The recent malfunction of GPS (satellite navigation) bombs in Iraq has raised the issue of jammers for GPS munitions. The GPS signal is very weak and jammers are not difficult or expensive to build. Russia openly sells GPS jammers to anyone with the cash. It is possible to build "jam resistant" GPS devices for bombs, but these are expensive, of unknown reliability and not yet in general use. The U.S. has already modified some of it's HARM (anti-radar) missiles to home in on GPS jammers. While radars are expensive, GPS jammers are so cheap that an enemy can have many of them. A defender can turn on additional GPS jammers as soon as some are hit with HARMs. The Air Force likes GPS guidance systems because they are relatively cheap and more accurate than the alternatives. GPS will also operate as well at night or in bad weather. But no one knows just how effective GPS jammers are until some defender makes widespread use of them. It is feared that Iraq is already using GPS jammers.