Artillery: U.S. Army Picks A 120mm Winner


April 28, 2010: After years of searching (and procrastinating), the U.S. Army has finally selected a GPS guided 120mm mortar shell. Recent tests of three such systems (two American and one Israeli) saw the U.S. ATK system win the contract. In development since 2006, ATK uses a guidance system that replaces the fuze (which is screwed into the front of the shell) with a larger unit containing the GPS and little wings that move to put the 120mm mortar shell closer to the target. Thus all you need to convert existing 120mm mortar shells to GPS guidance is the ATK fuzes (which handle the usual fuze functions, as in setting off the explosives in the shell, as well as the guidance functions.)

To use the ATK GPS system, you place each fuse into a device that transfers the target GPS coordinates, then screw the fuze into the shell, and fire the shell. It would also be possible to program each fuze once it is screwed into the shell, via a metal probe that would go into a hole in the fuze, transfer the data, and signal that that the transfer was accurately made. The GPS guided fuze will put the shell within six meters (and usually much less) of the coordinates entered.

Because of the GPS fuze, 120mm shells just got a lot cheaper and easier to use. This is particularly crucial for 120mm mortars, which are used by units close to the front lines, where not a lot of ammo can be carried, and resupply is riskier since the enemy is so close. Thus a guided 120mm shell means fewer shells getting fired to get the job done.

It’s about time, because the army has been working on a guided 120mm mortar shell for a long time. Three years ago, the U.S. sent laser guided 120mm mortar rounds to Iraq and Afghanistan for testing. The XM395 Precision Guided Mortar Munition had been in development for twelve years, and was almost cancelled at least once because of the delays. The 38 pound XM395 round has a range of 7.5 kilometers, and will land within a meter (three feet) of where the laser is pointed. This high accuracy is achieved because the XM395 uses laser guidance in addition to GPS. But this was more complex, expensive and difficult to use than the army required. What was needed was a mortar round that just provided consistent GPS accuracy (landing within 10 meters of the aiming point).

Unguided mortar shells cannot put the first round that close, and requires firing several rounds, and adjusting aim, before you get one on the target. A guided mortar round is very useful in urban warfare, where a miss will often kill civilians. The 120mm mortar round has about 2.2 kg (five pounds) of explosives, compared to 6.6 kg (15) pounds in a 155mm shell. The smaller explosive charges limits collateral damage to civilians. The XM395 was tested in Iraq and Afghanistan last year, but since it required someone nearby to use a laser designator, it was considered to have limited usefulness. Thus the push to get a GPS guided shell into service. Normally, an unguided 120mm shell will land anywhere within a 136 meter circle (on the first shot). The laser guided round will land within a one meter circle, and the GPS guided one with a ten meter circle. The GPS round is deemed the most useful, especially since the troops are satisfied with that degree of accuracy in GPS guided 155mm artillery shells, 227mm rockets and JDAM bombs.

Most U.S. infantry battalions are equipped with 120mm mortars. The army would like to get the GPS shell system into service by the end of the year. But the other two GPS shell manufacturers can challenge the results of the competition, and demand another opportunity to win. This can go on for a while.