Semiactive Homing

target (fig. 1-21, view A). The RF energy strikes the target and is reflected back to the missile. (This process is referred to as “illuminating the target.”) The missile seeker (receiving) antenna detects the reflected energy and provides it as an input to the missile guidance system. The guidance system processes the input, usually called the homing error signal, and develops target tracking and missile control information. Missile control causes the missile to fly a desired flight path. The effective range of the missile transmitter is somewhat limited because of its size (power output). For this reason, relatively long-range missiles, such as HARPOON, do not switch to active guidance until after midcourse guidance has positioned the missile so that the transmitter is within its effective range. Semiactive Homing.—In a semiactive homing system, the target is illuminated by a transmitter (an illuminator) on the launching site (fig. 1-21, view B). As with active homing, the transmitted RF is reflected by the target and picked up by the missile’s receiver. The fact that the transmitter’s size is not limited, as with active homing, allows a much greater range. The missile, throughout its flight, is between the target and the radar that illuminates the target. It will receive radiation from the launching ship, as well as reflections from the target. The missile must therefore have some means of distinguishing between the two signals, so that it can home on the target rather than on the launching ship. This can be done in several ways. For example, a highly directional antenna may be mounted in the nose of the missile; or the Doppler principle may be used to distinguish between the transmitter signal and the target echoes. Since the missile is receding from the transmitter and approaching the target, the echo signals will be of a higher frequency. Most SMS missiles use both of these methods. A drawback of this system is that the shipboard illumination is not free to engage another target while the missile is in flight. STANDARD SM-1 and SEA- SPARROW all use semi-active homing as their primary guidance; they do not use midcourse guidance. The STANDARD SM-2 uses midcourse guidance, and then semi-active homing only for terminal guidance. As a result, the SM-2 needs illumination from the ship only for the last few seconds of flight. 1-17 A. B. Figure 1-20.—Simplified command guidance systems: A. Radar/radio command; B. Beam rider.