The AGM-84 SLAM (Stand-off Land Attack Missile) is a land-attack derivative of the AGM-84 Harpoon, an anti-shipping missile. While current versions are air-launched, it may be adapted to a shorter-range land attack weapon for submarines, to complement the longer-range Tomahawks, although a submarine version probably would be fire-and-forget, not man-in-the-loop.
Land attack missiles have significantly different radar than anti-shipping missiles, due to the different problem of discriminating ground echoes from wave echoes.
Basic and incrementally improved SLAM
From the first version, it was a chimeric variant of the Harpoon, merging the body and engine of the Harpoon with an infrared seeker from the AGM-65 Maverick series. Man-in-the-loop guidance uses communications of the AGM-62 Walleye, and an AN/AWW-13 control pod on the launching aircraft.
After the first few SLAMs were delivered in 1990 and used in the Gulf War, the missile was changed to carry a WDU-18/B warhead in a new WAU-23/B warhead section. The impact fuze can actuate instantaneously or after a delay.
Work began in 1994 on the significantly improved AGM-84H SLAM-ER. It became operational in 2000. Existing SLAMs can be upgraded to the ER configuration.
Visually, the greatest change over SLAM is again technology, pop-out wings derived from those of the UGM-109 Tomahawk cruise missile. These swept wings add aerodynamic lift for increased range and aerodynamic control for maneuverability.
The H-model's AN/DSQ-61 guidance system, including mid course inertial and GPS guidance, was actually designed for this missile and not inherited from another one. It adds GPS to the man-in-the-loop infrared viewer, with a longer-range data link with better electronic protection. The guidance system also allows changing targets, or holding the current target lock while searching for a better impact point.
In the apparent SLAM tradition, the warhead is derived from that of the UGM-109 Tomahawk Block III. The 500-pound warhead, larger than on SLAM, is now contained in a titanium casing, which both increases penetration and then becomes reactive, the casing contributing to blast and heat effects. 
With deliveries starting in 2002, a further upgrade, Automatic Target Acquisition, has two new guidance capabilities. It offers a "fire and forget" capability where the missile can "remember" a seeker image and not require man-in-the-loop guidance for the entire path, although it can be brought back under human control at any time. It also has a "reattack" capability, in which it can decide it cannot find the target, and go into an orbiting flight pattern to reacquire it. In general, the electronics will be upgraded for electronic protection and increased accuracy. Existing missiles can be upgraded to this standard.
SLAM had been introduced as a means of attacking fixed land targets. On January 15, 2009, the SLAM Expanded Response (SLAM-ER) weapon system scored a direct hit against a remote-controlled, land-based moving target, in a test at the U.S. Naval Air Warfare Center at China Lake, Calif. 
In this test, an aircraft equipped with the Littoral Surveillance Radar System sent real-time targeting data to a F-18 Super Hornet aircraft, which relayed the data to a SLAM Expanded Response launched from a second Super Hornet. The missile hit a simulated mobile target traveling at approximately 12 mph in a desert environment. That environment had considerable background clutter, a classic problem for surface attack sensors. The moving target attack capability is awaiting U.S. Navy approval for procurement.
- Parsch, Andreas, Boeing (McDonnell Douglas) AGM/RGM/UGM-84 Harpoon, DesignationSystems.net
- Boeing, U.S. Navy Conducts Live Warhead Firing of Boeing SLAM ER
- AGM-84 Harpoon/SLAM [Stand-Off Land Attack Missile], Federation of American Scientists
- Boeing Global Strike Systems (February 18, 2009), Boeing SLAM ER Scores Direct Hit in Land-Based Moving Target Test