What are HARM, the air-to-surface missiles destroying Russian air-defence radar?

America’s supply of the powerful weapons to Ukraine has given its air force a telling advantage

Pentagon official confirmed what had previously only been hinted at: America is supplying Ukraine with High-speed Anti-Radiation Missiles (harm).

The missiles have been playing an important role in Ukraine’s spectacular offensive in Kharkiv province, as well as a separate attack in the south. Launched from an aircraft, harm homes in on and destroys air-defence radar. The missiles represent a serious challenge to Russia. Even if not fired, the threat they pose can force radar operators to turn off their sets and lie low. Ukraine’s air force, as a result, may not quite fly with impunity, but it has greater freedom than before.

During the second world war batteries of large-calibre anti-aircraft guns were deployed to counter bombers, but these became obsolete once jet bombers could fly faster and higher. Missile technology responded, and by the late 1940s the first radar-guided surface-to-air missiles (sam) appeared.

These evolved rapidly and could soon reach stratospheric altitudes and bring down the fastest jet. Bomber crews then had to learn to fly under the radar, or find ways of countering it, such as jamming by electronic warfare.

America pioneered Suppression of Enemy Air Defence tactics, or sead, in the war in Vietnam. “Wild Weasel” aircraft were given the task of destroying radar ahead of a wave of attack aircraft. The Wild Weasels had radar receivers to locate air defences and were initially armed with bombs and later special missiles which could target radar. harm is the latest incarnation of these air-to-surface missiles: a projectile of some 350 kg, with a range of some 90 miles (145km), able to locate and strike radar systems even after they have been turned off.

harm is so effective it is described not as suppression but as Destruction of Enemy Air Defence (dead) and has been used in wars in Libya, Iraq and the former Yugoslavia. Its presence in Ukraine was a surprise, however, because Ukraine’s air force flies Russian-made aircraft which are not compatible with nato weapons.

Physically attaching harm to a Ukrainian aircraft is not straightforward, since they use different systems attached to aircraft wings. It is possible that jury-rigged adapters have been deployed. Photographs allegedly taken in Ukraine show harm missiles attached to both MiG-29 and Su-27S jets.

The lack of a data interface between aircraft and missile presents another potential problem. Usually harm passes information from its own radar-seeker to the pilot, who then fires the missile at a radar system. harm does have a simpler pre-programmed mode in which the co-ordinates of an enemy radar site are entered before take-off. harm rigged this way cannot go after mobile or unexpected threats, only previously located radar.

Ukrainian pilots have employed harm “successfully”, says the Pentagon, which recently approved additional shipments. By now Russian sam radar operators may know to turn on their sets only when they glean from other sources that there is a Ukrainian plane nearby, to avoid being targeted.

Should they leave their radar on, harm will wreak havoc. That allows Ukrainian pilots or drone operators more freedom to carry out sorties with less risk of being shot down. The new American missiles are going some way to eroding the air superiority that Russia has so far enjoyed.