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Emergency
beacons
Australia is a huge continent, surrounded
by the vast Indian, Pacific and Southern
oceans. If you get lost in the outback or
have to abandon ship far out to sea,
you could be in very serious trouble.
T
wenty years ago you almost certainly would have
been in trouble. Rescuers might have searched for
days to find you – once they even knew you were
overdue.
Today the Cospas-Sarsat satellite system, set up by
Russia, Canada, France and the USA in 1982, takes much
of the search out of search and rescue.
A constellation of satellites quickly detects
signals from emergency radio beacons and alerts
search and rescue authorities around the world.
Since the system became fully operational in
1985, 29 other countries, including Australia,
have become involved and more than 11,000
people have been rescued. Carrying an emergency beacon means you can be certain help
will be on its way when you need it.
If you activate a beacon, it starts transmitting
a low-power radio signal. Satellites in geostationary and low earth orbits pick up the signal
and relay it to ground receiving stations, called
Local User Terminals (LUTs). The LUTs locate the
beacon position and pass it to Mission Control
Centres (MCCs) which coordinate the search and
rescue effort (Fig.1).
Beacons
Beacons come in many shapes and sizes. There
are Emergency Locator Transmitters (ELTs) fitted
in aircraft, Emergency Position Indicating Radio Beacons (EPIRBs) in ships, and hand-held
www.siliconchip.com.au
By
PETER HOLTHAM
Personal Locator Beacons (PLBs).
The oldest type of emergency beacon operates
on 121.5MHz (Table 1). They were originally
designed in the mid 1970s as ELTs for crashed aircraft. Nowadays there are about 600,000 low-cost
EPIRBs and PLBs also using this technology
worldwide.
Designed for detection by search aircraft
not satellites, their simple analog signal doesn’t tell the rescue authorities
who or what is in trouble, or exactly
where the emergency is. What is
worse, only about three in 100 alerts
worldwide are genuine.
Accidental or malicious activation, faults in the beacons,
non-beacon transmissions on
121.5MHz, even ‘hard’ landings by
aircraft with G-switch activated ELTs
cause the rest.
But each alarm must be tracked to its
source, wasting the time and resources
of search and rescue teams. Because the
false alarm rate is so high, Cospas-Sarsat
will stop processing signals from these
beacons after February 1st 2009, and they
will be obsolete.
Newer beacons, specifically designed for
detection and location by satellites, operate
on 406MHz (Table 2). Frequencies in the
March 2003 13
Fig 1: Basic Concept of the Cospas-Sarsat System
406-406.1MHz band are reserved solely for these beacons,
which helps minimise the number of false alarms.
The beacons transmit a 5W burst of radio frequency (RF)
every 50 seconds. The high power increases the chance
of detection, while the low duty cycle saves power and
allows more than 90 beacons to be operating at once in
view of one satellite.
Each burst of RF carries a digitally encoded message,
which identifies the owner of the beacon and its country
of origin. Search and Rescue authorities worldwide keep
a register of owners and can quickly make a phone call to
check if an emergency is genuine or not.
Most 406MHz beacons also include a 121.5MHz
transmitter and a flashing strobe light for search vessels to home in on during the last stages of a rescue.
Second-generation beacons, available since 1997, add
position data in the digital message, from an internal or
external GPS receiver.
Because the performance of the Cospas-Sarsat system
depends on the quality of the 406MHz beacons, manufacturers must get type-approval. Australian and New Zealand
Standard AS4280 describes the rigorous durability tests
a beacon must pass before it is approved for use. Only
two Australian companies have gained type-approval for
their beacons, and they manufacture them only for the
Defence Forces.
Table 2: 406MHz beacon data
Transmitted power
5W ± 2dB
Transmission life
at least 24 hours at minimum
temperature
50-100 mW peak effective radiated
power relative to a quarter wave
monopole
Frequency
406.025 ±0.005MHz
Modulation
phase modulation, bi-phase L data
encoding
Transmision life
48 hours
Transmission time
Frequency
121.5MHz ±6kHz
440ms (short message)
520ms (long message)
Modulation type
AM (amplitude modulation),
greater than 85%
Message length
112 bits (short)
144 bits (long)
Modulation
Swept audible tone, 300-1600Hz
(at least 700 Hz) at a rate of 2-4Hz
Message repetition time
50s
Operating temperature
-40 to +55°C
Table 1: 121.5MHz beacon data
Transmitted power
14 Silicon Chip
www.siliconchip.com.au
Fig 3: Approximate 121.5MHz Beacon Coverage from Australian and New Zealand LUTs.
Fig 2: Satellite in Polar Orbit Showing a Single Orbital
Plane.
Inmarsat, the organisation responsible for worldwide
ship-to-shore communication, also operates an EPIRB
tracking system using satellites as part of its commitment
to the safety of life at sea.
Inmarsat EPIRBs operate at 1.6GHz (Table 3). Like
406MHz beacons, they also transmit identification and
GPS-derived position information. Some also have a ‘will
to live’ feature – as soon as an Inmarsat land earth station
(LES) receives an emergency signal, it bounces it back to
the beacon. The beacon recognises its own code and shows
a telltale visual indication. Survivors in the water can see
that their distress signal has been received and that help
is on the way.
Satellites
The Cospas-Sarsat system uses low-earth orbiting
(LEOSAR) and geostationary (GEOSAR) satellites. The
LEOSARs are in polar orbits 800-1000km above the Earth.
They complete an orbit every 100 minutes or so, listening
for both 121.5MHz and 406MHz beacons.
The system uses a minimum of four LEOSARs to speed
Table 3: Inmarsat-E Beacon Data
Transmitted power
1W
Transmission life
48 hours minimum
Frequency
667 channels at 1.645GHz
Modulation
Frequency shift keying.
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up detection of activated beacons (Table 4). With a single
satellite, it takes at most one half rotation of the Earth
(twelve hours) for any location to pass under the orbital
plane (Fig.2).
A second satellite with its orbital plane at right angles to
the first reduces the time to six hours. Using four satellites
ensures the time taken to detect a beacon is less than one
hour at mid-latitudes and slightly longer nearer the equator
where the LEOSARs are more spread out.
With 121.5MHz beacon signals, LEOSARs simply act as
repeaters, relaying the signal to a LUT for processing. For
an emergency beacon to be noticed, a LEOSAR has to be in
view of the beacon and a LUT simultaneously for at least
four minutes. If it is not, the emergency will be missed until
a more suitable pass occurs, which can take several hours.
This constraint limits the use of these beacons to within
a 3000km radius of the LUT (Fig.3).
Table 4: LEOSAR Satellites
Satellite
Spacecraft
Status
Cospas-6
Nadezda-3
Operational
Cospas-8
Nadezda-5
Operational
Cospas-9
Nadezda-6
Operational
Sarsat-4
NOAA-11
Operational
Sarsat-6
NOAA-14
Operational
Sarsat-7
NOAA-15
Operational
Sarsat-8
NOAA-16
Operational
March 2003 15
Looking for all the world like the lighthouses of yesterday, Australia's two Local User Terminals (LUTs) are located at
Albany, WA (left) and Bundaberg, Qld (right). The inset in the middle is an on-ground shot inside a LUT antenna dome.
Because LEOSARs were specifically designed to detect
406MHz beacons, they do more than just relay the signal. An on-board Search and Rescue Processor (SARP)
decodes and time-stamps the beacon’s digital signal and
measures the Doppler shift (the change in frequency
caused by the relative movement between the satellite
and the beacon), locating the beacon to within 5km. In
95% of cases, the location is determined on the first orbit
after detection.
Second generation 406MHz beacons transmitting
GPS-derived position data can be located to within 120m.
In local mode, the satellites immediately transfer the
information to the 1545MHz downlink, for transmission
to any LUT that may be in view.
In global mode, the satellites also store the data in
memory and continuously re-broadcast it on the downlink
frequency. This means all LUTs tracking the satellite are
able to locate the beacon, giving the system global coverage. There is no waiting until the satellite can see both
the beacon and a LUT simultaneously, reducing the time
taken to launch a rescue.
Data storage gives the LEOSARs global but not continuous coverage; there may still be a delay before a satellite comes into view. So the Cospas-Sarsat system also
uses three geostationary satellites (GEOSARs) orbiting
Table 5: GEOSAR Satellites
Satellite
Status
GOES-8
Operational 75° W
GOES-10
Operational 135° W
GOES-11
In orbit spare
INSAT-2B
Operational 93.5° E
16 Silicon Chip
36,000km above the equator (Table 5). These communications and weather satellites carry 406MHz beacon receivers
as secondary payloads.
GEOSARs provide a continuous watch and can send an
alert as soon as a beacon is activated but there are some
disadvantages. As GEOSARs are stationary relative to the
Earth, there is no Doppler effect on the received signal to
provide position information. Unless it is encoded in the
digital message, LEOSARs must still be used for beacon
location.
Hilly ground or other obstructions can also hide the
GEOSARs from view, especially at high latitudes; neither
can they cover the polar regions, latitudes greater than 75°
(Fig.4). But even with this limitation, GEOSARS can see
about 97% of the Earth’s surface.
Four geostationary communication satellites spaced
around the equator detect Inmarsat EPIRBs. Like GEOSARs,
Inmarsat satellites cannot see the polar regions but this is
not a major problem as very little commercial shipping
enters these regions.
Local User Terminals (LUTs)
Local User Terminals are unmanned ground stations that
receive the downlink signal from the orbiting LEOSARS
as a 2400kbps data stream. They consist of an antenna, a
1545MHz receiver, a computer to process the data, and an
MCC interface. The antenna and receiver automatically
acquire, track and receive the downlink signal from all
non-conflicting LEOSAR passes.
There are 45 LEOSAR LUTs worldwide (Fig.5). Two are
in Australia, one on the east coast at Bundaberg and one
on the west coast at Albany. New Zealand has one LUT,
near Wellington, while to our north there are LUTs in Indonesia, Singapore and Guam. All LEOLUTs are expected
to be available 24 hours a day, every day, with less than
5% downtime a year.
www.siliconchip.com.au
Fig 4: Geosar Footprints
Fig 5: Worldwide location of Leoluts and Geoluts
Once a 121.5MHz beacon signal is received, the LUT
roughly fixes its position from the Doppler shift. Initially
two mirror-image positions are calculated, one on either
side of the satellite ground track. It takes until the next
orbit, 90 minutes later, to resolve the ambiguity and fix
the position to within 20km.
The data from 406MHz beacons is simpler to deal with,
since the Doppler shift is measured and time-tagged by
the SARP onboard the LEOSAR. Within minutes of the
satellite disappearing over the horizon, all stored data
has been processed and passed to the nearest Mission
Control Centre.
Another seven GEOLUTS worldwide receive
and process alerts relayed by GEOSAR satellites (Fig.5). In the southern hemisphere,
there are GEOLUTS in New Zealand
and Chile. Separate Land Earth Stations (LES) operated by Inmarsat
(including one in Perth) monitor
the signals from the Inmarsat-E
beacons.
Inmarsat Land Earth Stations. Because a 406MHz emergency is usually processed by more than one LUT, the
MCCs are networked together so that alerts can be rapidly
sorted and passed to the nearest search and rescue team
for action.
The MCC for the Australasian region is in Canberra and
is operated by Australian Search and Rescue (AusSAR),
part of the Australian Maritime Safety Authority. About
60 search and rescue specialists and support staff work
in the Centre, which operates 24 hours a day, 365 days
of the year.
As well as managing the Australasian segment of the
Cospas-Sarsat system, AusSAR coordinates Australia’s
Search and Rescue Region. Covering 53 million
square kilometres or one tenth of the Earth’s
surface, it includes the nation as well as vast
areas of the Indian, Pacific and Southern
Oceans.
The search and rescue teams coordinated
by the Centre come from the private sector,
the police, volunteer groups and the Defence
Forces.
How long will it be before you’re rescued
once you’ve switched on your beacon? AusSAR
has one of the best search and rescue response
times in the world. During 2000-2001, it took an
average of just 57 minutes to get a rescue underway
after receiving an alert. But how long it takes to get
to you will depend on where you are and whether
there are ships or aircraft nearby.
But unlike 20 years ago, at least you can be certain
SC
that you will be found.
Mission Control Centres (MCCs)
24 Mission Control
Centres around the
world receive alert
and location data
from LUTs, other MCCs and
The GlobalFix
406 is the next
generation of
EPIRB featuring an
internal GPS engine
to add latitude/longitude
coordinates to the emergency
signal. It is available in either Category
I (automatically deployable) or Category
II (manually deployable) models. The beacon’s self-test
features include a thorough analysis of the GPS’s circuitry
(each time you self-test the EPIRB, the GPS is tested as
well). When used in an emergency, the GlobalFix 406 will
automatically change its operating “red” flash to “green”,
to confirm the exact time the GPS coordinates are received
and re-broadcast in the EPIRB’s transmission.
(EPIRB pictures supplied by Tony Smith & John Bell, ACR
Electronics Inc.)
www.siliconchip.com.au
Acknowledgement:
Thanks for the assistance of Ben Mitchell of AusSAR in
preparing this feature.
Cospas
Sarsat
ELT
EPIRB
LES
LUT
MCC
PLB
Abbreviations
COsmicheskaya Sistyema Poiska Avariynich Sudov
(Space system for the search of vessels in distress)
Search and Rescue Satellite Aided Tracking
Emergency Locator Transmitter
Emergency Position Indicating Radio Beacon
Land Earth Station (Inmarsat)
Local User Terminal
Mission Control Centre
Personal Locator Beacon
March 2003 17
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