ELTs were originally intended for use on the 121.5-MHz frequency to alert air traffic control and aircraft monitoring the frequency. In 1982 a satellite-based monitoring system was implemented (COSPAS-SARSAT) to provide a better receiving source for these signals. As of February 1, 2009, the international COSPAS-SARSAT satellite system will discontinue satellite-based monitoring of the 121.5/243-MHz frequencies, in part because of a high number of false signals attributed with these frequencies. While there's no requirement in the United States to replace the first- and second-generation 121.5-MHz ELTs, after this date, 121.5/243-MHz distress signals transmitted from ELTs operating on the lower frequency will only be detected by ground-based receivers such as local airport facilities and air traffic control facilities or by overflying aircraft. It is important to note that after 2009, existing 121.5-MHz ELTs, although still legal from the FAA's perspective, will provide extremely limited assistance if an aircraft crashes, especially in a remote location.
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