Satellite Processing of 121.5/243 MHz

The International Cospas-Sarsat Program, a program that uses a satellite constellation to relay distress alerts to search and rescue authorities, announced at its 25th Council Session that it plans to terminate satellite processing of distress signals from 121.5/243 MHz emergency beacons on Feb. 1, 2009. Mariners, aviators, and individuals using emergency beacons will need to switch to those operating at 406 MHz if they want to be detected by satellites.

All TSO-C126 ELTs transmit on 121.5 and 406 MHz and most transmit on all 3 emergency frequencies (121.5/243.0 and 406 MHz). The ELT automatically activates during a crash and transmits the standard swept tone on 121.5 and 243.0 MHz. The ELT also transmits a 406.025 MHz encoded digital message to the Cospas/Sarsat satellite system, which allows for rapid identification and reduces Search and Rescue response time. Some of these ELTs may also be interfaced with an on-board GPS which enables latitude and longitude data to be transmitted as part of the 406.025 MHz message.

With an activated 121.5/243 MHz ELT, the typical search area may be a 15-20 kilometers radius. A 406 MHz ELT tightens that radius to 1-2 kilometers. When coupled to the aircraft navigation system (GPS, etc.) the position accuracy improves to approximately 100 meters.

The implication of this Cospas-Sarsat decision is that users of beacons that send distress alerts on 121.5 /243 MHz should eventually begin using beacons operating on 406 MHz if the alerts are to be detected and relayed via satellites. Meanwhile, anyone planning to buy a new distress beacon may wish to take the Cospas-Sarsat decision into account.