Emergency Locator Transmitters are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress. Strictly, they are radio beacons that interface with Cospas-Sarsat, the international satellite system for search and rescue (SAR). When activated, such beacons send out a distress signal that, when detected by non-geostationary satellites (the old 121.5 ELT signal), can be located by triangulation. In the case of the new 406 MHz beacons which transmit digital signals, the beacons can be uniquely identified almost instantly via geostationary satellites, and furthermore, a GPS position can be encoded into the signal (thus providing both instantaneous identification and position). Often using the initial position provided via the satellite system, the distress signals from the beacons can be homed by SAR aircraft and ground search parties who can in turn come to the aid of the concerned boat, aircraft, or people. The Cospas-Sarsat system operates 24 hours a day 365 days a year.
There are three types of distress radio beacons compatible with the Cospas-Sarsat system:
- EPIRBs (emergency position-indicating radio beacons) signal maritime distress.
- ELTs (emergency locator transmitters) signal aircraft distress.
- PLBs (personal locator beacons) are for personal use and are intended to indicate a person in distress who is away from normal emergency services.
The basic purpose of distress radio beacons (ELTs) is to get people rescued within the so-called "golden day" (the first 24 hours following a traumatic event) when the majority of survivors can still be saved.
Since the inception of Cospas-Sarsat in 1982, distress radio beacons have assisted in the rescue of over 24,500 people worldwide and over 6000 people in the United States alone. In 2006, distress radio beacons aided in the rescue of 1,881 people in 452 distress situations.
The beginnings of Sarsat date back to 1970 when a plane carrying two U.S. congressmen crashed in a remote region of Alaska. A massive search and rescue effort was mounted, but to this day, no trace of them or their aircraft has ever been found. In reaction to this tragedy, congress mandated that all aircraft in the United States carry an Emergency Locator Transmitter (ELT). This device was designed to automatically activate after a crash and transmit a homing signal. Since 1973, Emergency Locator Transmitters (ELTs) have been mandated on almost all civilian aircraft in the United States.
Since satellite technology was still in its infancy, the frequency selected for ELT transmissions was 121.5 MHz, the international aircraft distress frequency. This system worked, but had many limitations. The frequency was cluttered, there was no way to verify who the signal was originating from, and most importantly, another aircraft had to be within range to receive the signal.
After several years, the limitations of ELTs began to outweigh their benefits. At that time, a satellite based system was conceived. It would operate on a frequency reserved only for emergency beacons (406 MHz), it would have a digital signal that uniquely identified each beacon, and it would provide global coverage.
The SARSAT (Search and Rescue Satellite Aided Tracking) system was developed in a joint effort by the United States, Canada, and France. In the United States, the SARSAT system was developed by NASA. Once the system was functional, its operation was turned over to NOAA (National Oceanic and Atmospheric Administration) where it remains today.
As the system began to take hold, more and more emergency beacons found their way onto the market. ELTs continued to operate exclusively on 121.5 MHz, but maritime beacons (EPIRBs) were being built that operated on 406 MHz. The U.S. Coast Guard in their role as maritime search and rescue specialists immediately began to see the benefits of 406 MHz, and in 1990, took proactive steps to bring it into widespread usage. As a result, today there are over 75,000 EPIRBs in the NOAA 406 MHz Registration Database. With 406 MHz ELTs and PLBs the number of 406 MHz Emergency Beacons registered now totals over 82,000!
A similar system, COSPAS (a Russian acronym translating to: Space System for the Search of Vessels in Distress), was developed by the Soviet Union. The four nations, United States, Canada, France and the Soviet Union banded together in 1979 to form Cospas-Sarsat. In 1982, the first satellite was launched, and by 1984 the system was declared fully operational. These satellites, along with a network of ground stations and the U.S. Mission Control Center in Suitland, Md., are part of the International Cospas-Sarsat Program, whose mission is to relay distress signals to the international search and rescue community.
Although Cospas-Sarsat satellites were primarily designed to function on the much improved 406 MHz frequency, they still had to make a provision for the thousands of 121.5 MHz beacons already in use. For this reason, the satellites were designed to receive 121.5 MHz as well.
Since that time, however, 121.5 MHz beacons have become a source of chronic false alerts. Not to mention, they are not as accurate nor as advanced as the improved 406 MHz beacons. Because of their continual problems a decision was made internationally that on February 1, 2009 the Cospas-Sarsat satellites will no longer detect any 121.5 MHz beacons.
The Cospas-Sarsat organization also continued to grow. The four original member nations have now been joined by 36 other nations that operate 66 ground stations and 29 mission control centers worldwide or serve as Search and Rescue Points of Contact. Cospas-Sarsat continues to be a model of international cooperation. During the eighties, the Soviet Union and the United States were able to put aside their Cold War differences and tackle some tough technical questions. Today, new technology continues to evolve and the member nations are actively incorporating that technology into The Cospas-Sarsat System of Tomorrow
ELTs were the first emergency beacons developed and most U.S. civil aircraft are required to carry them. ELTs were intended for use on the 121.5 MHz frequency to alert aircraft flying overhead. Obviously, a major limitation to these is that another aircraft must be within range and listening to 121.5 MHz to receive the signal. One of the reasons the Cospas-Sarsat system was developed was to provide a better receiving source for these signals. Another reason was to provide location data for each activation (something that overflying aircraft were unable to do).
Different types of ELTs are currently in use. There are approximately 170,000 of the older generation 121.5 MHz ELTs in service. Unfortunately, these have proven to be highly ineffective. They have a 97% false alarm rate, activate properly in only 12% of crashes, and provide no identification data. In order to fix this problem 406 MHz ELTs were developed to work specifically with the Cospas-Sarsat system. These ELTs dramatically reduce the false alert impact on SAR resources, have a higher accident survivability success rate, and decrease the time required to reach accident victims by an average of 6 hours.
Presently, most aircraft operators are mandated to carry an ELT and have the option to choose between either a 121.5 MHz ELT or a 406 MHz ELT. The Federal Aviation Administration has studied the issue of mandating carriage of 406 MHz ELTs. The study indicates that 134 extra lives and millions of dollars in SAR resources could be saved per year. The only problem is that 406 MHz ELTs currently cost about $1,000 and 121.5 MHz ELTs cost around $250. It's easy to see one reason for the cost differential when you look at the numbers. However, no one can argue the importance of 406 MHz ELTs and the significant advantages they hold. There are currently 430,000 Cospas-Sarsat 406 MHz distress beacons in operation worldwide.
Due to the obvious advantages of 406 MHz beacons and the significant disadvantages to the older 121.5 MHz beacons, the International Cospas-Sarsat Program have made a decision to phase-out 121.5 MHz satellite alerting on February 1st, 2009. All pilots are highly encouraged both by NOAA and by the FAA to consider making the switch to 406!
Cospas-Sarsat made the decision to cease satellite processing on 121.5/243 MHz in response to guidance from the International Civil Aviation Organization (ICAO) and the International Maritime Organization (IMO). These United Nations organizations mandate safety requirements for aircraft and maritime vessels and have recognized the limitations of the 121.5/243 MHz beacons and the superior capabilities of the 406 MHz alerting system.
The digital 406 MHz beacons offer many advantages over analog 121.5/243 MHz beacons. With a 406 MHz beacon, the position of the distress can be relayed to rescue services more quickly, more reliably and with greater accuracy.
COMPARISON OF 406 MHz TO 121.5 MHz BEACON
Signal - 406 MHz Digital: unique identification, registration data provides information on the specific owner or aircraft
121.5 MHz Analog: no data encoded, higher false alert rate
Signal Power - 406MHz: 5 Watts pulse (strong output - 50 times stronger)
121.5 MHz: 0.1 Watts continuous (weak output - hard for satellites to detect)
Coverage - 406 MHz: Global (100% of the earth)
121.5 MHz: Regional (only 2/3 of the world is covered)
Position Accuracy - 406 MHz: Location within 1-3 NM, less than 6 square yards with GPS equipped beacons
121.5 MHz: Location within 12-15 NM (no GPS capability)
Alert Time - 406 MHz: Detection by geostationary satellites in less than 5 minutes worldwide
121.5 MHz: Waiting time for low earth orbiting satellite pass 45 minutes average
Doppler Position Ambiguity - 406 MHz: Resolution possible at first satellite pass
121.5 MHz: Two passes required to resolve position ambiguity
False Alerts - 406 MHz: 1 in 12 alerts are actual distress (low false alerts due to digital coded signals)
121.5 MHZ: Fewer than 2 in 1000 are actual distress (very high false alerts due to frequency interference)
With a 121.5/243 MHz beacon, only 1 alert out of every 500 alerts is a genuine distress situation. This has a significant effect on the resources of search and rescue (SAR) services. With 406 MHz beacons, false alerts have been considerably reduced (1 alert in 12 is genuine) and when properly registered can normally be resolved with a telephone call to the beacon owner using the encoded beacon identification. Consequently, real alerts can receive the attention they deserve.
When a 406 MHz beacon signal is received, SAR authorities can retrieve information from a registration database. This includes beacon owner contact information, emergency contact information, and vessel/aircraft identifying characteristics. Having this information allows SAR services to respond appropriately. Make sure your 406 MHz beacon is properly and accurately registered! To register, go to www.beaconregistration.noaa.gov.
