The U.S. Air Force is planning to install the microwave ADS, or Active Denial System (which transmits a searchlight sized beam of energy that makes people downrange feel like their skin is on fire) on several of its sixteen new AC-130J gunships. This would provide air force gunships with a non-lethal weapon for the first time. This may not work. That’s because ADS has been ready since 2007 but has never been used. In 2012 a possible reason for this was revealed. It seems that ADS performs poorly in the rain (or fog, mist, or snow). This is a common problem with microwaves and lasers, which are broken up by all these forms of precipitation. Tests of ADS during rainy conditions found that the pain turned to a warm comfortable feeling (especially in the cold and damp). Moreover, the microwaves only work on exposed skin, which means during cold or inclement weather (when people cover up) ADS is less likely to have much impact. Apparently it was believed that it would not take the locals long to figure all this out and improvise effective countermeasures. Thus this aspect of ADS, long suspected by scientists and engineers, was kept secret until 2012.
The ADS developers could not come up with any solutions to these climate problems and the system was never quite ready for prime time. It wasn't for lack of trying. In 2010 an ADS system was sent to Afghanistan but was eventually returned without being used. The proposed ROE (Rules of Engagement) for ADS were that anyone who kept coming after getting hit with microwave was assumed to have evil intent and could be killed. But it was pointed out that, in Afghanistan, people will get confused in stressful situations and run in the wrong direction. Troops had seen this happen often.
The microwave was believed to be particularly useful for terrorists who hide in crowds of women and children, using the human shields to get close enough to make an attack. This has been encountered in Somalia and Iraq. But not often enough in Afghanistan to give the one ADS there a chance to be used in action (as opposed to tests) for the first time. That was another unpleasant truth, there were simply not many situations where ADS was needed. Meanwhile a major, but unspoken reason, for not using ADS was commanders unwilling to take the media heat for employing a "death ray" on "innocent civilians."
In 2011 it was decided to radically redesign ADS to make it smaller, more reliable, and able to be used on the move. The Department of Defense had already spent nearly $60 million on ADS, so another $10 million or so seems a reasonable investment in something that almost works. ADS II was being designed to operate from moving aircraft, as well as moving ground vehicles and this appears to be the version the AC-130J will use. All this won't solve the problems with rain and fog, or lack of riots to disperse. The air force appears to see some uses for airborne ADS but is not releasing details.
The microwave powered ADS "radar dish" projects a "burn ray" that is about 1.3 meters (four feet) in diameter. It was supposed to be effective in fog, smoke, and rain, but wasn't. Under the right conditions, when pointed at people and turned on it creates a burning sensation on the skin of its victims, causing them to want to leave the area or at least greatly distracts them. The microwave weapon has a range of about 500 meters and that may have been increased in ADS II. ADS is carried on a hummer or Stryker, along with a machine-gun and other non-lethal weapons (like the sonic cannon LRAD).
ADS has been fired, in tests, over 2,500 times. Many of these firings were against human volunteers and the device performed as predicted, without any permanent damage. But generations of exposure to lurid science fiction descriptions of "death rays" has made the defense bureaucrats anxious over the negative public relations potential if something like ADS was actually used. From a publicity perspective, using more lethal "non-lethal-weapons" is preferable to deploying something safer but that could be described, however incorrectly, as a "death ray."