A typical system has two components. First, there are four ultraviolet detection sensors (weighing about four pounds each) mounted on different parts of the helicopter to detect an approaching missile. These sensor are linked to an 6-12 pound computer that contains software for determining that the object is indeed a missile and where it is headed. The detection computer is hooked to a countermeasures system using either flares and chaff (strips of metal foil), or a laser, to confuse the missiles guidance system (that is homing in the heat of the helicopters engines.) The countermeasures component weighs 30-50 pounds, depending on type or model. 

For over a decade there was is a debate going on in the military over whether to equip helicopters with the army developed ATIRCM/CMWS (Advanced Threat Infrared Countermeasures/Common Missile Warning System), that uses flares and chaff, or a system (DIRCM), that uses a laser to confuse the missiles guidance system. The DIRCM originally cost about 40 percent more than the ATIRCM/CMWS (which costs two million dollars per aircraft). But the cost difference has been shrinking. The ATIRCM/CMWS uses a proven technology, while the DIRCM's laser has slowly been gaining experience under combat conditions. The army initially equipped some of its Special Forces helicopters with DIRCM, and now there's a lot of momentum behind only using DIRCM in the future. So far, fewer than twenty American helicopters have been hit by missiles in Iraq and Afghanistan, and many more attempts have been foiled by missile countermeasures.