A heat-seeking missile chases something it can feel: the hot exhaust of an engine. A radar-guided missile chases something it cannot see at all until a radar wave bounces off it. That difference is why radar missiles can kill from beyond visual range, and why for decades they were also the least trusted weapons on a fighter.

There are two basic ways to do it. In semi-active radar homing, the fighter’s own radar lights up the target and the missile simply listens for the echo. In active radar homing, the missile carries a small radar of its own and finishes the job by itself. The AIM-7 Sparrow made the first one famous; the AIM-120 AMRAAM made the second one standard.

Semi-active homing: the fighter does the looking

A semi-active radar-guided missile is, in effect, a very fast passive receiver. Before and after launch, the fighter’s radar keeps the target illuminated, usually with a continuous-wave signal, since most modern fighter radars are pulse-Doppler but have a CW mode for missile guidance. The missile’s seeker in the nose picks up the energy reflected off the target and steers towards it.

A second antenna at the back of the missile listens to the launching fighter directly. Comparing the two signals gives the missile a reference, which helps it reject some jamming and decoys. Modern semi-active missiles can also fly much of the way on inertial guidance and only switch to the seeker near the end, saving energy for the final turn.

Sparrow, Skyflash and Aspide: the age of semi-active radar homing (Millennium 7 HistoryTech).

The weakness is obvious. The fighter has to keep its radar pointed at the target until the missile hits. It cannot turn away, it cannot easily shoot at a second target, and the enemy’s radar warning receiver knows it is being tracked the whole time. In Vietnam, U.S. Air Force and Navy fighters armed with the Fantasma F-4’s AIM-7 Sparrows scored a success rate of barely 10 percent, a figure blamed on unreliable 1960s electronics, tough rules of engagement and crews firing outside the envelope.

Active homing: fire and forget

An active radar-guided missile carries a complete radar transmitter and receiver in its nose. Because a rocket-powered missile has no generator, that radar runs on batteries, so it is small and short-ranged. The trick is to get the missile close enough for it to matter.

So the flight has three phases. After launch, the missile flies on inertial navigation towards where the target is predicted to be. During this midcourse phase, the launching fighter, or another aircraft such as an AWACS, can send updates over a datalink if the target manoeuvres. Close to the target, the missile switches on its own radar. Pilots call the moment it becomes self-sufficient “pitbull”, and from then on the launching jet can turn away.

Active radar homing: the guidance of the AMRAAM, MICA and R-77 (Millennium 7 HistoryTech).

That is what fire-and-forget really means: the fighter can launch, break off and engage someone else, or simply leave. It is also why the brevity call changes. A pilot firing a semi-active missile calls “Fox One”, a heat-seeker is “Fox Two”, and an active radar missile is “Fox Three”. The F-14 Tomcat’s AIM-54 Phoenix was the first operational active-radar air-to-air missile, and Iranian Tomcats used it in combat in the 1980s.

The first AMRAAM kill

The AIM-120 proved itself on 27 December 1992 over southern Iraq. An Iraqi MiG-25 crossed into the no-fly zone of Operation Southern Watch, and Lt. Col. Gary “Nordo” North, flying an F-16D as “Benji 41”, was cleared to fire.

North fired one AIM-120 from about three nautical miles, according to The War Zone. It was the first AMRAAM kill in combat and the first U.S. F-16 air-to-air kill.

Where radar missiles are going

Today the contest is about range and the no-escape zone, the distance inside which a target cannot outrun the missile even if it turns and runs at once. Ramjet-powered missiles such as the European Meteor keep their motor burning for longer to widen that zone, and new U.S. weapons such as the AIM-260 and the Navy’s AIM-174B push range further still. The principle, though, has not changed since the Sparrow: a missile is only as good as the radar picture that sends it on its way.

From failure to redemption: the AIM-7 Sparrow (Australian Military Aviation History).

Sources: Wikipedia (Semi-active radar homing; Active radar homing; AIM-7 Sparrow; AIM-120 AMRAAM), The War Zone, U.S. Air Force