The U.S. Army is funding Lockheed Martin to develop hardware and software for the Extended Area Protection and Survivability (EAPS) program. Under this program, Lockheed Martin has conducted the first guided test flight of the Miniature Hit-to-Kill (MHTK) interceptor rocket. The MHTK is designed to defeat incoming rocket, artillery, and mortar fire out to ranges of 3 - 4 km (1.9 - 2.5 miles).
More soldiers are killed on the battlefield by mortars than by any other weapon of war. Accordingly, high priority is given to methods of defeating mortar fire. This is one of the purposes of the EAPS program, which is essentially a next-generation miniaturized version of the Israeli Iron Dome missile defense system.
However, rather than removing incoming threats with explosive warheads, the EAPS system uses MHTK interceptors, somewhat like a low-altitude version of the U.S. National Ballistic Missile Defense System. These interceptors are very small and highly agile rockets, only 69 cm (27 in) in length, about 3.8 cm (1.5 in) in diameter (not counting the fins), and weighing about 2.3 kg (5 lb). The MHTK is powered by a Nammo Talley rocket engine.
The MHTK rockets contain a tungsten penetrator and a semi-active radar guidance system that guides the rockets to strike targets which are illuminated by a ground-based radar. While this goal may seem fantastic, it is worth remembering that semi-actively-guided .50 caliber bullets have been developed that home on a laser-illuminated target.
The EAPS test was held March 22 at White Sands Missile Range in New Mexico. The exercise was designed to test the MHTK interceptor in a realistic field scenario, where an enemy mortar is launched at an MHTK-protected area.
When a ground-based radar detects the mortar round, it's tracked as it approaches the protected area. The mortar round is illuminated by a high frequency radar while one or more MHTK interceptors are launched vertically from a NLOS (Non-Line-Of-Sight) launcher on a trajectory from which the interceptor can detect the reflected illumination from the mortar round.
The MHTK interceptor being tested maneuvered to pass close by the target (this was not an interception test), and as it did so, it returned data to the fire control system. In addition to measuring the performance of the interceptor, which performed in accordance with expectations, this was the first time that the entire intercept system was tested as a unified whole. An intercept flight test is planned for later in 2013.
Sources: Lockheed Martin and Aviation Week
Which brings me to Jeffery's point - if the weapon has a cyclic rate of fire of say 6000 rpm and you fire a 3 second burst per mortar round engaged, that's 300 rounds fired, only a few of which may hit. What happens to the other 290 odd rounds? Have you considered the collateral damage to surrounding populations and forces?
Mortar rounds as Dekarate has pointed out are very cheap - as are mortar tubes. This is a very expensive system, BUT hopefully the number of missiles needing to be fired would be less than 5 to 10 (as that would be as many rounds as the mortar team would be able to get away before counter battery fire guided by using Mortar locating radar such as the AN/TPS 49 will quickly wipe out the offending tube and crew. Even insurgents aren't stupid - if none of their rounds get though, and they are wiped out they will be forced to use alternate methods which may be more expensive and or difficult to enable.
Laser/energy based systems are the long term solution, but may be limited for multiple projectiles unless the directed energy is sufficiently high.
For the short to medium term, a battery of mini missiles provide a solution.
However all these systems can't defend against a directional air burst mortar that sends a narrow cone of dense shrapnel towards the intended destination. Someone needs to invent enegy shields. :b
Hitting a 75mm wide target at a range of 500m with a 7.62mm X 51mm (308) round is not difficult. Using a computer controlled, radar guided, machine gun that is in a powered stabilized mount to hit a target on a ballistic target does not seem an insurmountable task. The smallest gun I would use is a 12.7mm (50BMG).
On a conventional battlefield like WWII making the the bullets come down on enemy territory (Avoiding civilian populated areas) or in cleared fallout zones. However bullets do not have to come down with lethal force. Making bullets that will develop on unaerodynamic tumble or placing a tiny explosive charge that will reshape the projectile to generate aerodynamic high drag is easily doable and I see no real difficulty in making bullets that burn to dust before falling back to earth.
But hitting a 82mm target at 500m when it's moving at a high rate of speed is a STUPENDOUSLY difficult task. Recall WW2 where anti-aircraft guns had to shoot *thousands* of rounds in order to hit air planes (obviously *much larger* than mortar bombs).