The primary mission of Airborne Early Warning (AEW) is to detect, classify, and track distant air targets, and to direct the simultaneous interception of multiple threat forces. However, the AEW system also performs (often simultaneously) other tasks, such as the coordination of search and rescue and airborne rendezvous control (e.g., airborne-tanker join-ups).

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 this chapter, AEW requirements and operational concepts are used to apply on AEW platform selection. There are various factors to be considered - first factor is defense area if it is a task force at sea. The second factor to be considered is the threat or target to be detected. Basing requirements become a factor in platform selection.

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.

This chapter includes discussions on the average radar cross section (RCS) of targets and clutter, their fluctuation properties, and the detection of targets in the presence of noise and clutter. It specifically addresses, when possible, the causes for variation in the RCS of targets and clutter, and the statistical principles used in radar echo analysis. Example RCS values applicable to AEW are given for targets and clutter. Statistical target and clutter models that have proved useful for radar system analyses are also discussed. Finally, basic background material and sample problems are included on the detection of targets in the presence of noise and clutter.

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 the basic issues of tracking airborne targets (aircraft, missiles, and so on) with a radar operating in a track-while-scan mode. Although multitarget tracking radars are of primary interest, the issues associated with tracking a single target will be addressed first. These issues are addressed first because most multitarget track algorithms act as 'executives' that pass information to subordinate singletarget track algorithms. Finally, the multiple-target tracking problem and associated algorithms will be presented.

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.

IFF systems have been around for a number of years and have proved their usefulness on all types of aircraft and ships. New systems such as Mark XV are being developed to improve interoperability, performance, security, and increased antijam ability. However, IFF by itself does not solve the identification problem. ESM is a valuable adjunct to the surveillance radar for the early detection, classification, and identification of electromagnetic emissions in order for surveillance operators to concentrate all the available resources toward identifying potential threats. Future ESM systems are expected to be highly integrated, totally automatic, and incorporate expert and knowledge-based systems to handle the very high pulse data rates and complex modulation waveforms of the future. Communications is the key element to command and control. Satellite communications will enable the AEW platform to receive data rapidly from a variety of sensors. Because of advances in high-speed processing, data can now be processed aboard space-based sensor platforms and then communicated directly to the AEW platform. Self-protection systems will be an integral part of future AEW platforms. These countermeasures systems will be multifunctional so that they can be changed as the threat changes. IR sensor technology will continue to mature. Staring focal plane arrays with up to 100,000 detector elements will be developed and deployed aboard future space-based infrared sensor systems. The combined capability of radar, ESM, IFF, IR, and other national assets should provide future AEW platforms with the capability to detect AEW targets in adverse weather with a high degree of confidence.

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.

Aerodynamically shaped, tethered balloons, commonly referred to as aerostats, provide high-endurance and cost-effective platforms for many airborne early warning applications. This chapter describes modern aerostats and support systems, their advantages and problems, and the radar sensors used on board.

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.

This chapter discusses the basics of statistics focusing on probability density functions and distributions.

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