Fighter Combat - Tactics and Maneuvering

FIGHTER WEAPONS 37
home on reciprocating engine exhaust. Because modern aircraft can travel
at speeds comparable to or even faster than the speed of sound, acoustic
homing tends to result in inefficient lag-pursuit trajectories and is seldom
used. This method may, however, be very effective against slow, noisy
aircraft such as helicopters.
Passive homing systems are often designed to follow pure-pursuit
trajectories, since target LOS is usually the only input to the guidance
system. It is possible, however, with only this information, to compute a
lead-collision course by a process known as "proportional navigation."
This involves turning the missile until a heading is found which stops the
target's apparent LOS drift rate. By maintaining this constant lead angle,
the missile will theoretically fly a straight path to intercept a nonmaneuvering
target. In actuality the lead required to stop the LOS drift rate
depends on target speed and aspect, as well as missile speed (note: no range
dependence). For a nonmaneuvering target (constant speed and TAA), the
lead required for a proportional-navigation course varies with missile
speed. Figure 1-8 illustrates the resultant flight path of a boost-glide missile
initially launched directly at the target. Immediately at launch the
missile senses the target drifting to the right of its nose and turns right to
stop the LOS rate (apparent target drift across the horizon) by time "2,"
establishing an intercept course. At this point the missile is still accelerating
and its speed advantage over the target is small, requiring a rather large
lead angle. As the missile's speed continues to increase, however, it requires
less lead to maintain the constant LOS, and it turns back toward the
INITIAL.
LOS
STABILIZED
LOS
TARGET
LEAD-COLLISION
COURSE
TARGET
TRACK
Figure 1-8. Missile Velocity Effects on Lead-Collision Trajectory
MOTOR
BURNOUT
PROPORTIONAL-
NAVIGATION COURSE