The basic purpose of the emergency beacons is to get people rescued within the "golden day" when the majority of survivors can still be saved.
Between 1982 and 2002, these systems enabled the rescue of 14,700 people. As of 2002, there are roughly 82,000 registered beacons,and over 500,000 of the older unregistered type.
Most beacons are brightly-colored, waterproof, fit in a cube about 30cm on a side, and weigh 2-5 kg. They can be purchased from marine suppliers, aircraft refitters (and in Australia and Alaska) hiking supply stores. The units have a useful life of 10 years, operate across a range of conditions (-40°C to 40°C), and transmit for 24 to 48 hours. As of 2003 the cost varies from US$139 to US$1200, with varying performances (see below).
There are two types: manually activated, and automatically activated.
In the U.S., offshore beacons are rescued by the Coast Guard, or on-shore beacons by local search and rescue services in Alaska. In the U.S. there are no published notification systems for other locations.
In the U.S. no special license is required beyond purchase and registration.
In some jurisdictions, larger boats and ships are required to carry them.
Registration
Modern emergency beacons transmit a serial number. When the beacon is purchased this number should be registered with the relevant national authority. Registration provides the national authority with phone numbers to call, and a description of the vessel signaling. The phone numbers can give most of the information needed for the rescue. Also, they provide an easy way for the system to check and eliminate false alarms.
How they work
All the systems work something like this: The beacon transmits to a satellite. The satellite transmits the beacon's data, including location, to its ground control station. The satellite's ground station forwards the data to a national authority. The national authority forwards the data to a rescuing authority. The rescuing authority makes the rescue. Once the satellite data is in, the forwarding processes are less than a minute to any signatory nation.
There are several systems in use, with beacons of varying expense, different types of satellites and varying performance.
The most modern 406 MHz beacons with GPS (US$ 1200 in 2002) locate a beacon to 100 meters, anywhere in the world, and send a serial number so the government authority can look-up phone numbers to notify next-of-kin in four minutes, with rescue commencing shortly afterward. The GPS system permits unmoving, wide-view geosynchronous communications satellites to report the location (doppler location, see below, is not needed). If any of the system breaks down, yet the beacon works, it defaults to the performance of the compatible intermediate technology beacon, below.
An intermediate technology 406 MHz beacon (US$ 900-500) has world-wide coverage, locates within 2 km. (12.5 sq. km search area), notifies kin and rescuers in 2 hours maximum (46 min avg.), and has a serial number to look up phone numbers, etc. This can take up to two hours because it has to use moving weather satellites to locate the beacon. To help locate the beacon, the beacon's frequency is controlled to 2 parts per billion, and its power is a hefty five watts.
Both of the above types of beacons usually include an auxiliary 25 milliwatt beacon at 121.5 MHz to guide rescue aircraft.
The oldest, cheapest (US$ 139) beacons send an anonymous warble at 121.5 MHz. They work over only 60% of the earth, require up to 6 hours for notification, locate within 20 km (search area of 1214 sq. km.) and are anonymous. Coverage is partial because the satellite has to be in view of both the beacon and a ground station at the same time - the satellite does not store and forward the beacon's position. Coverage in polar and south-hemisphere areas is bad. The frequency is the standard aviation emergency frequency, so false alarms are common. To reduce false alarms, a beacon is confirmed by a second satellite pass, which slows notification to 4 hours. Also, the beacons can't be located as well because their frequency is only accurate to 50 parts per million, and they send only 75 milliwatts.
By international agreement, these original 121.5 MHz (civil) and 243 MHz (military) beacons will no longer be sensed by satellites starting in 2009.
Note that even the oldest system is an immense improvement in safety.
When the beacon has no GPS receiver, the system locates the beacon from its doppler shift as received by the quickly-moving satellite. Basically, the frequency received varies depending on the speed of the beacon relative tot he satellite. The amount of doppler is proportional to the range and bearing to the satellite. The instant the beacon's doppler shift changes from high to low indicates the time when the bearing from the beacon to the satellite's ground track is 90 degrees. The side of the satellite track is determined because the rate of change of the doppler is faster when the Earth is turning towards the satellite track.
In order to handle multiple beacons, modern 406 MHz beacons transmit in bursts, and remain silent for a few seconds. This also conserves transmitter power.
The Russians developed the original system, and its success drove the desire to develop the improved 406 MHz system. The original system is a brilliant adaptation to the low quality beacons, originally designed to aid air searches. It uses just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listen to each satellite as long as it is above the horizon. Doppler is used to locate the beacon(s). Multiple beacons are separated when a computer program performs a fourier transform on the signal. Also, two satellite passes per beacon are used. This eliminates false alarms by using two measurements to verify the beacon's location from two different bearings. This prevents false alarms from VHF channels that affect a single satellite. Regrettably, the second satellite pass almost doubles the average time to notification of the rescuing authority. However the notification time is much less than a day.
Satellites used
Receivers are auxiliary systems mounted on several types of satellites. This substantially reduces the program's cost.
The weather satellites that carry the SARSAT receivers are in "ball of yarn" orbits, inclined at 99 degrees. The longest period that any can be out of line-of-sight of a beacon is about two hours.
The first satellite constellation was launched by Soviet Russia. Making it available internationally was a substantial and generous humanitarian act.
Some geosynchronous satellites have receivers. Since they see the Earth as a whole, they see the beacon immediately, but have no motion, and thus no doppler frequency shift to locate it. If the beacon transmits GPS data, the geosynchronous satellites give the fastest response.
History
The original impetus to the program in the U.S. was the loss of two congressmen in the Alaskan wilderness in 1970. A massive search effort failed to locate them. The result was a U.S. law mandating that all aircraft carry an emergency locator transmitter. Technical and organizationaly improvements followed.
COSPAS/SARSAT is an international organization that has been a model of international cooperation, even during the Cold War. SARSAT means Search And Rescue SATellite. COSPAS is a Russian acronym that means SARSAT. A consortium of Russia, the U.S., Canada and France formed the organization in 1982. Since then 29 others have joined.
COSPAS/SARSAT defines standards for beacons, auxiliary equipment to be mounted on conforming weather and communication satellites, and communications methods. The satellites communicate the beacon data to their ground stations, which forward it to main control centers of each nation that can initiate a rescue effort.
The older beacons just made a warble on 121.5 MHz, the standard aircraft emergency frequency. They gave many false alarms, because this frequency can be used in other ways. With these beacons, standard practice is to confirm the beacon with a second satellite pass. This can take up to four hours.
To combat false alarms, modern 406 MHz beacons send a serial number. By international agreement, the frequency is dedicated only to emergency beacons.
The satellite control center uses the registration number to route notification to the main control center of the beacon's owner's home country. The center looks up the registration number to get contact phone numbers, vessel description etc. Then, they call the phone numbers.
A phone call eliminates most false alarms, and can often give enough information to permit scrambling a helicopter even if the alarm was received by a geostationary satellite, and the doppler satellite's locating pass is not yet complete.
Another problem was that the older 121.5 MHz beacons had low power (75 milliwatts) and less-accurate frequencies (only 50 parts per million). The search area was therefore more than 1315 square kilometers.
The newer 406.025 MHz beacons have 5 watts of power, and two parts per billion of frequency accuracy, which reduces the search area to less than 20 square kilometers. If the new beacon sends coordinates from a GPS receiver, the search area is only 0.0003 square kilometers (300 square meters).
With the 121.5 MHz EPIRBs, alarms in major metropolitan areas are usually investigated by a volunteer search-and-rescue organization, or a low-priority police call. Alarms offshore, or in wilderness areas get much higher priority.
The U.S. Coast Guard once promoted an emergency beacon on maritime VHF emergency channels. It now promotes the superior COSPAS/SARSAT system, and no longer services emergency beacons on maritime VHF frequencies.
References:
COSPAS-SARSAT, Document C/S T.001 October 99
RTCM, Standard for 406 MHz Satellite EPIRBs
FCC, Part 80 and GMDSS
MED, 0735/2001