Airborne Early Warning (AEW)

The detection range of search radar is limited by the curvature of the earth, making it difficult for ground based radar to detect very low level flying aircraft. One solution is to carry Airborne Early Warning (AEW) radar systems aboard search aircraft. However, existing AEW surveillance and interceptor radar systems in general have difficulty reliably detecting and therefore tracking low altitude targets at a large enough range to permit effective fire control responses. In spite of such difficulty, detection of these targets improves with better control over antenna characteristics, especially with respect to pointing angles, sidelobe suppression and operating frequencies.

During periods when military budgets and aircraft fleet sizes are shrinking, systems that serve to cost effectively increase the utility of the remaining weapons can still undergo procurement growth. The increased situational awareness and battle field management provided by Airborne Early Warning (AEW) radar is one such force multiplier. The primary role of an AEW aircraft is the long-range detection of airborne targets. As potent new airborne threats, such as low flying cruise missiles, reduce the timelines that traditional air defense systems have to react, the utility of an AEW system's long-range surveillance capabilities to recover the lost time is clear. Fundamentally, these new targets stress the principal performance capabilities of an AEW radar sensor leveling new requirements on these systems to deal with this advanced threat.

These increased requirements have led to world-wide, substantive work in the development of radar upgrades to existing AEW aircraft, such as the U.S. Navy's E-2C Hawkeye and the U.S. Air Force's E-3A AWACS, as well as new systems and platforms, such as the Swedish Air Force's ERIEYE. The required increases in sensitivity, resolution, and the associated data rates that stem from these performance improvements will have profound impact on the way these systems are operated and how they perform in various environments. As these increasingly capable systems evolve, AEW radar will be expected to take on additional missions and perform other surveillance functions in the pursuit of dominant battle field awareness.

In the early 1970's, studies directed by NATO's major military commanders showed that an airborneearly warning (AEW) radar system would significantly enhance the Alliance's air defense capability.In December 1978, the Defense Planing Committee signed a Memorandum of Understanding to buy andoperate a NATO-owned AEW system. With this decision the member nations embarked in NATO's largest commonly funded acquisition program. Various AEW systems were considered before NATO selected the E-3A aircraft. The NATO Airborne Early Warning & Control (NAEW&C) Force was established in January 1980. The NAEW&CF mixed force consists of two operational elements (Components): the NATO E-3A Component at Geilenkirchen, Germany with 17 Boeing NATO E-3A aircraft and a second component, No. 8 (Airborne Early Warning) Squadron of the British Royal Air Force (RAF) at Waddington, United Kingdom, with 7 Boeing E-3D aircraft. The French fly their own AEW fleet of 4 E-3Fs.

Aircraft early warning (AEW) surveillance radar systems use both electronic phased array and mechanically rotating antenna structures. Although phased array radar systems have distinct advantages, aircraft mounted single antenna systems cannot generally search 360.degree. in azimuth. Nor can they reposition themselves quickly so as to cover critical sectors and skip sectors that are not of interest. In the case of mechanically rotating antennas, large dome structures are customary and this produces drag--reducing aircraft performance. Reliability problems become apparent in rotating mechanical antenna structures due to the fact that RF power and analog signals must pass through a rotating joint. An additional problem is the customary fixed dwell time of sector surveillance thereby precluding increased dwell for critical sectors.

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.

Suitable couplings and slip ring assemblies may be provided in order to connect the antenna array contained in the radome to suitable stationary sources of electrical energy while, concurrently, enabling the pick-up of signals received by the antenna array and to transmit the signals to stationary signal processing component and/or display consoles which are located in the cabin of the aircraft. Moreover, a suitable cooling fluid may also be transmitted to the antenna components contained in the radome through the intermediary of the hollow shaft mounting and supporting the radome-antenna installation for rotation. However, the components for supplying electrical energy to the antenna array and picking up the signals derived therefrom, in addition to the heat exchange structure for circulation of a cooling fluid for the rotating components of the radome-antenna structure are normally stationary components mounted in the interior of the aircraft.

Although this is generally adequate and satisfactory for utilization with currently employed low or moderately powered airborne radome-antenna surveillance systems, which generally employ complex rotary couplings to transfer electrical power and/or radio frequency signals between the revolving radome and the stationary equipment contained in the aircraft, the development of much more sophisticated and higher-powered surveillance systems, particularly of the airborne radome type, has rendered the use of such rotary couplings for the transfer of electrical power and signals between rotating and stationary components to be extremely inefficient in view of significant and frequently untenable electrical losses ordinarily encountered with the rotary couplings which are currently designed in such radome installations.

High-powered systems of this type which are presently being contemplated for installation in airborne rotatable radome-antenna structures may necessitate the generating and distribution of electrical power at a level which is a multiple of that in presently utilized systems, and may conceivably incorporate an excess of fifty or even more separately controllable electrical circuits within the rotatable radome. Such electrical circuits must be able to be accommodated in the rotary coupling, and hence signifies a greater potential for encountering electrical losses and signal distortions in the radio frequency signals which are received from the antenna array contained in the radome.