EPIRB Fundamentals: 121 vs. 406 MHZ - SailNet Community
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EPIRB Fundamentals: 121 vs. 406 MHZ

Three months after the Titanic sank in 1912 SOS was adopted as the international distress call. The signal was adopted because it was easily recognizable in the Morse Code -- three dots, three dashes and three dots -- and not because SOS stood for anything such as "save our souls" or "save our ship," as is popularly believed.

If you don't know your Morse Code not to worry. Morse Code officially went out of use last February 1, as the dots and dashes had years ago given way to a satellite-aided tracking (Sarsat ) search and rescue system that led to the Epirb, or emergency position indicating radio beacon. The satellite system can pinpoint the location of a vessel signaling for help within 200 meters. Distress signals are beamed from a boat or ship to a satellite, which relays the alert to a rescue coordinating station on the ground. Since the satellites fly over the poles on every orbit, coverage is best there and least at the equator. In the mid-latitudes the average waiting time for a satellite pass is 30 to 45 minutes.

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), a device designed to automatically activate after a crash and transmit a homing signal.

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 United States, Canada and France developed the Sarsat system in a joint effort. In the United States, NASA developed the Sarsat system. Once the system was functional, its operation was turned over to NOAA 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 Epirbs, maritime beacons that operated on 406 MHz, were built. In their role as maritime search and rescue specialists, the U.S. Coast Guard 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 now more than 33,000 Epirbs in the NOAA 406 MHz registration database.

The Soviet Union developed a similar system, called Cospas. 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. 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.

There are important differences between the two frequencies:

rapid fix 4061. The 406 MHz signal is digital and can be stored aboard the satellite for later relay to the next available ground station, thereby giving it global capability and a 1-hour average notification time. The 121.5 MHz signal is analog and is not stored aboard the satellite. The satellite must see the beacon and the ground station simultaneously for a 121.5 MHz transmission to be detected; thereby giving an average notification time of 6 hours.

2. The 406 MHz signal contains digital information unique to each beacon, which provides a link to information, contained in a registration database. The registration information helps the Sar forces identify the vessel or aircraft in distress and greatly speeds up response time. The 121.5 MHz beacon is not capable of data encoding.

3. The 406 MHz beacons transmit a 5-watt burst of data for half a second every 50 seconds, creating a very stable signal that can be more accurately fixed in comparison with the 121.5 MHz signal. By using Doppler shift, the 406 MHz beacon's location can be pinpointed to a two nautical-mile radius whereas the 121.5 MHz signal has a 12 nautical-mile radius. The newest 406 MHz Epirbs have a GPS receiver and transmits its exact coordinates to a .05 nautical mile radius.

As you might suspect, there are also important differences in the prices. A 121.5 MHz beacon is about $200, whereas the 406 MHz Epirb is closer to $2,000 and with the GPS option the price jumps to over $5,000. Today's newer 406 MHz beacons also transmit on the 121.5 MHz frequency and come in several categories. Category I Epirbs are automatic float free, while the Category II must be manually activated and released.

The Cospas-Sarsat system provides a tremendous resource for protecting the lives of mariners that was unthinkable prior to the space age. With a Epirb rescue forces can be quickly summoned from anywhere on earth 24 hours a day, 365 days a year.

If you go offshore or do long-distance cruising you should have an Epirb on board. This is another piece of equipment that helps to bring peace of mind and no matter how expensive, they are still cheep when compared with the safety of your crew. Let's just hope that you never have to use them.

Jim Sexton is offline  
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