The AGM-88 High-Speed Anti-Radiation Missile (HARM) is an air-to-surface tactical missile designed to seek out and destroy enemy radar-equipped air defense systems. The AGM-88 can detect, attack and destroy a target with minimum aircrew input. The proportional guidance system that homes in on enemy radar emissions has a fixed antenna and seeker head in the missile's nose.
The HARM can be used in three different operational modes, known as Pre-Briefed (PB), Target Of Opportunity (TOO), and Self-Protect (SP).
Pre-Briefed: the long range of the AGM-88 is used to launch the missile on a lofted trajectory toward a known threat. When the HARM reaches lock-on range, and detects the radar emission, it can home on the target. If the target radar has been switched off before any lock could be acquired, the missile destroys itself to avoid possible friendly casualties by the impact of the now unguided missile.
Self-Protect: the aircraft's radar warning receiver is used to detect enemy emissions. The CP-1001B/AWG HARM Command Launch Computer (CLC) then decides which target to attack, transmits the data to the missile, and launches the AGM-88.
Target Of Opportunity: it means that the seeker of the AGM-88 itself has detected a target, and the missile can be fired manually if the radar emission is identified as a threat.
In SP and TOO modes, the AGM-88 can even be fired at targets behind the launching aicraft, although this of course significantly reduces the missile's range. The AGM-88 missile has an inbuilt inertial system, so that whenever it has acquired a lock once, it will continue towards the target even if the emitter is shut down (although the CEP is larger in this case).
The AGM-88A is the baseline version of the HARM. The initial version is classed as Block I. The Block II upgrade had a new seeker with software in an EEPROM, which could be reprogrammed for new types of threats at short notice.
This variant had the Block II seeker from the beginning, but had improved computer hardware in its WGU-2B/B guidance section, compatible with the forthcoming Block III software. This Block III update improved the in-flight reprogramming (a.k.a. flexing) capabilities of the AGM-88B, as well as the PB mode targeting capabilities. Block III update required fully powering up the missile, the U.S. Navy decided to retain its Block II missiles on aircraft carriers for safety reasons (powering up live missiles in the shops below deck was considered too risky).
The next upgrade of the HARM is designated AGM-88C. The major hardware improvement was a new WDU-37/B warhead with 12800 tungsten alloy fragments and a revised explosive charge, which significantly enhanced the lethality of the missile. The AGM-88C was initially produced with Block IV software in the upgraded WGU-2C/B guidance section. The WGU-2C/B used a single antenna instead of the previous two, and has a much more powerful signal processor. Block IV software was updated to counter the latest threats, and increased TOO mode capability by doubling the seeker range sensitivity. The Block V software update introduced home-on-jam capability, including the option to home on jammers which try to disrupt the ever more important GPS navigation system (used by many of the latest guided weapons).
AGM-88Cs upgraded to Block VI standard are known as AGM-88D. This upgrade is an international collaboration by the U.S. (Raytheon), Germany (BGT), and Italy (Alenia). The main improvement of HARM Block VI is the incorporation of a GPS navigation system. This greatly increases accuracy when radar lock is lost after emitter switch-off, because the GPS guidance keeps the missile within a narrow box towards the last known emitter position.
The AGM-88E, also known as Advanced Anti-Radiation Guided Missile (AARGM) is a further improved Block VI missile, which uses not only the AGM-88D's GPS but also a MMW (Millimeter Wave) active radar seeker for terminal homing in its new WGU-48/B guidance section. The MMW seeker employs active target recognition algorithms, and therefore be able to strike not only the radar emitter, but also e.g. the control vehicle of the site.