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latter, the physical characteristics of the radio frequency and the output signal mean
that there is coarser resolution with beacons operating on this frequency compared to
those operating on the higher frequency.
Australia, through AusSAR, is responsible for operating the regional Cospas-Sarsat
ground segment in the South West Pacific region.This is done by monitoring satellite
intercepted signals from three ground stations in Albany (WA), Bundaberg (QLD), and
Wellington (NZ). With 121.5 MHz signals, the three elements in the process (ie the
beacon, the satellite and the ground station) must be in view of each other. This
introduces delays in the SAR system responding. With later technology 406 MHz
signals, the satellite has the capacity to time tag the digital information and repeat it
when it is next interrogated by a ground station or pass the information via satellites
in geo-stationery orbit over the equator to provide a near instantaneous alerting
function.
BEACON TERMINOLOGY
There have been a number of conventions used in the past to describe the various
types of distress beacons that have been available in the market place. The current
practice is to use Electronic Locator Transmitter (ELT) to describe those that are fitted
to an aircraft, Emergency Position Indicating Radio Beacon (EPIRB) to describe those
that are designed to float when immersed in water, and Personal Locator Beacon (PLB)
to describe the portable units that are designed for personal use. Many GA operators
carry the PLB variant.
320
Civil Aviation Safety Authority Australia
EMERGENCY PROCEDURES DISTRESS BEACONS
09/2001
4
EMERGENCY ACTIVATION
SECTION
COMPATABILITY OF OLDER TECHNOLOGY BEACONS
The 1960s saw the emergence of aviation distress beacons that operated on 121.5
MHz. These beacons meet the FAA TSO C91 standard and provide an audible tone on
the frequency with the likelihood that other aircraft or air traffic services in the area
would intercept it and become aware that an aircraft is in distress. A large number of
aircraft still operating in Australia are fitted with this standard of ELT. These older
beacons are not covered by the Cospas-Sarsat system and continue to rely on the
aviation sector for SAR alerting purposes.
When a decision was taken to extend the Cospas-Sarsat system to include
121.5 MHz, the standard pertaining to aviation beacons was revisited and a
new standard (FAA TSO C91A) was set making the beacon emission suitable
for intercept by satellite. The FAA standard for 406 MHz beacons is TSO C126.
These standards are reflected in CAR 252A.
COMPARISON OF DISTRESS BEACONS
The 121.5 MHz beacons in current production are relatively lightweight and
inexpensive. They provide an affordable alternative to the more expensive 406 MHz
beacons (which are now available with an embedded GPS so that they can
automatically report the beacon position in digital form via the satellite system when
activated). A comparison of the two beacon technologies is shown in
Table 1.
As a result of the location of the three ground stations servicing the Australian region,
there are approximately fifty satellite passes serviced per day by AusSAR which results
in a typical coverage area and average times for detection of a 121.5 MHz beacon.
321
Civil Aviation Safety Authority Australia
EMERGENCY PROCEDURES DISTRESS BEACONS
09/2001
4
EMERGENCY ACTIVATION
SECTION
322
Civil Aviation Safety Authority Australia
EMERGENCY PROCEDURES DISTRESS BEACONS
09/2001
4
EMERGENCY ACTIVATION
SECTION
The major implications for general aviation aircraft operating in Australia using 121.5
MHz beacons is that if the beacon is of the older type, then there is a reliance on
other aircraft to detect the 121.5 MHz signal and raise the alarm. This may be
problematic in many parts of Australia as only the larger commercial aircraft regularly
monitor this frequency. If the beacon is Cospas-Sarsat compatible, the system will
generally detect the signal but produce an ambiguous fix position either side of the
satellite pass. Follow-on passes, collateral information, or the use of aircraft to
investigate both possible positions are used to refine the correct distress beacon
position.
This evolution takes time and the accuracy of the Cospas-Sarsat derived position is less
accurate than with the more technically advanced 406 MHz beacon which usually
provides an accurate position on the first pass. These beacons are also encoded with
the details of the registered owner and, through the GEOSAR supplementary
repeaters, provide near instantaneous advice that an emergency situation exists prior
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