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 offboard 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, so duplicating this hardware on the missile itself is wasted. 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 pulse Doppler 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. This could leave the launch aircraft vulnerable to counterattack, as well giving the target's electronic warning systems time to detect the attack and engage countermeasures. Recent-generation SARH weapons have superior electronic counter-countermeasure (ECCM) capability, but the system still has fundamental limitations. The modern trend is towards missiles such as AIM-120 AMRAAM or MICA, which use an initial lock-on by the firing aircraft to illuminate the target and then steer the missile to the target area by inertial guidance before activating a terminal guidance system (based on active radar or infrared homing) for the final attack. Some of these weapons, like AMRAAM, allow the firing aircraft to update the missile with mid-course updates via datalink, but otherwise allow the weapon to be "fire and forget" (a.k.a. "launch and leave"), allowing the attacker to move on to other targets.

The combat record of SARH missiles has been unimpressive. During the Vietnam War, USAF and US Navy fighters armed with AIM-7 Sparrow attained a success rate of barely 10%. Even though some of the failures were attributable to mechanical failure or pilot error, the intrinsic accuracy of these weapons is relatively low, and they are better suited to interceptor missions than to dogfighting.