Airborne Early Warning - AEW

With the rapid advance in the development of weapons delivery systems, the early detection of airborne targets is increasingly critical to a successful defense. Efforts to increase the range of surface based radars have encountered numerous problems. For example, low flying aircraft are often shielded by the horizon from surface antennae in high frequency line-of-sight radars, and the physical size of the antenna and the enormous power supplies necessary for the transmission of low frequency radar energy are significant practical limitations. An additional problem has been the atmospheric trapping of the radar energy due to temperature inversion and other atmospheric conditions.

To avoid these problems, the prior art has resorted to the use of radar pickets or outposts spaced from the defended area. By using aircraft as the radar platform for these outposts, the problem of atmospheric trapping is reduced, and higher frequency signals requiring less power and physical antenna size can be utilized due to the effective increase in the height of the antenna with respect to the horizon. However, while the clutter of ground and sea return is a limiting factor only at ranges in the order of 5 to 10 miles for upward looking and outward looking surface based radars, airborne downward looking radars are severely limited by the clutter. This problem is particularly acute where the target is in close proximity to the ground.

Antenna arrays for aircraft surveillance radar systems require high gain, wide-band operation and low aerodynamic loading with full 360 degree azimuth scanning capability. Such radar systems can be mounted on aircraft which operate in flight in an Airborne Early Warning (AEW) system. The designs of prior AEW type antenna arrays have encumbered this type of antenna arrays with general inflexibility, excessive aerodynamic loading, disadvantages of mechanical scanning operation, and/or limited electronic scanning capability.

In prior antenna arrays having electronic scanning, large apertures placed parallel to the air stream have employed high-gain radiating elements and have thus provided high antenna gain with aerodynamic loading limited through control of the antenna frontal area. Dorsal fins, billboards attached to the sides of aircraft, triad phased and other structures have housed AEW type antenna arrays, but none of these approaches has been entirely successful in meeting gain, aerodynamic, and electronic scanning objectives together.

A basic problem results from the fact that placement of large apertures parallel to the air stream provides required high antenna gain but simultaneously makes it difficult to produce full-azimuth electronic scanning. Electronic steering systems in most prior art systems cannot provide beam steering sufficiently forward or aft to achieve 360 degree scanning primarily because of the side-looking location of radiating elements in the arrays placed parallel to the air stream.

The utilization of integral radome-antenna structures, and particularly such types of structures which are rotatably mounted on aircraft and employed as so-called airborne early warning systems (AEW) is well-known in the technology, and has successfully found widespread applications in conjunction with military surveillance aircraft,, especially aircraft adapted to be launched from naval carriers. In various instances, as currently utilized in military aircraft, such radome-antenna structures are mounted positions so as to be superimposed above the fuselage of the aircraft, although conceivably also being suspendable from below the fuselage, and incorporate a depending shaft structure, generally hollow in nature, extending downwardly from the radome into the fuselage of the aircraft, and wherein the shaft is operatively connected to a suitable drive arrangement for simultaneously rotating the shaft about the longitudinal axis thereof and the radome-antenna structure at specified speeds of rotation.

An alternative to the rotating antenna is a dorsal fin array. The dorsal fin array is thin, light and requires no moving parts. This array consists of two conventional, electronically scanned antenna (ESA) arrays positioned back-to-back. Each of the ESA arrays usually can scan .+-.60.degree. for a combined total of 240.degree., short of the desired 360.degree.. Placing an array on either end of the back-to-back configuration, due to size constraints, won't allow these end arrays to provide nearly as much directive gain as the side-looking arrays, hence limiting the radar detection range.

In blue water environments, airborne early warning or AEW aircraft, through the use of a comparatively large antenna, a rather low speed platform and a high performance clutter cancellation circuit have been successful in reducing the display of radar energy reflected from the sea to a point where the system is effective.

Additional problems, however, arise in the overland environment in that the ground return from an enormous number of stationary reflectors, buildings, cliffs, fences and the like, may completely saturate the equipment and obscure the far smaller return from aerodynamically shaped and often much smaller targets such as aircraft. In the near land environment, the large return from the side lobes of the antenna is often sufficient to produce a "ring around" effect, thereby severely limiting the effectiveness of the system even when the main beam of the antenna is pointed at sea.

The prior art has utilized the concept of doppler frequency shift in the separation of the stationary from the moving targets. In these prior art moving target indicator (MTI) systems, the change in doppler frequency due to the speed of the radar platform, the wavelength of the energy transmitted, and the bearing of the reflector measured from the normal to the velocity vector are calculated or detected. These doppler frequencies are then attenuated by the radar signal processor to remove the return from ground targets in the main beam. It is necessary to continually retune the moving target doppler filters to attenuate the ground return doppler frequency bands associated with the antenna direction as the antenna rotates. This tuning operation is commonly referred to as TACCAR.

Stationary target echoes in the main beam of the antenna system have been detected on the basis of their amplitude relative to aircraft echoes as a 60 to 70 db difference in amplitude is comparatively frequent in many areas. Those echoes of sufficient magnitude to override the main beam clutter attenuation are clearly stationary and they are detected and the doppler filters inhibited at that range to prevent a main beam false alarm. This technique is generally known as main beam blanking.

A greater problem occurs with respect to side lobes. Main beam clutter is lower in power and narrower in doppler spectrum as the width of the main beam is narrowed, and main beam clutter can thereby be reduced. However, a reduction in the width of the main beam produces higher side lobes and thus more side lobe clutter.

The magnitude of the ground return signals from stationary targets is often so large relative to the desired moving target signals that the attenuation of the side lobes is also overcome and false alarms are generated. Once a known stationary target has been detected or identified as having sufficient strength to override the side lobe attenuation, the prior art system doppler filters have been inhibited at the calculated range to the target and the false alarm thus prohibited. Inhibiting the doppler filters at a given range unfortunately also precludes the detection of any target at that range, thus seriously reducing the effectiveness of the system.

The advent of increasingly complex and sophisticated offensive and defensive weapon systems and supersonic jet propelled aircraft has vastly increased the requirements of the radar return signal processors for accumulating the received data and for discriminating between false targets and real targets. In so doing, these radar processors have become extremely complex and sensitive to variations in the return signal strength.

Because of these highly sensitive signal processors which are necessary for reliable target discrimination, the magnitude of the difficulties described above have been significantly increased where the search area of the radar includes strong clutter creating area such as foliage, ocean waves and discrete ground reflectors such as buildings. These clutter producing objects can cause wide variations in the clutter signal strength and may produce target-like signals which exceed the dynamic range of the processor.

Prior efforts to reduce the effects of clutter have been directed primarily in the direction of limiting or clamping of the radar return signal to the dynamic range of the radar processors. This solution, however, is unsatisfactory since the limiting of the signal may also obscure much target information in the radar return signals, particularly where the targets are small and airborne.

The problem is especially acute in an airborne moving target indicator (AMTI) system in which the radar platform is itself an airborne aircraft and the doppler search radar is of the look-down type. Weight and physical size also impose practical limitations on the complexity of any system which must be airborne. The amplitude of the clutter signals in an AMTI system varies widely with range and the foliage backscatter on such a radar return signal has been measured to vary as much as 40 dbs. Discrete objects such as buildings, etc. have been found to produce return signals which vary as much as 70 dbs in excess of adjacent return signals.

An important aspect of AMTI radar systems performance involves their capability of detecting airborne targets against high clutter backgrounds and in severe jamming environments; and this capability is a direct function of their antenna mainlobe-to-sidelobe response ratio. Since targets in the main beam can have extremely small radar cross sections, they must compete with clutter returns both in the main beam and in those side lobes which might occur in the same range cell as the target. With high or medium pulse repetition frequency (PRF) doppler radar mechanizations, close-in, strong sidelobe clutter returns can totally obscure targets at longer ranges because of their inherent range ambiguity.

In low PRF systems, where range is unambiguous, the target in the main beam needs only to compete with sidelobe clutter which occurs at the same range. However, strong fixed sidelobe clutter discrete signals at other ranges are frequently detected and will be treated as bona fide moving targets because of the AMTI/time averaged clutter coherent airborne radar (TACCAR) mechanization typically employed in such radars for minimizing beam clutter spread and thus, they will be tracked until their lack of target validity is established. This is a significant problem in such radars due to these sidelobe discrete signals overloading the track processor.