Semi-active radar

Semi-active radar homing (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external (“offboard”) source — as it reflects off the target(in contrast to active radar homing, which uses an active radar: transceiver). Semi-active missile systems use bistatic continuous-wave radar.

The NATO brevity code for a semi-active radar homing missile launch is Fox One.

The basic concept of SARH is that since almost all detection and tracking systems consist of a radar system, duplicating this hardware on the missile itself is redundant. The weight of a transmitter reduces the range of any flying object, so passive systems have greater reach. In addition, the resolution of a radar is strongly related to the physical size of the antenna, and 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 reflected from the target and point itself in the right direction. Additionally, the missile will listen rearward to the launch platform's transmitted signal as a reference, enabling it to avoid some kinds of radar jamming distractions offered by the target.

The SARH system determines the closing velocity using the flight path geometry shown in Figure 1. The closing velocity is used to set the frequency location for the CW receive signal shown at the bottom of the diagram (spectrum). Antenna offset angle of the missile antenna is set after the target is acquired by the missile seeker using the spectrum location set using closing speed. The missile seeker antenna is a monopulse radar receiver that produces angle error measurements using that fixed position. Flight path is controlled by producing navigation input to the steering system (tail fins or gimbaled rocket) using angle errors produced by the antenna. This steers the body of the missile to hold the target near the centerline of the antenna while the antenna is held in a fixed position. The offset angle geometry is determined by flight dynamics using missile speed, target speed, and separation distance.

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