Air to Air Missiles

The operation of conventional air-to-air missile guidance systems used in missiles with infrared seeking sensors or radars is well known. Most of these missiles operate in a "fire and forget mode". At the time of launching of a missile from an aircraft the target is brought into the field of view of the sensor, or radar antenna, of the missile. Once the missile is launched it uses its self-guidance system to track and then home in on the target by maintaining the target at the center of its field of view by continually updating the missile's flight path.

Perhaps the most challenging of all guidance and control problems is that of a tactical air-to-air missile in pursuit of a highly maneuverable target aircraft. The problem presented to this missile may be divided into several parts which include the estimation of target motion, the generation of guidance commands to optimally steer the missile toward target intercept and control of the coupled, nonlinear, multivariable, uncertain dynamics of the air-to-air missile. Each portion of this problem, i.e., estimation, guidance and control, is inherently nonlinear and time varying, and a solution of all three problem parts combines to comprise a complex integrated system.

Combat performance in Southeast Asia continued to indicate a need for an in-depth examination of the entire process by which Air-to-Air missile systems are acquired and employed. Captain Frank W. Ault 7/1310 U.S . Navy NAVAIRSYSCOM Code 001, was been directed to conduct such a review in order to identify any and all areas where improvements can and should be made. The Report noted that " ... history shows that a development program generally costs the Government whatever the costs actually are: if not in dollars (as is usually the case), then in time, or in the quality of the final product.... the Government, in the interplay of profit incentives versus high integrity image, gets usually only what it is able to specify in detail and fund adequately. By and large, industry will produce as 'high' a "high quality product" as is requested and funded..."

"Numerous missiles fired in combat have missed because they were fired out of envelope at law altitude against a relatively small maneuvering target by a U.S. fighter aircraft with a missile control system computer mechanized for a high altitude, non-rnaneuvering bomber. ..... A primary reason for less-than-desired combat performance of air-to-air missile systems in Southeast Asia is their design optimization for a high altitude engagement against a non-maneuvering, large (bomber) target. Consequently, they exhibit important limitations in a low-altitude fighter-to-fighter engagement. "Dogfight" modifications to the SPARROW (AIM-7E2) and SIDEWIIJDER (AIM-9D) and the improved "dogfight" capability of the AIM-7F, will overcome some, but not all, of these and a true "dogfight" missile will require a new development program.

"(2) Because of the complexity of the basic missile design and its close dependence on proper functioning of the aircraft's missile control system, the probability of a successful SPARROW shoot is lower than that for SIDEWINDER. This has been validated in both CONUS and combat firicgs. The inherently greater capabilities of the AIM-7 (viz. allweather, all-aspect, greater range, larger warhead kill-radius) - but significantly poorer (than the AIM-91)) combat performance - merit the highest priority attention to deficiency correction....

" The complexity and unreliability resulting from the adaptation of a rail-launched missile (AIM-7) to an ejection launcher is the primary contributor to the 25% misfire rate experienced in combat firings. A solution for the poor motor fire record of the SPARROW is urgently required.... The AIM-7C and AIM-7D are both inferior performers to their successors and are not desired for combat by either the Navy or the Air Force....

"Despite a renewed emphasis on air combat maneuvering (ACM) training since the commencement of hostilities in Southeast Asia much of this effort has been wasted because it did not stress one of the key elements of the problem: missile envelope recognition / identification at low altitude. Since the missile control system computers are not properly mechanized for a low-altitude maneuvering target, firing envelope recognition is largely by "eyeball and intuition." ACM exercises conducted on an instrumented range at NavMisCen, Pt. Mugu by experienced fighter pilots in VX-4 revealed that about half of the simulated missile shots were being made 'out of envelope.' ACM practice on an instrumented range can materially improve performance....

"The performance of shipboard missile assembly, handling, and loading crews suffers from lack of command emphasis on training as well as dilution of attention and lack of appreciation of the importance of proper missile assembly, loading and handling, caused by concurrent comnitment to other ordnance operations. Missiles treated like bombs frequently perform like bombs."

For the specific example of missile's equipped with infrared seeking sensors the process of updating the missile's flight path is as follows. At the time of launching the sensor is directed substantially towards the target so that an infrared radiating "hot" spot of the target is located at, or near, the center of its field of view. As the target moves away from the center of the field of view of the missile's sensor so that the missile's flight path correspondingly moves off target, the sensor rotates independently of the missile's body to bring the target's infrared radiating hot spot back into the center of its field of view.

A signal representative of the spatial rotation angle through which the sensor rotated during this manoeuvre is transmitted to a control unit which in turn operates the missile's steering system which, by way of a non-limiting example, activates the missile's fins to re-align the missile thereby ensuring that its flight path is again on target. This procedure of rotation of the missile's sensor and re-aligning of the missile, has to be performed continuously, or quasi-continuously, since a missile cannot make sudden changes in direction, i.e., its flight path is always smooth, even though the missile's sensor is fitted on gimbals that allow for fairly large angles of rotation.

Missiles fitted with sensors that are capable of rotating independently of the missile and therefore "seeing" targets that are off boresight are termed "off-boresight missiles". The angle through which the seeker rotates from boresight is termed the "off-boresight angle". For a review of the properties of various high performance short-range off-boresight missiles see: Aviation Week and Space Technology, pp. 36-49, Oct. 16, 1995, the field of view of the sensor is relatively small (about 3°). Hence, in order for the missile not to completely "lose sight" of the target, the updating of the missile's flight path has to be continuously performed. The process involved in updating an air-to-air missile equipped with a radar system is similar, the main difference being that in this case the target is maintained at the center of the field of view of the radar's antenna by maintaining a maximum target echo as received by the radar system.

There are, therefore, clearly a number of serious drawbacks with conventional fire and forget missiles. For a start, the missile has to be in a "seek mode" from the moment of launching. Hence, for such missiles the target cannot be outside the field of view of the missile either at the time of launching of the missile, or at any time after launching. This means that targets outside of the field of view of the missile, e.g., behind or on a side of the aircraft (i.e., at an angle of greater than 90° from boresight), on which the missile is mounted, cannot be acquired by the missile at the time of launching.

Another drawback of such missiles is that should the target escape completely from the missile's field of view after launch, there is no way to set the missile back on a homing flight path toward the target. Another well known drawback is the susceptibility of these missiles to counter measures, which normally take the form of flares for infrared seeking missiles and chaff for missile's equipped with radars. In both of these cases the counter measures act as decoys which, as far as the missiles are concerned, are valid targets.

Although future missile systems are planned to include various Counter Counter Measures (CCM), e.g. CCM's which utilize micro processors for comparing various characteristics of the decoy with those of the target (e.g., for the infra red sensor case these characteristics could be, the spectrum, intensity and velocity of the radiation emitted by a flare and by the exhaust of the target), they would require mounting an appropriate sub-system on a missile. Needles to say, that existing missiles would have to be fitted with such a sub-system in order to enjoy decoy counter counter-measure capability.

To penetrate enemy territory, a bomber must fly at low altitudes and utilize electronic counter measures (ECM) to reduce the effectiveness of radar detection and tracking. There is no active defense other than a radar-directed tail gun system. The effectiveness of the gun has been demonstrated in Southeast Asia, where two interceptors were destroyed without corresponding loss of bombers. Unfortunately, at low altitudes, the gun has questionable fire control radar performance and the bomber is reduced to a passive, undefended target. Once an interceptor succeeds in gaining visual or infrared contact, he may proceed virtually without interference to an optimum weapon launching position.

The most critical attack area for a bomber is the rear quadrant, particularly with infrared guided missiles. The higher closing velocities and line-of-sight rates from the front and side quadrants will significantly reduce missile kill probabilities. Typical launch envelopes for a Mach 0.9 interceptor against a bomber are 2.2 nautical miles (n.m.) over±45° tail aspect angle, or for a more advanced infrared guided weapon 3.0 n.m. over a±60° aximuthal sector.

To offset bomber vulnerability, a number of defense missiles have been proposed. The major problem has been getting these missiles turned into the rear quadrant. They must be either launched forward and turned 180° after launch, or launched to the rear with an awkward stability transition through zero speed. In order to execute these maneuvers and retain rear quadrant range, some of the resultant missiles have been as large as offensive missiles which impacts the bomber's offensive payload. These missiles inherently have greater performance in the front and side quadrants and have been matched with fire control radars to exploit the full missile capability. As a result, the threat detection, fire control, and missile guidance systems have become inordinately complex, sophisticated and costly.