Fighter Combat - Tactics and Maneuvering

38 FIGHTER WEAPONS
target to correct. After motor burnout the missile begins to decelerate and
once again must increase its lead to complete the intercept. The ideal
proportional-navigation course (or lead-collision course) for a constantspeed
missile with about the same average speed is shown for reference.
For a constant-speed missile, a proportional-navigation course is identical
to a lead-collision trajectory against a nonmaneuvering target.
Proportional navigation assumes that the target is traveling in a straight
line at any moment; should the target be maneuvering, constant leadangle
changes are required. The "perfect" lead-collision course is a straight
path to the intercept point, but since the target is generally free to change
its maneuver during the missile's flight, this intercept point is difficult to
predict. It is usually not greatly advantageous for the missile to attempt to
predict 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 a
cooperative target that continues to emit the energy required for homing.
Semi-active homing avoids this problem by having the missile home on
reflected energy that is provided by another source, often the launch
platform. The missile can derive LOS information from the reflected
beam; or, by comparing the characteristics of the reflected beam with
those of the same beam received directly from the guidance platform, it
may also be able to compute target range, closure, and maneuver, for
guidance and warhead-fuzing purposes. Although this guidance system
provides capability against uncooperative targets, one of its major problems
is greatly increased complexity, and added complexity usually results
in reduced reliability. Essentially this technique requires two separate,
properly operating tracking systems (one in the missile, the other in the
launch platform) to be successful. Another serious drawback is the requirement
for target illumination by the guidance platform throughout the
missile TOP. This requirement makes the illuminator vulnerable to passive-homing
weapons, and with airborne illuminators it often restricts the
maneuvering options of the aircraft providing target illumination. As is
explained later, predictable flight paths greatly increase vulnerability in air
combat situations, and destruction of the illuminator effectively terminates
its missile threat.
An active-homing system provides the source of illuminating energy in
the missile itself. Although this method requires a more complex, a larger,
and a more expensive missile, the total guidance system is no more
involved than the semi-active system, and in some ways it is simpler and
more reliable. It also gives the launching platform "launch-and-leave"
capability, as do passive systems. One disadvantage, however, is the possibility
of reduced target detection and tracking ranges. The maximum range
of radar systems using a given power and level of technology is proportional
to the area of the antenna. Since the missile is usually limited by
size to carrying a smaller antenna than a launching aircraft or groundbased
system can support, semi-active horning may provide greater maximum
launch ranges than active homing.