Semi-active radar homing
From Citizendium, the Citizens' Compendium
Semi-active radar homing (SARH) is a technique for guided missiles, usually surface-to-air (SAM) or air-to-air (AAM). A SARH missile has a passive radar guidance receiver, and expects to have an external source keep an active radar transmitter pointed at the target.
The technique now tends to be used more by SAMs, especially area-coverage naval missiles. For example, the U.S. Navy's RIM-156 Standard SM-2 gets general guidance from the multifunction AN/SPY-1 or AN/SPY-2 radar aboard an AEGIS-equipped air defense ship, but the final target illumination is done by an AN/SPG-62 radar. The latter only need to point at the target for the final seconds before detonation, so the combination of radars can manage a large number of missiles and targets.
SARH, while used on older AAMs such as the AIM-7 Sparrow, is not a popular method for fighter-launched missiles, since the launching aircraft has to keep its nose radar pointed at the target during the full missile trajectory. Staying on such a course makes the launcher increasingly exposed to counterfire from its targets.
Some SARH missiles do have a home-on-jam capability so that they will attack an enemy electronic warfare system, or have infrared guidance for the terminal phase of their trajectory; the detonating fuze, not infrequently, is infrared.