01.17.2006 12:00 AM
Freeview Set-Top Box Calls for Help on 121.5 MHz Distress Frequency
Several U.K. Web sites reported on a renegade Freeview set-top box in England that transmitted a signal on the international distress frequency of 121.5 MHz. The signal was close enough to the distress signal sent by emergency beacons that it was intercepted by a satellite orbiting the Earth. While the satellite that received the signal wasn't mentioned, it was likely part of the Cospas-Sarsat System. The Royal Air Force was dispatched to search for the beacon near Portsmouth Harbour before it was determined the signal was coming from dry land.

False alerts on the 121.5 MHz emergency frequency aren't unusual. According the SARSAT Web site, no data is encoded in the analog beacon signal, increasing the chances of false alerts. The problem has become so bad that Cospas-Sarsat plans to stop satellite processing of 121.5/243 MHz signals Feb. 1, 2009.

Cospas-Sarsat said only one in 50 alerts is a genuine distress situation. The 121.5 MHz beacons are being replaced by higher power 406 MHz beacons (5 watts versus 0.1 watt) that will use a digitally modulated signal. This signal includes unique information that can link it to registration data on the owner, vessel or aircraft. Without GPS data, the position of the 406 MHz beacon can be traced to 5 km, compared with 20 km for the 121.5 MHz beacons. There are several other advantages detailed in the announcement of the Cospas-Sarsat Phase-Out of 121.5/243 MHz Alerting Services.

For additional information on the Freeview box incident, see Digibox sparked RAF rescue mission in The Scotsman and Freeview box sparks RAF rescue mission in the Telegraph. For a more humorous look at the incident, see Digi-TV box bid to signal alien invasion blocked by RAF choppers -- Or was it...? in The Register.


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