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120mm, Mortar, GPS, Make It Happen
by James Dunnigan
April 8, 2010

After years of searching (and procrastinating), the U.S. Army is finally going to choose a GPS guided 120mm mortar shell from one of three suppliers (two American and one Israeli). All the systems are similar. For example, one of the American systems is the RCGM (Roll-Controlled Guided Mortar). This one works by using a special fuze that includes a GPS unit 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 RCGM is the RCGM fuzes (which handle the usual fuze functions, as in setting off the explosives in the shell, as well as the guidance functions.)

To use RCGM, 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.

In any event, guided 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. And RCGM is not the first attempt to produce a guided 120mm mortar round. There are several, so the army can afford to put the three best candidates through a round of competitive tests.

ItÂ’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. 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.

Every U.S. infantry battalion is equipped with 120mm mortars. The army expects to select a GPS shell system by the end of the year, and quickly get it into service.


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