EPIRBS
	Emergency Position Indicating Radio Beacons have been with us for a number of years. Their name explains their function. EPIRBs are used for transmitting a distress alert and then for guiding the rescuers to the site. An operating EPIRB indicates that one or more persons are in distress and that they may or may not have the ability to receive radio transmissions.
	Types of EPIRBs
	The earliest EPIRBs transmitted on the aircraft emergency frequencies of 121.5 MHz and/or 243 MHz. They simply transmitted a signal, usually an alternating tone, to be received on the emergency frequency on a normal aircraft radio - 121.5 MHz being the civil emergency frequency, and 243 MHz the military equivalent. Indeed, their early development was more for aircraft than for ships. For some reason, if they are fitted to aircraft, they are known as Emergency Location Transponders. As this name suggests, they were not thought of as a way to raise the alarm but more as a way to find the distressed aircraft or ship. Such a beacon could only be used to raise the alarm if an aircraft happened to pass within range and had its radio tuned to the right frequency. With the dawning of the space age this concept changed. It was realised that if there was a satellite, or better still several satellites listening on these frequencies, then an EPIRB could be used to raise the alarm in a distress situation and it soon became apparent that the position of the transmitting beacon could be fixed from the satellite with a fair degree of accuracy. In 1979, the then-USSR signed a treaty of co-operation with the USA, Canada and France, to establish a satellite system to aid in search and rescue. Cospas-Sarsat was born. Cospas is an acronym for the Russian words meaning Space System for the Search of Vessels in Distress, and Sarsat is the acronym for Search And Rescue SATellite. Trials began in 1982 and the system was declared operational in 1985. In 1990, Russia assumed responsibility for the former Soviet segment. There is always a minimum of four satellites in the system. The Russians keep at least two operational satellites in near polar orbits, at an altitude of about 1,000 kilometres. Each carries equipment for detecting and analysing EPIRB transmissions on 121.5 and 406 MHz. Additionally, there are always at least two US NOAA weather satellites which are in sun-synchronous near-polar orbits at an altitude of about 850 kilometres, each carrying a set of Search And Rescue instruments provided by Canada and France, also monitoring both frequencies. From its inception, the Cospas-Sarsat system was designed to monitor two frequencies. The frequency of 121.5 MHz was retained for two reasons. Existing EPIRBs could use the system, and search aircraft would still be able to use their VHF radio equipment to home in on the victim's signal from a 121.5 MHz EPIRB. An additional higher frequency of 406 MHz was assigned for specialised satellite EPIRBs. This higher frequency could be made more stable, which makes fixing the position from the satellite much easier. The signal on this frequency incorporates a coded message to provide identification of the vessel to whom the EPIRB is licensed. The best EPIRB would be one that can transmit on both frequencies - 406 MHz to give the satellite the best chance of getting a good fix on the EPIRB's position and 121.5 MHz for any searching aircraft to home on. On the ground there are some 22 Local User Terminals located in various countries. Each of these monitors each satellite as it passes. If a satellite detects an EPIRB signal and it is within 'sight' of an LUT, it will immediately relay the information to that ground station. If there is no LUT within range and the satellite receives a 406 MHz signal, then the satellite can store the information and transmit it to the first LUT that comes into range. It is not able to do this for signals received on 121.5 MHz. If the satellite is not in range of a LUT when it receives a 121.5 MHz signal it will not be relayed. This effectively limits the use of 121.5 MHz EPIRBs via the satellites to a range of about 1,600 miles from any LUT. With the 22 available LUTs much of the world is covered, but there are some gaps, particularly in the Southern Ocean from where the signal will not be relayed. Because these satellites are in a comparatively low orbit - compare these altitudes with those of the Inmarsats at 36,000 kilometres - they can receive relatively low powered transmissions. An average 406 MHz EPIRB transmits with a power of about five watts and a 121.5 MHz EPIRB is usually under one watt. This means that the EPIRBs can be small because they do not require great big batteries to power them. Each satellite completes an orbit about every 100 minutes. The satellite will pass from horizon to horizon in about 15 minutes. During its orbit each satellite can receive signals from a swathe some 400 kilometres wide. As the earth turns beneath the satellites they each pass over a new strip every orbit. The maximum delay that a distressed vessel should experience in waiting for a satellite to pass within range is about 90 minutes. When it is activated, a 406 MHz EPIRB transmits a short coded message every 50 seconds. Each half-second burst indicates the identity of the vessel to whom the EPIRB is registered, but like the transmissions on 121.5 MHz it is still a very simple signal from a relatively unsophisticated transmitter. It is the satellite that does all the clever stuff. When the satellite detects a signal on either frequency it monitors the signal, measuring its frequency very accurately. Because of the speed of the satellite, and the low orbit enhances this effect, the frequency appears to change due to the Doppler Effect. This is the same effect that makes a train whistle sound as if it is changing pitch as the train comes towards us and then passes. The satellite is able to calculate the position of the EPIRB by measuring the Doppler shift. There is one slight problem. The calculation will produce two positions - one the mirror of the other, either side of the satellite's track. For a transmission on 121.5 MHz, the satellite will probably have to wait until the next pass to solve the ambiguity. Trouble starts if the next orbit does not pass close enough to the EPIRB to receive the signal. If this happens, the whole process has to start again with the next satellite. The signal from a 406 MHz EPIRB is much more stable, allowing more accurate measurements to be taken. The satellite resolves the ambiguity by doing an extra calculation taking the rotation of the earth into consideration. For 90% of the 406 MHz signals it receives, the satellite can fix the position of the EPIRB to within about five kilometres (a little over 2.5 miles) with only one pass. By comparison, the probable error in the position fix for a 121.5 MHz EPIRB is about seventeen kilometres (about 9 miles) and even at that it is likely to take two passes of the satellite to achieve the fix. The Cospas-Sarsat system can process up to ninety 406 MHz beacons which are activated at the same time - the mind boggles at what could cause such an occurrence! They are limited to handling a maximum of ten 121.5 MHz beacons simultaneously, but this limitation is unlikely to be a problem too often. The Cospas-Sarsat organisation is currently experimenting with a 406 MHz payload on a geostationary satellite. This would have the advantage of instant alerting - no waiting for a satellite to come over the horizon. But a geostationary satellite cannot, with present technology, fix the position of an EPIRB by examining the signal. The EPIRB would have to broadcast its position to the satellite, perhaps using a built-in GPS receiver. This is exactly the technique adopted by the Innarsat-E EPIRBs. They operate on one of the normal Inmarsat frequencies of 1.6 GHz. The EPIRB must either be programmed with its position, or more normally, they contain an integrated GPS receiver. This makes the unit more expensive and bigger than a 406 MHz unit. Because they are transmitting a much more complicated message, and it has to travel 36,000 kilometres instead of a mere 850-1,000 kilometres, they must be much more powerful. This also makes them bigger and more expensive. The benefit is that the alert is near enough instantaneous and, as well as an accurate position, other information can be included in the message to assist any rescuers. Compulsory GMDSS vessels must carry an EPIRB as part of the equipment list. For vessels limited to Area A1, this can be a VHF EPIRB. For vessels in any of the other areas A2, 3, and 4, it must be an EPIRB which can work through one of the satellite systems. Both the Cospas-Sarsat 406 MHz and the Inmarsat E EPIRBs are approved by the GMDSS.
	Operation of EPIRBs and Test Procedures
	For compulsory vessels, the EPIRB must be fitted in a 'float free' mounting. In such a mounting the EPIRB is held in place by a Hydrostatic Release Unit. This senses the increasing water pressure if a vessel sinks and at a predetermined depth the HRU releases the mount, allowing the EPIRB to float to the surface. A well designed mount will also cause the EPIRB to switch itself on as it is released, so it will start to operate automatically if the vessel sinks. For non-compulsory vessels such a mount is good to have, in case the vessel sinks very quickly before anybody has time to get the EPIRB from its stowage. EPIRBs should be examined for physical damage on a weekly basis. If the EPIRB is retained in its mount by an HRU, then the expiration date or service date on the HRU should be noted. The expiration date of the EPIRB's battery should also be noted. This is usually given on the maker's label or another plate affixed to the EPIRB. Most EPIRBs have a test switch. This should be operated on a weekly basis or in accordance with the manufacturer's recommendations. The test switch is usually spring loaded so it cannot be left on inadvertently and thus flatten the battery. A light will indicate that the test circuits are operating correctly. For compulsory vessels these tests must be logged each week. When operating a self-test, no signals are radiated and the EPIRB must not be tested by actual operation. If it is accidentally activated in the transmit mode, then it should be turned off at once and the false alarm cancelled by calling the nearest Coast Station and by making a safety call on the VHF radio for the benefit of any vessels in the vicinity. When used in an emergency, some EPIRBs must be floating in the water for their antenna to operate at peak efficiency. The maker's instructions will say if their EPIRB should be operating afloat or if it can be kept inside the liferaft. In either event, once the EPIRB is activated in a distress situation leave it switched on until the batteries are exhausted or until you have been rescued. There have been many cases reported where people kept turning the EPIRB on and off in an attempt to prolong the life of the battery. As we have already seen, except for an Inmarsat E EPIRB there can be a considerable delay before a satellite passes close enough to receive the signal from an EPIRB. If the satellite needs two passes to fix the position and the operator has turned the EPIRB off before the second pass, at best it will delay the satellite from fixing the position. It could also cause an unnecessary inaccuracy in the calculated position or even cause the satellite to dismiss the signal as erroneous. Once you have turned it on, leave it on. A new development, which as yet is not part of the GMDSS, is the STAtus RECording System. STAREC is somewhat akin to the 'Black Box' system of aircraft. It is being developed to explain the mysteries of vessels which vanish without trace. The STAREC unit is interfaced with various sensors mounted in the vessel and it records their readings at regular intervals. It retains the last several hours of readings, replacing the earliest recordings with the latest. The unit is self-contained with its own position fixing system and its own Innarsat-C terminal. In a distress situation the unit can either be jettisoned or it can float free, and on activation it will automatically contact a pre-programmed station. This could be a Rescue Co-ordination Centre or the perhaps the Owner's office. The unit will transmit a brief message, basically saying that it has been activated. It can then be interrogated and all the recorded data downloaded without the unit itself having to be located or retrieved. All relative systems on board the ship can be monitored and the recorded data should explain what went wrong when the distress situation arose. In the future, these units may well become GMDSS compulsory for some vessels.