Building Basics: The ELT-Past and Future

The problem with the 121.5 frequency and the legacy. ELTs that provide signals to it comes down to one issue: reliability. There are problems with false alarms, ...
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building basics

The ELT–Past and Future ELT changes aim to improve safety G EO RGE R. WIL HEL MSEN

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he transponder and emergency locator transmitter (ELT) are two pieces of avionics in the average plane that go largely unappreciated by the pilot. Unlike the avionics that allow the pilot to communicate and navigate, these two devices serve only to inform others of your position, either while in flight or after an accident. In the case of the ELT, the unit is little more than a nuisance until the unthinkable happens, and the airplane crashes. The ELT became a required piece of flight equipment as a result of the loss of two congressmen in October 1972. While flying to attend a political event, House Majority Leader Hale Boggs and Rep. Nick Begich were lost when their Cessna 310 disappeared on a flight from Anchorage to Juneau, Alaska. After the ensuing search was unable to find the missing plane or occupants, a new law was created that mandated the installation of an ELT in nearly all aircraft. Some planes were excluded based on their type of operations, but for the most part, if you were flying a small plane, you had to install one. The conventional ELT system operates on a frequency of 121.5 MHz, which made perfect sense since that frequency

was the original international distress frequency. However, signal strength, frequency clutter, and other issues quickly led to the expansion of the system to include the 121.5 and 243 MHz frequencies. Signals from ELTs are currently sensed from satellites, which triangulate the location of the beacon to provide the approximate location of the downed aircraft. From there, search and rescue teams attempt to find the aircraft and its occupants in time to save the lives of those on board. The satellites are intended to detect faint signals, and dispatch rescuers even before a plane is identified as overdue, which in turn increases the potential that a plane can be found. However, problems with the system make this a challenge in the real world. The problem with the 121.5 frequency and the legacy ELTs that provide signals to it comes down to one issue: reliability. There are problems with false alarms, with studies showing the false alarm rate is 99 percent. That means that out of every 100 signals that are received and processed by the satellites, only one constitutes an actual downed aircraft. Detailed studies provided some of the sources of these false alarms, which ranged from

Detailed studies provided some of the sources of these false alarms, which ranged from pizza ovens to sports stadium scoreboards, to ATM machines, and even the occasional plane with a hard landing.

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building basics

Image Courtesy Of Cospas-Sarsat

Image Courtesy Of NASA

> The 406 MHz COSPAS-SARSAT system employs satellites in geostationary orbits that cover most of the planet, augmented by low-Earth orbit satellites in polar oribits. The signals are downlinked to local user terminals, where they are processed and the decision is made on where to launch search and rescue operations.

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pizza ovens to sports stadium scoreboards, to ATM machines, and even the occasional plane with a hard landing. There are issues with the rate at which the units actually initiate, with the older units failing to activate when intended about twothirds of the time. Evidence also suggests the units broadcast a satellite-usable post-crash signal only a tenth of the time. When you add those challenges in with the signal strength and the average time it takes to triangulate the location of a downed aircraft, you can see where the need for new ELTs is based. This means the satellite monitoring system has to weed through hundreds of false signals every day, trying to separate the valid signals from the background noise. The original search system, dubbed SARSAT, was founded by the United States, France, and Canada, and was developed by NASA. In addition to being used by aircraft, the system is also used by hikers, boaters, and other outdoor enthusiasts. Alerts from non-aviation sources far outnumber those from aircraft. The former Soviet Union developed its own system dubbed COSPAS, and both systems joined together, COSPASSARSAT, to improve search-and-rescue capabilities in 1979. The first satellite was launched for the new system in 1982, with the system reaching full operation two years later.

The New Standard Studies of ELT effectiveness made it clear there was a problem. The international community that oversees the operation of the search-and-rescue satellite network asked for a new standard that would help to eliminate the false signals and improve search capabilities. This need for improved performance is what drove the current changeover to the new

system. As a result, the International COSPAS-SARSAT Program will stop processing the old analog 121.5/243 MHz signals and require the use of 406 MHz ELTs by February 1, 2009. The new ELTs are a significant improvement in technology over what was previously available. The digital transmitter broadcasts in bursts at 5 watts, which is a huge improvement over the analog 0.1 watt transmit power of the 121.5 frequency ELTs. The stronger signal helps overcome some of the shortcomings of the traditional model, such as signal blanking by aircraft wreckage or terrain.

The digital transmitter broadcasts in bursts at 5 watts, which is a huge improvement over the analog 0.1 watt transmit power of the 121.5 frequency ELTs. Equally important, the 406 MHz system is not subject to most of the false positives that have plagued the 121.5 MHz system. This means that when a 406 MHz frequency ELT alarm is received, the search and rescue teams can be dispatched promptly, instead of being delayed by the review process to validate whether a signal is real or the result of someone scoring a touchdown somewhere. The new system will provide worldwide coverage, and will allow aircraft positions to be pinpointed to within 2 nautical miles—better if the ELT is connected to a GPS receiver. It also allows false alarms to be screened more easily, since the identity of the owner of the specific unit broadcasting an alert will be on file. When you contrast the 2 nautical mile radius identification range with EAA Sport Aviation

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The Artex ME406 ELT can transmit on both the current 121.5 and the new 406 Mhz frequencies through the same antenna at an affordable price.

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the original 121.5 MHz ELT’s ability to locate a downed aircraft within an approximate 12 nm radius, you can see where your chances of being picked up are improved. Worst case, you have less than 12.6 square nautical miles that must be searched to find a downed aircraft, which is a big improvement over the 452.4 square nautical miles “pinpointed” by the previous system. Obviously, the new standard provides a better chance to find the plane than the old system ever would. Many of the 406 MHz systems are designed to be connected to a GPS system, and some have their own GPS receivers. A GPS signal will further improve the accuracy of the search from a 2 nm radius to around 100 meters. This would make it even more likely that the search and res-

cue teams would be able to find you in time to save the lives of those on board after a crash.

Gearing Up for 406 MHz Since the timeline has been set, the best approach is to start planning now to install a 406 MHz ELT in early 2009—less than three years away. To ease this transition for pilots, Artex has created a “plug-and-play” replacement that will allow you to upgrade to the new 406 MHz standard with a minimal installation expense, which in turn will allow owners of aircraft with such ELTs to make the transition at a reduced price. Other manufacturers are expected to follow this trend in the future. If it is any consolation, all other

users of emergency locators—also known as personal locator beacons— are facing the same issues. They face the same deadline to convert to the 406 MHz models as their aviation counterparts, and are working to meet that deadline.

Where they originally started out at more than $5,000 plus installation, prices have dropped for many models into the $1,500 range, making them more realistically priced for pilots who operate on tight budgets to be able to afford to install them. When you think about an ELT, you probably think of it as something that your skills as a pilot should allow you to never need. However, while your efforts will result in the unit consuming batteries only, all it takes is one unexpected event such as a lightning strike, a random engine failure, or a gasket that leaks and causes your engine to run out of fuel or oil, and you may be thanking the new ELT for allowing rescuers to find you quickly.

A GPS signal will further improve the accuracy of the search from a 2 nm radius to around 100 meters. Increased demand for 406 MHz ELTs has induced more manufacturers to get into the market with products. The demand, along with the competition of the new manufacturers, has caused the prices of 406 MHz models to plummet since their appearance in aircraft in 1994.

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