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COMPARISON OF THE 406 MHz AND 121.5 MHz DISTRESS BEACONS
The following table compares 406 MHz and 121.5
MHz beacons in these critical areas:
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406
MHz Beacons
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121.5 MHz
Beacons
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Coverage:
o Global
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o Ground station
dependent; ground
stations have an effective radius of about 1800 nm (2300 km).
Both ground station and beacon must be in satellite footprint.
Current coverage is about two-thirds of the world.
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False Alerts:
o All alerts come
from beacons. Satellite
beacon transmissions are digital, coded signals. Satellites
process only encoded data, other signals are rejected.
o About 1 in 12
alerts are actual distress.
o 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 POCs prior to launching SAR assets.
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o 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!
o Fewer than 2 in
1000 alerts and 2 in 100 composite alerts are actual distress.
o Since 121.5 MHz
beacons transmit anonymously, the only way to ascertain the
situation is to dispatch resources to investigate -- a costly
disadvantage.
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Alerting:
o First alert
warrants launch of SAR assets. Earlier launches puts assets on
scene sooner--Average 3 hrs saved in maritime, 6 hrs in inland.
o Average initial
detection/alerting by orbiting satellites is about 45 minutes.
o Average subsequent
satellite passes every 60 minutes.
o Vessel/aircraft
ID, point of contact information provided with alerts allows
rapid verification or stand-down.
o Allows false alert
follow -up to continuously improve system integrity/reliability.
o Near instantaneous
detection by geostationary satellites. System provides
world-wide coverage.
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o High false alert
rate makes first-alert launch unfeasible.
Absent independent
distress information means RCCs must wait for additional alert
information.
o Same as 406 MHz.
o Same as 406 MHz.
o Alerts are
anonymous. 121.5 MHz analog technology not capable of
transmitting data.
o No false alert
follow -up capability.
o No GEO detection
capability = no instantaneous detection.
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Position
Information:
o 1-3 nm (2-5 km)
accuracy on average.
Position calculated by Doppler shift analysis.
o Less than 100
yard accuracy with GPS-equipped beacons. GPS position
processed with initial alert. Major beacon enhancement.
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o 12-15 nm (15-25
km) accuracy on
average. Position calculated by Doppler shift analysis.
o No GPS
capability.
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Locating the
Target:
o Superior alert
(non-GPS) position accuracy limits initial search area to
about 25 sq. nm (65 sq. km).
o GPS-equipped
beacons reduce search area to a significantly smaller area.
o 121.5 MHz homing
signal facilitates target location by radio detection finder
equipped search units.
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o Initial position
uncertainty result in 500 sq. nm
(800 sq. km)
search area on average.
o No GPS
capability.
o Same as 406 MHz.
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Power Output:
o 5.0 Watts (Strong
power output)
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o 0.1 Watt (Weaker power
output) – Hard for satellites to detect
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Cost:
o Average cost is
$1000 (GPS-equipped EPIRB)
o Average cost is
$500 (Personal Locator Beacon)
o Average cost is
$1500.00 - $3000.00 (ELT
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o Average cost is
$200.00 - $400.00 (EPIRB)
o Average cost is
$600.00 - $1200.00 (ELT)
o 121.5 MHz
beacons are being phased out
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Have you ever wondered how
the 406 system differs from 121.5 system? Check out this link
to the NOAA website
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