sidelobe antennas, and the determination of target altitude. Each of these subjects is of considerable importance to the design of future airborne early warning (AEW) radars.

The first airborne radar systems were developed during World War II, starting with the British AI (airborne intercept) VHF (200 MHz) radar used in their Beaufighter and Blenheim twin engine fighter/bomber. Airborne radar designs evolved quickly into three distinct categories, airborne early warning (AEW), fighter intercept, and bombing/navigation. Later developments included airborne radars for surface surveillance and weather avoidance. The need for supporting systems evolved at the same time radars were developed. The first support system needed was electronic communications. This was followed by the need to determine whether the object detected was friendly, neutral, or the enemy, resulting in identification, friend or foe (IFF). The need for command and control of intercepting fighter aircraft led to the development of better navigation systems. Then, all fighters being used for attack knew where the target was located because they had basically the same reference grid, or had achieved 'grid lock.' Electronic intercept systems capable of detecting and identifying electromagnetic emissions from aircraft were added to provide an independent means of target identification and location. Infrared and television systems have also been used to provide positive means of target identification.

This chapter provides a discussion of the fundamentals of target recognition technology and concepts. Emphasis is placed on recognition techniques as applied to radar signatures, for the AEW systems radar will in general provide the most reliable, long-range, all-weather, all-scenario data upon which to base the recognition process.

The book includes the appendices, acronyms and symbols about the airborne early warning system concepts.