The four founding Party States led development of the 406-MHz marine EPIRB for detection by the system. The EPIRB was seen as a key advancement in SAR technology in the perilous maritime environment. Prior to the founding of Cospas-Sarsat, the civilian aviation community had already been using the 121.5 MHz frequency for distress, while the military aviation community utilized 243.0 MHz as the primary distress frequency with the 121.5 MHz frequency as the alternate. ELTs for general aviation aircraft were constructed to transmit on 121.5 MHz, a frequency monitored by airliners and other aircraft. Military aircraft beacons were manufactured to transmit at 243.0 MHz, in the band commonly used by military aviation. Early in its history, the Cospas-Sarsat system was engineered to detect beacon-alerts transmitted at 406 MHz, 121.5 MHz and 243.0 MHz. Because of a large number of false alerts, and the inability to uniquely identify such beacons because of their old, analogue technology, the Cospas-Sarsat system beginning in 2009 stopped receiving alerts from beacons operating at 121.5 MHz and 243.0 MHz, and now only receives and processes alerts from modern, digital 406-MHz beacons. Many ELTs include both a 406-MHz transmitter for satellite detection and a 121.5 MHz transmitter that can be received by local search crews using direction-finding equipment.