Here's What You Need to Remember: An active radar homing missile differs from a semi-active radar homing missile in that the missile’s seeker has its own radar transmitter to accompany a receiver. Semi-active missiles only have a receiver and require the launching aircraft to use its own radar to “designate” an enemy aircraft for the missile to engage.

At the end of the Cold War, there were major revolutions in the technology used for aerial combat. The most visible one was stealth, with the iconic diamond shaped F-117 and B-2 stealth bombers and the F-22 stealth fighter. But a second revolution was underway, with major implications for the USAF and other air forces. The active radar homing (ARH) air-to-air missile began to replace semi-active radar homing (SARH) air-to-air missiles as the primary medium and long-range missiles that armed every fighter.

But how exactly did this revolution occur? Why were the 1980s a big turning point? What’s the future of active radar homing missiles?

An active radar homing missile differs from a semi-active radar homing missile in that the missile’s seeker has its own radar transmitter to accompany a receiver. Semi-active missiles only have a receiver and require the launching aircraft to use its own radar to “designate” an enemy aircraft for the missile to engage.

This burdens SARH with a lot of limitations. The launching aircraft must provide a radar signal, so it must keep its radar pointed at the enemy aircraft. While radars have a limited range of motion for search, generally an aircraft must keep flying at an enemy aircraft to guide a SARH missile.

It also means that if an aircraft is destroyed or the launching aircraft is forced to maneuver by other missiles or aircraft, then the missile will break lock and go “dumb”. The range of SARH missiles also can be limited by the range of the launching aircraft: if the radar is too weak or if the enemy aircraft is too far out, a SARH seeker may not be able to attain the initial lock before launch. Later SARH missiles have lock-on after launch (LOAL) capability, that mitigates but doesn’t eliminate this limitation.

Active radar homing missiles solve almost all these issues. The aircraft can be free to maneuver after launching a missile at enemy aircraft, as the missile will fly until it goes “pitbull,” activating its seeker and homing in on the aircraft by itself. Missiles can also be fired in “maddog” mode, with the seeker locking onto the first radar contact it reaches. These tactics can allow for fighters to “shoot and shoot” effectively, allowing a fast enough aircraft to avoid an engagement entirely after launching its missiles.

While they only recently became widespread, the concept has been around for a long time. The first attempt came in the 1950s when the United States attempted to develop a variant of the AIM-7 Sparrow SARH missile called the Sparrow II that would have an ARH seeker. This failed, due to the inability of the technology at the time to produce a radar seeker that could have both a transmitter and receiver that could fit into the diameter of the medium-range and weight Sparrow missile.

The first real operational ARH missile was the AIM-54 Phoenix. The “long arm” of the U.S. Navy’s fighter fleet, the AIM-54 needed to have active radar homing due to its role as a super-long range (over 190 kilometers) air-to-air missile. The AIM-54 was meant to defeat Soviet bomber fleets before they could launch their anti-ship cruise missiles at the Navy’s ships.

As a result, it needed an ARH seeker to get good accuracy against its targets at the terminal phase (within the last nineteen kilometers of the missile’s flight) when it was closest to the enemy aircraft, although SARH was still used to provide midcourse guidance to the missile for the long shots. For shorter range shots, the AIM-54 could be launched with ARH active from the start, allowing the aircraft to maneuver immediately after firing the missile, with the AIM-54 guiding itself.

The missile was well suited for an ARH seeker, having a large diameter in order to accommodate the massive rocket motor needed for the long range, and heavy warhead needed to kill Soviet bombers.

However, due to the Cold War staying cold, the AIM-54 never was able to fulfill its proper role in combat. A few were fired during Operation Southern Watch but missed their targets due to the age of the missiles and their use against tactical fighter targets, not the bombers they were originally intended to destroy.

AIM-54s were also provided with the F-14As to Iran prior to the Islamic Revolution in 1979. Accounts of Iranian F-14 usage suggests that the AIM-54s were rather effective, downing Iraqi bombers and even some fighters at extended range.

Despite the successes of the AIM-54, it was still a big heavy missile designed for use against relatively non-maneuvering targets. The first real successful ARH missile would come almost fifteen years later in the form of the AIM-120 Advanced Medium Range Air to Air Missile (AMRAAM).

The AIM-120 was created to replace the AIM-7 Sparrow with a newer, higher performance missile. As electronics technology had advanced, an ARH seeker that could fit into a smaller missile was now possible. But even with new electronics, the development of such an advanced missile faced protracted problems: The missile was in pre-production in 1984 but only reached low-rate production in 1988 and initial operational capability in 1991.

The AMRAAM finally had its combat debut during Operation Southern Watch, downing an Iraqi MiG-25 in 1992. Since then, it has gone on to serve admirably over former Yugoslavia, Iraq and Syria (in 2017). Notably, in the Syrian engagement, the ARH using radar homing was able to secure the kill when the AIM-9X infrared-guided missile was unable to successfully track the target.

The missile’s compact seeker has also been adapted to land and naval usage, arming the Norwegian/American NASAMS Surface to Air missile and the naval SM-6 multirole missile, all of which take advantage of the accurate terminal guidance of the ARH seeker.

What’s next for AMRAAM and ARH? Recent updates to the AMRAAM have given it enhanced datalink capabilities, allowing more aircraft to send data to the seeker and for the seeker to give what it sees to its launching platform and other aircraft.

The radars in ARH missiles have also seen upgrades. The Japanese AAM-4B ARH missile is the first ARH missile in the world with an AESA seeker, which can increase the slew rate of the radar, allowing it to potentially be more powerful and track targets faster.

Another innovation that’s making ARH missiles more deadly is the use of ramjets to provide continuous thrust during a missile’s flight. One of the main advantages of ARH is how the missile’s lock tends to get more accurate as the missile gets closer to the target. Conversely, with traditional rocket engines, the missile tends to have the least energy for maneuvering when it is closest to the target. Pairing ARH with ramjet engines, as MBDA has done in the Meteor missile, could potentially be a lethal combination as it can take advantage of both technologies.

Charlie Gao studied political and computer science at Grinnell College and is a frequent commentator on defense and national-security issues. This article first appeared in 2018.

Image: Reuters