AN/APG Fire Control Radar
Pilots of US military aircraft carrying semi-active missiles experienced difficulty in launching the missiles against enemy fighters in an air combat maneuvering (ACM) environment. The semi-acitive missile weapon systems currently employed were designed to intercept enemy bombers attempting to destroy or otherwise threaten the U.S. fleet and/or continental defenses. In the mechanization of these systems, the look angle of the search and track fire control radar was established to cover the forward sector of the launching aircraft. In most cases this sector consisted of a cone of microwave illumination .+-.60.degree. around the aircraft datum line. Missiles can be launched against the airborne target whenever the system indicates to the pilot that the target is in range and that the aircraft is headed in the proper lead pursuit course. These launches were made against non-maneuvering targets at ranges of 3 to 15 miles. As the systems were designed to combat non-maneuvering targets, they performed adequately.
In an ACM environment they are operating in air space which is occupied by commercial and friendly military aircraft as well as enemy aircraft. Because of the necessity of positive identification all targets must be visually identified prior to launching a missile. This requirement changes the scenario from a nonmaneuvering airborne intercept (AI) environment to an ACM environment. The usual ACM situation develops when the interceptor is tracking the enemy target at long range up to the point of visual identification at which time or shortly thereafter the target passes to the rear of the interceptor exceeding the gimbal limits of the radar. Immediately, the target turns back 180.degree. toward the interceptor as the interceptor continues on its course. After flying the course for a calculated distance, the interceptor turns back toward the target endeavoring to get into a missile firing position.
Often, the length of time required for the radar system to re-acquire the target, which can only be accomplished when the target has reached a position in space that falls within the gimbal limits of the interceptor radar, precludes a successful missile launch from being executed. As the target and interceptor turn to get into firing position, the interceptor pilot must wait until the target is within the radar gimbal limits. Then the radar must re-acquire the target, settle down to track the target, generate signals relative to closing rates and compute the proper launch point in angle and range. In the classic case there is not enough time as the aircraft approach each other to execute a successful launch of the missile.
Multi-mode radar systems (meaning systems that may perform different functions, either simultaneously or in a rapid sequence) incorporate directional antennas which may be required to scan in many different ways. If such a system is to be airborne, as by a high performance aircraft, the problem of providing a satisfactory scanning technique is particularly difficult to solve. In such an application, the location of a directional antenna is, for aerodynamic reasons, restricted to the interior of a streamlined radome making up the nose section of the aircraft. With a scanning antenna so located, the limit of the scanning field of a mechanically scanned beam is in the order of 60.degree. from the longitudinal centerline of the aircraft. A scanning field of such limited size is too small for many modes of operation. Further, if rapid scanning in azimuth and elevation is required, it is necessary to provide a relatively large, heavy and powerful mechanical scanning mechanism. Such a scanning mechanism, obviously, is detrimental to the optimum capability of the radar and the aircraft.
If a mechanical scanning mechanism is replaced by any known electronic scanner (to permit rapid scanning), other types of problems are encountered. For example, because the width of the beam from a phased array antenna increases with scan angle, antenna gain decreases. Thus, at a scan angle of say 60.degree., the beamwidth doubles as compared to the beamwidth at broadside. Nevertheless, because a beam from a phased array antenna may be scanned so much more quickly than the beam from a mechanically scanned directional antenna, some kind of phased array antenna is required for multi-mode airborne radar.
If a phased array antenna is to be mounted in a streamlined radome in a high performance aircraft, several problems unique to such an installation are encountered. First, it is necessary, to avoid the occurrence of grating lobes within the scanning field, to place the individual antenna elements of a phased array as closely together as possible. Further, the type of feed used to illuminate a phased array is important, it being necessary to use some kind of constrained back feed in order to avoid antenna blockage. Any known "space fed" system must be folded to fit inside the radome, thereby creating subsequent alignment and efficiency problems; and any known "radial feed" prevents optimum disposition of the antenna elements in the array.
The difficulties mentioned hereinbefore are multiplied when operational requirements dictate that the radar in an aircraft combine high power and angular discrimination capabilities. To meet power requirements, a maximum amount of radio frequency energy, (concomitant with a satisfactory beam shape) must be radiated from each one of the antenna elements. To permit such a maximum amount of radio frequency energy to be radiated, it is necessary, in the present state of the art, to cool the antenna elements and associated control circuitry. Such cooling must be as effective at high as at low altitudes, with the result that a positive way of cooling at any operational altitude be provided. To meet both requirements, the radar beam must be narrow and well formed, implying that there be a large number of antenna elements and that the power to each be controllable. To meet angular discrimination requirements for many applications it is highly desirable that the radar be a monopulse radar. Any known constrained feed for a monopulse radar entails the extensive use of waveguide transmission lines and conventional couplers.
Any satisfactory "All Weather Tactical Strike System" (AWTSS) must, in addition to possessing a guidance capability for air-to-ground guided missiles, have a capability of functioning as a navigational aid for the aircraft carrying the AWTSS and as a target acquisition and tracking means. All of the required functions, further, must be carried out in a hostile environment, meaning when the aircraft is maneuvering violently to avoid interdicting fire and when electronic countermeasures are being taken by the enemy.
|Join the GlobalSecurity.org mailing list|