Radar jammer
From Halopedia, the Halo wiki
The Radar Jammer is an equipment that scrambles nearby players' motion sensors when deployed. While it can create a chaos for enemies, it has the same effect on teammates, be careful!
References were found to it in the Halo 3 Beta code[1] and its effect was seen in leaked beta footage. It has now been confirmed in the June issue of GamePro.
