Fighter Combat - Tactics and Maneuvering

FIGHTER WEAPONS 43
Of course, this technique also eliminates any returns from real targets
having about the same closure, which includes those with beam aspects
(approximately 90° TAA). MBC is less of a problem with high-altitude
targets or when the radar is looking up at the target. By not blanking out
the MBC, radar missiles may retain a capability under such conditions
against targets with beam aspects.
Because Doppler radars only detect relative motion, targets flying in
nearly the same direction at about the same speed as the host aircraft may
not be detected either. Since side-lobe clutter (SLC) is associated with
closing speeds equal to or less than the host aircraft's own airspeed, it too
may be eliminated. But because this procedure would limit detectable
targets to those with forward aspects, and SLC is usually fairly weak, this is
generally not done. Doppler SLC does, however, limit detection ranges
when the host aircraft is in the target's rear hemisphere. The amount of
this degradation is largely dependent on the host aircraft's altitude.
Doppler's great advantage is in detecting targets with high closure
(forward aspects), in which case clutter is not a problem even when the
radar is looking down. This leads to radars with so-called "look-down"
capability. A missile directed by such a system is said to have "shootdown"
capability. A given Doppler radar is limited, however, in the band of
return frequencies it can detect. It is theoretically possible, therefore, for a
target to be closing or opening too fast to be detected.
Besides detection problems, various types of missile seekers have other
limitations. Most missiles that employ proportional-navigation techniques
require a movable seeker to keep track of the target. Such seekers
have physical stops in all directions, called gimbal limits, which restrict
their field of view and therefore limit the amount of lead the missile may
develop while the seeker points at the target. If the seeker bumps the
gimbal limit, the missile usually loses its guidance capability. Such situations
most often develop when the missile's speed advantage over the
target is low and the target LOS rate is high. This may occur early in the
missile's flight, before it has accelerated fully, with a high target LOS rate.
It also becomes a problem near maximum range, when the missile has
decelerated greatly and must pull more and more lead to maintain a
stationary target LOS.
Although the gimbal limit may be bumped in a hard-turning intercept
with a maneuvering target when the missile's turn capability cannot quite
stop the target LOS drift, this situation more often leads to exceeding the
seeker's tracking-rate limit. Missile seekers are usually gyro-stabilized to
point along a fixed line in space, much like the needle of a magnetic
compass. The body of the missile is then free to turn about the "fixed"
seeker. Such motion causes little problem and generally is limited only by
the missile's turn capability and the seeker's gimbal limits. If the seeker's
LOS must be changed, however, because of changing target LOS, its gyro
must be precessed. The rate at which this can be accomplished (known as
the target's maximum gyro tracking rate) is limited, and it is often dependent
on the target's signal-to-noise ratio.