The U.S. National Distress System (NDRS) was established more than 30 years ago as a VHF-FM-based radio communication system that has a range of up to 20 nautical miles (40 km) along most of the U.S. shoreline for the United States Coast Guard. While this system has served the Coast Guard well over the years, it consists of out-of-date and non-standard equipment with many limitations. This system is being replaced by Rescue 21.