The Switch to 406 MHZ

February 04, 2009
Written by Carole Smith
More than just a frequency change

On February 1, 2009, the international satellite system for search and rescue, COSPAS-SARSAT, will complete its transition to the digital era. After 26 years of listening for analog distress beacons transmitting on 121.5 MHz and 243 MHz, the system will soon monitor digital 406 MHz signals only.
Why the change? Quite simply, analog beacon technology pre-dates the satellite age and has limited capability. After a quarter-century, it is time to embrace the full potential of this life-saving system.
Emergency locator transmitters (ELTs) were first developed in the 1950s to help locate military aircraft, and were later mandated for civilian use. The marine community also had emergency position-indicating radio beacons transmitting on 121.5 MHz. After being reported overdue, signals from these beacons helped rescuers find the distressed craft.
In the early 1980s, the COSPAS-SARSAT system was born after Canada, the United States, France, and Russia recognized the great advantages offered by satellite surveillance of these emergency signals. Satellites could automatically detect these beacons, calculate their position, and relay the information to a rescue coordination centre even before aircraft or vessels were reported overdue. The first rescue assisted by COSPAS-SARSAT occurred in September 1982, just a few days after system testing began. Three lives were saved when a satellite detected the ELT from a downed aircraft in the mountains of British Columbia, 90 kilometres off its planned route.
By the mid-1990s, the COSPAS-SARSAT system was providing its “lifeline to survival” to over half-a-million aviation, maritime, and land-based users around the globe. Digital 406 MHz beacons designed specifically to work with satellite technology were now available. Recognizing these benefits, the International Civil Aviation Organization and the International Maritime Organization urged COSPAS-SARSAT to invest fully in the 406 MHz system. In October 2000, a unanimous decision was made to end the satellite processing of 121.5 MHz analog signals on February 1, 2009.
So what does the 406 MHz system offer that 121.5 MHz does not?
Unlike their analog cousins, emergency beacons transmitting on 406 MHz work with COSPAS-SARSAT’s geostationary satellites. These satellites continuously monitor the globe between 70° N and 70° S, detecting and relaying beacon alerts almost immediately.
406 MHz beacons are also optimized for use with COSPAS-SARSAT’s lowearth orbiting satellites, which travel overhead the poles and “see” into areas that may otherwise be shielded from the geostationary satellites. Unlike 121.5 MHz models, data from 406 MHz beacons can be stored on board these low-earth orbiting satellites, and downloaded when the next ground station comes into view. This reduces the number of satellite passes required to calculate a position. And, thanks to greater frequency stability, 406 MHz beacons may be located within 2-5 km on the first satellite pass, rather than the 15-25 km accuracy generated by 121.5 MHz beacons.
Transmitting at 5 Watts, 406 MHz beacons send a signal that is at least fifty times stronger than analog beacons. They are also equipped with a 121.5 MHz homing frequency, to help rescuers go the final distance when visibility is reduced by darkness, fog, or vegetation.
Being digital, 406 MHz beacons are uniquely coded and can be registered to a specific aircraft, vessel, or individual. This enables search and rescue authorities to start a response even while a final location is being calculated. False alerts can also be resolved by contacting the owner, without needing to send rescue units to investigate.
The greater alerting and locating capability, and the positive identification offered by 406 MHz beacons bring indisputable benefits. In the much-publicized case of a light aircraft forced down on the Arctic ice in early December 2008, signals from the aircraft’s 406 MHz ELT were successfully transmitted in the few minutes before the aircraft broke through the ice and sank.
The eight-year transition period to exclusive monitoring of 406 MHz by COSPAS-SARSAT is now coming to a close. In Canada, users of marine and land beacons have already moved to 406 MHz technology. While some in the aviation community have installed a 406 MHz ELT, many have not, citing concerns about cost and reliability. Do ELTs function 100 per cent of the time? No, but neither do any other safety devices. While some promising alternative technologies are emerging, none yet offer the same performance capabilities of COSPAS-SARSAT’s 406 MHz system, nor the extensive publicly-funded infrastructure that backs it up.
Regulatory requirements aside, it is hoped that aircraft owners will recognize that COSPAS-SARSAT’s switch to 406 MHz is more than just a frequency change, and will choose to continue taking advantage of this unique lifeline to survival. The National Search and Rescue Secretariat is Canada’s representative to the COSPAS-SARSAT Council.

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