“The Safety Board is concerned that, although the United States supports the termination of 121.5-MHz signal processing, there are an estimated 180,000 general aviation 121.5-MHz ELT’s [Emergency Locator Transmitters] in service with no requirement to upgrade to a 406-MHz ELT when the satellite service is terminated in February 2009,” the National Transportation Safety Board (NTSB) warned the Federal Aviation Administration (FAA) in a 7-page letter of 4 September 2007.