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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. www.siliconchip.com.au 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