Semi-active radar homing
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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. Vympel R-33 AA missile for MiG-31 interceptor uses SARH as the main type of guidance (with supplement of inertial guidance on initial stage).
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. Because most SARH missiles require guidance during their entire flight, only one target per radar emitter can be engaged at a time. The F-14 Tomcat with the AWG-9/AIM-54 Phoenix was the only US fighter until AMRAAM that could fire up to 6 missles simultaneously against 6 different targets, and then turn for home as soon as the missles were away, with all radar missle capable US fighters standardized on the Sparrow medium range missle.
Recent-generation SARH weapons have superior electronic counter-countermeasure (ECCM) capability, but the system still has fundamental limitations. Some newer missiles, such as the Standard SM-2 incorporate terminal semi-active radar homing (TSARH). TSARH missiles use inertial guidance for most of their flight, only activating their SARH system for the final attack. This can keep the target from realising it is under attack until shortly before the missile strikes. Since the missile only requires guidance during the terminal phase, each radar emitter can be used to engage more targets. Some of these weapons, like the SM-2, allow the firing aircraft to update the missile with mid-course updates via datalink.
The combat record of SARH missiles was unimpressive during the Vietnam War. USAF and US Navy fighters armed with AIM-7 Sparrow attained a success rate of barely 10%, which tended to amplify the effect of deleting the gun on most F-4 Phantoms, which carried 4 Sparrows. Some of the failures were attributable to mechanical failure of 1960s era electronics which could be disturbed by pulling a cart over uneven pavement, or pilot error, the intrinsic accuracy of these weapons was relatively low relative to Sidewinder and guns. However, since Desert Storm, most F-15 Eagle combat victories have been scored with the Sparrow at BVR ranges, including one fast, high flying MiG-25 Foxbat by an Israeli plane.
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