How the U.S. Military’s New Ship-Killing Missile Turns Targets' Radar Against Them

[F-22Raptor]

The Pentagon’s newest anti-ship missile uses technology from the B-2 stealth bomber to home in on and sink enemy ships. The Long Range Anti-Ship Missile (LRASM), the U.S. Military’s first new anti-ship missile in more than thirty years, homes in on the very radar signals an enemy ship broadcasts to defend itself. The result is a missile that can work in so-called “denied” environments when navigational assets such as the Global Positioning System are unavailable.

The Navy’s main anti-ship missile, the Harpoon, first entered service in 1977. Called an "anti-ship cruise missile," the Harpoon launched from a ship, aircraft, or submarine, and uses an active radar system to detect enemy ships and home in on them. Although progressively updated over the years, Harpoon has grown pretty long in the tooth and is due for a replacement.

The active radar guidance system on many older anti-ship missiles has always been problematic. Anti-ship missiles are launched in the enemy’s direction and fly close to the surface to the water—in the case of Harpoon just thirty feet above the wavetops—in order to stay off the enemy’s radar screens for as long as possible.

The problem is, a lot can change between when the missile is launched and the missile arrives in the target zone. Target ships can be miles away from their original location by then, creating what is called an “area of uncertainty”—a circle on the map where the target is, its exact location unknown. This can result in a clean miss, or worse, hitting the wrong target. In 1987, an Iraqi Mirage fighter jet launched two Exocet anti-ship missiles into the busy Persian Gulf, hoping to strike Iranian oil tankers. Instead of an Iranian tanker, the French-made missiles found the guided missile frigate USS Stark, killing 37 U.S. Navy sailors.

LRASM, as the missile’s project manager Joe Mancini explained to Popular Mechanics, does things differently. Instead of using an active radar system to locate its targets, LRASM uses a passive sensor to sniff out and home in. LRASM doesn’t emit any electromagnetic signals that would allow the enemy to detect it ahead of time. Instead, the missile turns the tables by homing in on enemy shipboard radars.

Imagine two people playing tag in a dark room where the person who is 'it' is empty-handed, but his opponent has a flashlight. In order to see the person who is 'it,' the defender has to turn on his flashlight. After all, he can’t defend against what he can’t see. The trick is that when the flashlight is on, 'it' has a guiding light to aim for.

This puts the enemy commander in an impossible position: As long as he wants to defend himself, he must keep his radars on and in doing so, feed LRASM the data to find him. This will even work in poor weather and against ships with stealthy anti-radar shaping, as the missile doesn’t rely on radar to find its target.

LRASM’s guidance system is based upon so-called electronics support measures (ESM) equipment originally made for the F-22 Raptor, F-35 Joint Strike Fighter, and B-2 Spirit bomber, and the equipment went through an extensive miniaturization effort to fit it into a guided missile. ESM collects and analyzes enemy electromagnetic signals, such as radar broadcasts, to identify and help counter them. LRASM uses this equipment to sort out all of the electromagnetic noise, identify the unique radar signals of its target, and home in for the kill.

LRASM’s wide angle “floodlight” antenna can scan a wider arc in front of the missile for enemy radar emissions than radar can, reducing the area of uncertainty problem. The missile is smart enough, via artificial intelligence, to detect new threats in its flight path, classify them, and fly around them. Once it gets close enough to, say, an enemy task force center around an aircraft carrier, it can compare the ships in front of it to an onboard library of enemy ships, making sure it strikes an aircraft carrier and not a frigate. It can even strike a certain part of a ship, such as the aircraft carrier’s island or a destroyer’s missile magazine, to do maximum damage.

In addition to its guidance capability, LRASM has a range in excess of 200 miles, or three times as long as the older Harpoon missile. It’s also built with low-observable technologies, making itself harder to detect via radar. The 1,000 blast fragmentation warhead is twice the size of Harpoon’s, and a single missile would likely cripple a 9,000-ton destroyer-sized ship.

It’s taken the Navy decades to finally pick a new anti-ship missile, but LRASM’s unique guidance system should prove resilient against modern anti-ship missile defenses, almost all of which rely on radar to be effective. In July the Pentagon announced it was buying 23 missiles for use on Navy and Air Force aircraft, and hundreds if not thousands will likely follow.

http://www.popularmechanics.com/mil...anti-ship-missile-targeting-system-explained/