Anti-shipping missiles are a subset of guided missile, which may be surface-to-surface missiles (either land- or ship-launched), air-to-surface missiles, and underwater-to-surface missiles. For this article, they can be assumed to be attacking a ship target beyond visual range of the launching platform, which requires them to have a target sensor that can pick out a ship against the interference of moving water.
By this definition, the German Fritz-X was not technically an anti-shipping missile, because the operator, aboard a bomber aircraft, visually tracked both the missile and the target. From the perspective of the launching platform, however, manned kamikaze aircraft were anti-shipping missiles, because the final selection of a target and intercepting it depended on the senses of the suicide pilot, not sensors on the launching aircraft or airfield.
There has been a trend to make anti-shipping missiles fly at very low, "sea-skimming" altitudes. While that gives the target a harder defensive problem, since the missile may not be detected until it is very close, the missile has the reverse problem: it is harder to detect the target from low altitude. Methods to help the missile find the target can include midcourse guidance corrections from a ship-launched helicopter, long-range reconnaissance aircraft, or a submerged submarine near the target.
While early and effective anti-shipping missiles were subsonic, the latest families of sea-skimming missiles are very fast, again to deprive the target of defensive time. Close-in defensive systems are migrating from autocannon to missiles, in the hope of intercepting farther away, with more time for additional shots if the first misses. In many air defense applications, not just against anti-shipping missiles, the defender fires the surface-to-air missiles in pairs, to increase the probability of a hit.
Guidance challenges and techniques
While anti-shipping missiles are launched by surface, subsurface, or air platforms, their design requirements differ from anti-surface missiles intended for use over land. Their guidance systems must cope with the constantly changing radar return from the ocean, yet be able to recognize a ship surrounded by radar "clutter". It is quite common to have different guidance systems for steering to the general area of the target, and then for terminal guidance to attack the target.
Many anti-shipping missiles fly at extremely low altitude, and are called "sea-skimming". Just as the ocean interferes with the missile's own guidance system, it also interferes with the enemy's warning and fire control radar. A sea-skimming missile will also enter the enemy radar field of view only when close to the target, when there is minimal time to engage it.
Several common missiles use intertial or GPS guidance for navigating from the launcher to the target area. They may combine guidance in azimuth with a radar altimeter for staying just above the water. The Exocet, for example, launches with the combination of inertial navigation for bearing and a radar altimeter for altitude.
The U.S. AGM-84 Harpoon, however, has two modes available: bearing-only launch (BOL), where the radar seeker guides the entire flight, and an initial inertial mode. The inertial mode can be programmed to fly a "zigzag" or "dogleg" course, so several Harpoons can be sent at a target, and complicate its defense problem by simultaneous attacks from different directions.
Inertial launch, whether straight or complex, gives no indication of the launching platform's position, which is revealed with all-the-way radar.
The West refers to several different Russian missiles, of the Moskit series, by the NATO designation SS-N-22 SUNBURN. They have been informally called the "Harpoonski," but have superior performance to the U.S. Harpoon.
|Moskit P270, 3M80||Ship-launched||Mach 2+|
|Moskit 9M80E||Ship-launched,||Mach 2+ extended range|
|Moskit 3M82||Ship-launched||Mach 2+ to 3|
|Moskit ASM-MMS, Kh-4||Air-launched||Mach 2+|
- Sea Eagle
The U.S. makes one dedicated anti-shipping missile, the Harpoon, which has different designations depending on its launch mode; AGM/RGM/UGM-84 Harpoon which are, respectively, air-launched, ship-launched and submarine-launched. Ship-launched Harpoon became operational first, in 1977, so the article name is RGM-84 Harpoon. Air-launched and submarine-launched versions came in 1979 and 1981.  The Harpoon, in turn, has a land-attack variant, the AGM-84 SLAM series of stand-off Land Attack missiles.
In addition, however, U.S. Navy SH-60 Seahawk helicopters carry the AGM-119 Penguin air-to-surface missile, made by the Norwegian firm Kongsberg and used by NATO nations in air, ship, and land-launched versions. The series of Standard SM-2 missiles have a secondary anti-shipping capability, either in direct fire or indirect fire.