Fighter Combat

38 FIGHTER WEAPONStarget to correct. After motor burnout the missile begins to decelerate andonce again must increase its lead to complete the intercept. The idealproportional-navigation course (or lead-collision course) for a constantspeedmissile with about the same average speed is shown for reference.For a constant-speed missile, a proportional-navigation course is identicalto a lead-collision trajectory against a nonmaneuvering target.Proportional navigation assumes that the target is traveling in a straightline at any moment; should the target be maneuvering, constant leadanglechanges are required. The "perfect" lead-collision course is a straightpath to the intercept point, but since the target is generally free to changeits maneuver during the missile's flight, this intercept point is difficult topredict. It is usually not greatly advantageous for the missile to attempt topredict the impact point for maneuvering targets, so many "lead-collision"guidance systems actually use proportional-navigation principles.One of the major drawbacks of passive homing is its dependence on acooperative target that continues to emit the energy required for homing.Semi-active homing avoids this problem by having the missile home onreflected energy that is provided by another source, often the launchplatform. The missile can derive LOS information from the reflectedbeam; or, by comparing the characteristics of the reflected beam withthose of the same beam received directly from the guidance platform, itmay also be able to compute target range, closure, and maneuver, forguidance and warhead-fuzing purposes. Although this guidance systemprovides capability against uncooperative targets, one of its major problemsis greatly increased complexity, and added complexity usually resultsin reduced reliability. Essentially this technique requires two separate,properly operating tracking systems (one in the missile, the other in thelaunch platform) to be successful. Another serious drawback is the requirementfor target illumination by the guidance platform throughout themissile TOP. This requirement makes the illuminator vulnerable to passive-homingweapons, and with airborne illuminators it often restricts themaneuvering options of the aircraft providing target illumination. As isexplained later, predictable flight paths greatly increase vulnerability in aircombat situations, and destruction of the illuminator effectively terminatesits missile threat.An active-homing system provides the source of illuminating energy inthe missile itself. Although this method requires a more complex, a larger,and a more expensive missile, the total guidance system is no moreinvolved than the semi-active system, and in some ways it is simpler andmore reliable. It also gives the launching platform "launch-and-leave"capability, as do passive systems. One disadvantage, however, is the possibilityof reduced target detection and tracking ranges. The maximum rangeof radar systems using a given power and level of technology is proportionalto the area of the antenna. Since the missile is usually limited bysize to carrying a smaller antenna than a launching aircraft or groundbasedsystem can support, semi-active horning may provide greater maximumlaunch ranges than active homing.