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Why 121.5 MHz Will Always Be Needed

The 121.5 MHz frequency for alerting to the Cospas-Sarsat satellites was phased out completely on the 1st of February in 2009.  The decision to do this was approved in February 1999 with a 10-year phase-out period.  Cospas-Sarsat does not require 121.5 MHz homing on any 406 MHz beacon. Individual member nations have put into place regulations requiring any 406 beacon (ELT, PLB, or EPIRB) sold, manufactured or used in that country to have a 121.5 MHz homer.  The following countries have major beacon manufactures who are all Cospas-Sarsat member nations and they all have regulations requiring 121.5 homing on all beacons. The USA, Australia, Canada, The UK, The European Union, China, and Russia.

The majority of these countries national regulations require the 121.5 MHz homer to have a signal strength of 50-100 mW.  The primary exception to this is the US, which requires all PLBs sold in the US to have a maximum 25 mW homer while ELTs and EPIRBs remain at 100mW. This means for ground search scenarios, TrueNorth Rescue Inc. has ensured that our Polaris RDF has enough sensitivity to pick up the weaker 25 mW homing transmission at the greatest range being 25 miles line of sight.

A lesser known fact about all Cospas-Sarsat 406 MHz beacons including commercial is that the GPS accuracy associated with these beacons is 100 meters.  Cospas-Sarsat knowingly cut short the GPS data string to equal 1 second of 1 degree of Latitude, or, 100 meters on average due to other data requirements.  As people typically don’t get in trouble in wide open areas, 100 meters can be a lot of territory to cover.  This is especially true in mountainous regions, heavily forested areas, and at sea during the night with less than optimal search conditions.  Therefore, even with GPS, the need for a 121.5 MHz homer is still critical to go that last 100 meters when time is of the essence.

One last factor related to GPS on 406 MHz beacons is that unlike the more capable handheld GPS units available that can receive and report a GPS position very reliably, a beacon (PLB, EPIRB or ELT) is designed foremost to get an emergency satellite signal out. Although a standalone GPS unit can get a position fix with one or two GPS satellites in view, a beacon normally requires three to four GPS satellites in view at the same time to acquire a fix. This clearly shows that the GPS functionality is secondary to the alerting function for beacons. Because in scenarios where there is a restricted view of the sky the GPS on the beacon may not acquire a fix.  This reduces the position accuracy from a GPS-aided 100 meters to a Doppler calculated 1.5 miles on average.