406 MHz vs. 121.5 MHz Distress Alerting

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The table below covers the most significant differences between a 406 MHz distress beacon and a 121.5 MHz only distress beacon

406 MHz 121.5 MHz POWER OUTPUT 5.0 Watts - Easy to detect, punches through overhead cover and improves accuracy 0.1 Watt - Hard for satellites to detect and reduces accuracy COVERAGE Global via low-earth orbiting satellites Between 70° North & South latitudes via geostationary satellites (provides nearly instantaneous detection) Ground station dependent; ground stations have an effective radius of about 1800 nm (2300 km). Both ground station and beacon must be in view of the low-earth orbiting satellite, within this footprint. Current coverage is about two-thirds of the world. ALERTING First location alert warrants launch of SAR assets. Earlier launches puts assets on scene sooner - an average 3 hours saved in maritime, 6 hours saved in inland. Average initial detection/alerting by orbiting satellites is about 45 minutes, maximum 90 minutes. Average subsequent satellite passes every 60 minutes, maximum 90 minutes. Beacon ID combined with registration data and point of contact information allows rapid verification and launch or stand-down. Allows false alert follow-up to continuously improve system integrity/reliability. Near instantaneous detection by geostationary satellites. Beacon ID combined with registration data and point of contact information allows potential for near real-time immediate launch of SAR assets even without beacon derived location information. High false alert rate makes first-alert launch unfeasible. Absent independent distress information means RCCs must wait for additional alert information. Average initial detection/alerting by orbiting satellites is about 45 minutes, maximum 90 minutes. Same as 406 MHz. Average subsequent satellite passes every 60 minutes, maximum 90 minutes. Same as 406 MHz. Alerts are anonymous. 121.5 MHz analog technology not capable of transmitting data. No false alert follow-up capability. No geostationary satellite detection capability = no instantaneous detection. POSITION INFORMATION 1-3 nm (2-5 km) accuracy on average. Position calculated by Doppler shift analysis. Position information on first satellite pass. Better than 300 feet accuracy with GPS equipped beacons. GPS position processed with initial alert, near instantaneous via geostationary satellites. 12-15 nm (15-25 km) accuracy on average. Position calculated by Doppler shift analysis. Requires minimum of two satellite passes. No GPS capability. SURVIVOR LOCATION Position accuracy (non-GPS) limits initial search area to about 25 sq. nm (65 sq. km) or better. GPS equipped beacons reduce search area to virtually pinpoint area. 121.5 MHz homing signal facilitates final short-range survivor location by radio detection finder equipped search units. Initial position uncertainty result in 500 sq. nm (800 sq. km) search area on average. No GPS capability. 121.5 MHz signal facilitates survivor location by radio detection finder equipped search units. Larger search area makes this more difficult and problematic. FALSE ALERTS All alerts come from beacons. Satellite beacon transmissions are digital, coded signals. Satellites process only encoded data, other signals are rejected. About 1 in 12 alerts are actual distress. Beacon-unique coding/registration allow rapid incident corroboration. Registration mandatory since 1994. 90% beacons registered. About 70% of false alerts are resolved by a phone or radio call to registration point of contacts prior to launching SAR assets. Only about 1 in 5 alerts come from beacons. Satellites cannot discern beacon signals from many non-beacon sources. Beacons transmit anonymously with no unique identifier. Non-beacon interferers have included ATM machines, pizza ovens, and stadium scoreboards. Fewer than 2 in 1000 alerts and 2 in 100 composite alerts are actual distress. Since 121.5 MHz beacons transmit anonymously, the only way to ascertain the situation is to dispatch resources to investigate -- a costly disadvantage that puts SAR crews at risk unnecessarily.. High false alert rate makes first-alert launch unfeasible. THE FUTURE International standard for the foreseeable future. Next generation system already being fielded is 100% backward compatible and results in improved accuracy and shorter alert times with current available beacons. Satellite processing will cease on February 1, 2009. Use of 121.5 MHz EPIRBs by U.S. boaters is illegal effective January 1, 2007.

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URL: http://www.equipped.org/406_vs_1215
First Published on ETS: June 28, 2003
 

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