Semi-active homing guidance
The example shown above illustrates the guidance method used on an
air-to-air missile like Sparrow. This missile relies on radar energy
transmitted by the launch aircraft to track and home in on the target. This
system is also sometimes referred to as bistatic meaning that the radar
waves that intercept the target and those reflected back to the missile are
at different angles to one another.
However, it should be noted that semi-active guidance is used by other
types of seekers besides radar. Laser-guided weapons like the Paveway series
can also be considered semi-active weapons because the laser energy these
bombs track as they steer towards a target is supplied by an external
source. The source could be a laser designation pod on the launch aircraft,
on a second aircraft, or aimed by a soldier on the ground.
Semi-active radar homing
Semi-active radar homing, or SARH, is a
common type of missile guidance system, perhaps the most common type for
longer range air-to-air and ground-to-air missile systems. The name refers
to the missile itself being a passive detector, while an off board radar
provides a signal for the missile guidance system to "listen to" when it
reflects off the target.
The basic concept of SARH is that
almost all detection and tracking systems consist of a radar system In
addition, the resolution of a radar is strongly related to the physical size
of the antenna, in the small nose cone of a missile there isn't enough room
to provide the sort of accuracy needed for guidance. Instead the larger
radar dish on the ground or launch aircraft will provide the needed signal
and tracking logic, and the missile simply has to listen to the signal and
point itself in the right direction.
Contrast this with
beam riding systems,
in which the radar is pointed at the target and the missile keeps itself
centered in the beam by listening to the signal at the rear of the missile
body. In the SARH system the missile listens for the reflected signal at the
nose, and is still responsible for providing some sort of "lead" guidance.
The advantages are twofold. One is that a radar signal is "fan shaped"
growing larger, and therefore less accurate, with distance. This means that
the beam riding system is not terribly accurate at long ranges, while SARH
is largely independent of range and grows more accurate as it approaches the
target -- the "source" of the signal it listens for. Another addition is
that a beam riding system must accurately track the target at high speeds,
typically requiring one radar for tracking and another "tighter" beam for
guidance. The SARH system needs only one radar set to a wider pattern.
SARH systems use continuous-wave radar
for guidance. Even though most modern fighter radars are
sets, most have a CW function to guide radar missiles. A few Soviet
aircraft, such as some versions of the MiG-23 and MiG-27, used an auxiliary
guidance pod or aerial to provide a CW signal.
SARH missiles require the tracking
radar to lock on to the target and then illuminate it for the entire
duration of the missile's flight.
Active Homing Guidance
Active homing works just like semi-active except that the tracking energy
is now both transmitted by and received by the missile itself. No external
source is needed. It is for this reason that active homing missiles are
often called "fire-and-forget" because the launch aircraft does not need to
continue illuminating the target after the missile is launched.