The Harpoon is an all-weather, over-the-horizon, anti-ship missile, developed and manufactured by McDonnell Douglas (now Boeing Defense, Space & Security). The Standoff Land Attack Missile (SLAM) is a land-attack variant.
The regular Harpoon uses active radar homing and flies just above the water to evade defenses. The missile can be launched from:
Fixed-wing aircraft (the AGM-84, without the solid-fuel rocket booster)
Surface ships (the RGM-84, fitted with a solid-fuel rocket booster that detaches when expended, to allow the missile's main turbojet to maintain flight)
Submarines (the UGM-84, fitted with a solid-fuel rocket booster and encapsulated in a container to enable submerged launch through a torpedo tube);
Coastal defense batteries, from which it would be fired with a solid-fuel rocket booster.
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