Pilots and crew of aircraft that went down use CSEL radios to let rescue aircraft know where they are. This is especially critical if you go down in hostile territory. The AN/PRC-112G uses GPS and satellite phone technology to send a brief ("burst") transmission of the radios (and downed pilots) location via satellite. When the rescue chopper is close enough (within line-of-sight) the AN/PRC-112G provides encrypted (the enemy can't listen in) two way radio capability to get any essential information from the downed pilot before the pickup is made. The AN/PRC-112Gs batteries are good for 96 hours of use.
The AN/PRC-112 was developed in the 1980s based on experience during the Vietnam War. CSARs first appeared early in World War II, when the Germans invented a hand cranked rescue radio (with a range of 300 kilometers). One was captured by the British in 1941 and copied. The design was passed on to the U.S., who produced their own version of it. After World War II new designs appeared that were smaller, lighter, and more capable than the original 15 kg (33 pound) World War II models.
The handheld AN/PRC-112 is being replaced by the more compact and 10 percent heavier AN/PRQ-7. Weighing .9 kg (32 ounces), the AN/PRQ-7 incorporates GPS, a beacon that can be picked up by satellite or nearby aircraft, and a two way radio. The AN/PRQ-7 batteries also last longer (over 450 hours) and the entire unit is more rugged and reliable. Over 50,000 AN/PRQ-7s have been bought since it was introduced 14 years ago.
While CSEL radios have gotten better over the last four decades, they have also been used less and less. American warplanes and helicopters have become more reliable and American domination of the air has grown, as has the defensive capabilities of these aircraft. But aircraft, usually helicopters, still go down because of component failures or enemy action and the CSELs still get used.