Emergency position-indicating rescue beacon

Emergency position-indicating rescue beacons (EPIRB) are small radio transmitters that some satellites and search and rescue aircraft can use locate people or boats needing rescue. They are a component of the Global Maritime Distress Safety System.

On aircraft, the same device is more usually called an emergency locator transmitter. (ELT) When carried by a person, it is called a personal locator beacon. (PLB) See the U.S. center's website (http://www.sarsat.noaa.gov).

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 30 cm on a side, and weigh 2-5 kg. They can be purchased from marine suppliers, aircraft refitters, and (in Australia and the United States) 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$3000, with varying performances (see below).

There are two types: manually activated, and automatically activated.

In the U.S., offshore beacons are investigated and victims rescued by the Coast Guard. On-shore beacons are investigated by local search and rescue services in Alaska. The Air Force Rescue Coordination Center is charged with land-based emergency signals, usually dispatching volunteer members from The United States Air Force Auxiliary Civil Air Patrol. In the U.S. there are no published notification systems for other locations.

In the U.S. no special license is required, but serial-number registration is required. Nearly all U.S.-registered aircraft are required to carry an ELT. In some jurisdictions, larger boats and ships are required to carry them.

Contents

1 Current types 2 Obsolete types 3 Registration 4 How they work 5 Satellites used 6 History 7 See also 8 References 9 External links

Current types

Current EPIRBs are generally divided into three classes; Category I, Category II, and Class B (or Category B).

• Category I EPIRBs are considered the best but are also the most costly. Category I EPIRBs can be either activated manually or set to activate automatically in the event of a disaster at sea. These EPIRBs are generally housed in a specially designed bracket on deck and the buoyant beacon is designed to rise to the surface and emit two signals, an emergency homing signal on 121.5 MHz and a digital identification code on 406 MHz that can be used to identify the stricken vessel.

• Category II EPIRBs are similar to Category I EPIRBs but are generally manual activation only. Category II EPIRBs are also generally less costly averaging less than US$1,000.

• Class B EPIRBs, also called Category B or "Mini B", operate a 121.5 MHz homing signal only and are usually manual activation only units. They are the cheapest units but also the least capable. Due to their limitations, Category B EPIRBs are slowly being phased out. The International COSPAR-SARSAT program will no longer monitor Category B EPIRB signals as of February 1, 2009. Although the U.S. Coast Guard no longer recommends them, they remain in wide use.

Obsolete types

There are also several older types of EPIRB devices which are no longer recommended for use.

• Class A - A 121.5 MHz automatic activation unit. Due to limited signal coverage and possible lengthy delays in signal recognition, the U.S. Coast Guard no longer recommends use of this type.

• Class C - Operates on VHF channel 15/16. Designed for small crafts operating close to shore, this type was only recognized in the United States. Use of these units was phased out in 1999.

• Class S - A 121.5 MHz unit similar to Class B but is often included as an integral part of a lifeboat or survival suit. Their use is no longer recommended by the U.S. Coast Guard.

• Inmarsat E, entered service in 1997. The unit is an automatic activation unit operating on 1646 MHz and detectable by the Inmarsat geostationary satellite system. This class of EPIRB was approved by the Global Maritime Distress Safety System (GMDSS), but not by the United States. In September 2004, Inmarsat announced that it was terminating its Inmarsat E EPIRB service as of December 2006 due to a lack of interest in the maritime community.

Furthermore, the U.S. Coast Guard recommend that no EPIRB of any type manufactured before 1989 be used.

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 signaling vessel, including its home port. The registration can give much of the information needed for starting the rescue. Also, they provide an easy way for the notification services to check and eliminate false alarms quickly.

How they work

All the systems work something like this: A beacon is activated by a crash, a sinking, or manually. The beacon's transmission is picked up by one or more satellites. The satellite transmits the beacon's signal to its ground control station. The satellite's ground station processes the signals and forwards the data, including approximate location, to a national authority. The national authority forwards the data to a rescuing authority. The rescuing authority uses its own receiving equipment to locate the beacon and makes the rescue or recovery. 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-$3000 in 2002) locate a beacon with a precision of 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 stationary, 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 km² 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 km²) 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 satellites do 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, and there is interference from other electronic and electrical systems, 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 75-100 milliwatts of power.

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 satellites. Basically, the frequency received varies depending on the speed of the beacon relative to the 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 simultaneous 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 shift 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 before 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 all satellites can be out of line-of-sight of a beacon is about two hours.

The first satellite constellation was launched in the early 1970s by Soviet Russia, Canada, France and the U.S.

Some geosynchronous satellites have beacon receivers. Since end of 2003 there are four such geostationary satellites (GEOSAR) that cover more than 80% of the surface of the earth. As with all geosynchronous satellites, they are located above the equator. The GEOSAR satellites do not cover the polar caps.

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. However, if the beacon transmits GPS data, the geosynchronous satellites give nearly instantaneous 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 organizational 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, ground stations, 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 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.

See also

Search and rescue radar transponder

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

External links

• NOAA SARSAT website (http://www.sarsat.noaa.gov/)
• NOAA notice of planned phasing out of 121.5/243 MHz beacons in 2009 (http://www.sarsat.noaa.gov/121phaseout.pdf)