The idea of outfitting every aircraft with an ELT started in 1970, when a plane carrying two congressmen went down in Alaska. The Alaska Rescue Coordination Center mounted a huge search but was unsuccessful. The plane has still not been found. Soon after the incident, it was mandated that all aircraft be required to have an ELT.

121.5 MHz was chosen as the international aircraft distress frequency. The idea was good, but the system had many limitations. For example, there was no way of telling where the signal was originating from. The frequency was often cluttered, and another aircraft had to be within range to receive the signal. However, satellite technology was in the growing stage and better systems were in the works.

Before long, the SARSAT (Search and Rescue Satellite Aided Tracking) system was developed by NASA. This system operates on 406 MHz. The distress signal is relayed by satellite to ground receiving stations. Every 50 seconds, a five watt signal is sent out in a half-second burst. The transmission is coded with information about the unit and the pilot. Search and Rescue is given the owner's name, telephone number, and the type of aircraft. Such information cannot be provided by 121.5 MHz beacons.

Because it uses geostationary satellites, the 406 MHz alert is received as soon as the beacon is activated. Plus, some of these beacons can digitally transmit GPS positions with an accuracy of within 100 meters. With a 121.5 MHz beacon, a polar orbiting satellite must be overhead, which may take up to two hours. And because these signals are analog, GPS coordinates cannot be sent.

The US Coast Guard long ago saw the potential of the 406 MHz beacon, and in 1990 they started measures to bring it into widespread usage. The aviation industry has been slower to catch on; only one company is producing this beacon, and it has only recently been released.

Without any doubt, the 406 MHz ELT is the best route for any aircraft owner. The information that the signal provides helps Search and Rescue distinguish between false alarms and a true emergencies. There are so many false alarms in a given day from 121.5 MHz beacons that a real one may be ignored.

The following are some suggestions on how you can prevent a false alarm. By following the simple steps you can increase the effectiveness of this system. Your life may depend on it.

1. Always tune 121.5 MHz on your comm before shutting down. If you hear a siren, turn off your ELT. Call Flight Service Station and report the false alarm immediately.
2. When you replace your ELT, spend the extra money and upgrade to a 406 MHz.
3. Regularly maintain your ELT. Go to your local avionics shop for an ELT test. Low batteries can give erroneous readings or trigger a false alarm.

It's important to have a good, working ELT when flying, but it is also important to always file a flight plan and, when it is available, always use flight following. That way, Search and Rescue will know where you are and can get to you quickly in an emergency.

Alisa Christenson, Santa Fe Avionics. 505-474-3606 .