Airframe and Powerplant

Getting a Better Signal

January 1, 2005

Why a new ELT may be a good investment

"The wreckage was 28.39 miles west of Lancaster at 34 degrees 50 minutes north latitude and 118 degrees 46 minutes west longitude about 5,086 feet mean sea level. The FAA Rancho Murrieta Automated Flight Service Station reported an emergency locator transmitter (ELT) [signal] to the Civil Air Patrol about 1530. According to the Air Force Rescue Coordination Center, because of satellite alignment requirements, an ELT signal may be delayed 2 or 3 hours before being received or confirmed." — NTSB airplane accident report

The ELT in your airplane is your ally. But most of us are allied with an old, tired ELT that may not be of much help when we need it most. Fortunately for the folks who are charged with reacting to ELT signals, and those who truly need help because of an airplane mishap, there are better allies available and costs are coming down.

ELTs were first installed in the early 1970s and by 1974 were required equipment in most general aviation airplanes. These first-generation ELTs transmit a warbling tone on 121.5 MHz when the unit is manually turned on by a pilot or crewmember or when an internal G (gravity) switch is activated by a sudden deceleration. Like many good ideas that are mandated by the government, the installation of ELTs seemed like a reasonable solution to the problem of locating airplane crash sites. First-generation ELTs and the battery packs that powered them were quickly rushed to market and before long the FAA started writing airworthiness directives to address shortcomings in these products.

The rush to mandate also created more problems simply because there was no proven method of locating an ELT signal. Search pilots attempted to locate signals by flying patterns and listening for changes in the ELTs' signal strength. These techniques are the build-and-fade method, using either a signal strength meter or VHF receiver squelch, and the wing-shadow method, in which the pilot would fly a track until the ELT signal was lost or blanked by the wing (ELT signals are in the VHF frequency band and therefore are line-of-sight signals). Signal loss would tell the pilot to turn 90 degrees right (or left) and search in the quadrant that was blanked by the wing. These coarse techniques combined with weak signal strength and loose frequency band tolerances in early ELT transmitters created chaos.

These first-generation ELTs — and make no mistake, many of us still have the first iterations installed in our airplanes — met the standards in original Technical Standard Order (TSO), C91. The first satellite with ELT signal-detecting abilities was launched by the Russians in 1982 — the U.S. version followed a year later. In 1988 four countries — the Soviet Union, the United States, France, and Canada — signed the International Cospas-Sarsat Programme Agreement to manage a satellite system to detect emergency beacons. The Cospas-Sarsat satellite system will be explained later.

The big difference

The standards in TSO-C91 apply to two-frequency (121.5 and 243 MHz) ELTs and refer to Radio Technical Commission for Aeronautics (RTCA) Document 183. The minimum standard for power output for these first-generation ELTs is only 50 milliwatts (50 thousandths of one watt) and the transmitter signal tolerances are wide. An apt analogy to describe the early ELTs is a dim flashlight with a dirty lens. The first-generation ELT signals are not strong nor are they particularly well focused.

Today the array of satellites that detect signals from 121.5-MHz ELTs obtains fixes on these signals, but because of the wide frequency tolerances they are only able to narrow the location of the signal down to a broad search area. Depending on whom you believe, this area can range in size from 12 to 29 miles on each side. Imagine driving 20 miles and then turning 90 degrees before driving another 20 miles. How long would it take to search for a downed airplane within the boundaries of the box you just drove. The size of the box is huge, especially when the new-generation 406-MHz ELTs (with more power and tighter transmitter frequency controls) cut that search area to a box that's one to three miles per side. Then there are the 406-MHz ELTs that interface with aircraft-borne GPS equipment or that have their own internal GPS receiver and are able to transmit a GPS location. These ELTs narrow the search area to a football field-size box. Using the flashlight analogy again, the 406-MHz transmitters shine a much brighter light through a much cleaner lens.

How are we doing?

Information supplied by Air Force Maj. Allan Knox, the assistant director of operations at the Air Force Rescue Coordination Center (AFRCC) at Langley Air Force Base in Virginia, proved that ELTs are busily transmitting every day. Unfortunately almost all of these signals are false alarms. The average number of false alarms generated by 121.5-MHz ELTs in 2003 was 531 per month or 18 per day. Only 4 percent of the signals were transmitted by airplanes actually in distress.

A second iteration of the TSO for ELTs was written in 1985. The new TSO required a few improvements such as environmentally sealed G switches, but there was no requirement to retire units built under the earlier TSO (C91). Maintenance shops could still install the first-generation ELTs in new ELT installations until June 21, 1995, when FAR 91.207 was amended — after that date all new ELT installations had to comply with the newer (TSO-C91a) standards. The standards your aircraft's ELT were built to are listed on the ELT data plate. This change also mandated maintenance requirements for ELTs, requiring maintenance personnel to inspect annually each ELT by testing G-switch operation, testing for the presence of a radiated signal, and inspecting for proper installation, functioning of controls, and battery corrosion. This annual requires a logbook entry specifying that the tests and inspections spelled out in 91.207(d)(1), (2), (3), and (4) have been completed.

The bulldog on 24-hour watch

The satellite network that detects ELT beacon signals consists of both polar-orbiting satellite and geostationary satellite arrays. Seven polar-orbiting satellites form a system called low earth orbit search and rescue (Leosar). When one of these satellites detects an ELT signal (121.5, 243, or 406 MHz) the signal is immediately relayed to a ground receiving station known as a local user terminal (LUT). So far, so good. But even at best, there's a minimum lapse of one and a half hours before the ELT can be reasonably located (each satellite orbits every 100 minutes but because of the array of satellites, each spot on Earth is scanned by at least one satellite every 45 minutes — it takes at least two passes to get a good ELT fix). This time can increase. Since orbiting satellites have a relatively small viewing window (3,720 miles or 6,000 kilometers wide) and the possibility exists that there may not be a LUT within the viewing window during every pass, it may take more than one orbit before the relayed signal from the ELT is picked up by a LUT.

Since the Leosar satellites move in relation to the Earth's surface, the locations of ELT signals are detected by measuring the relative movement (Doppler shift) of the satellite over the stationary signal. The big drawback to the 121.5-ELT signal is the length of time before what can best be described as a fuzzy fix is determined, and the amazing number of false alarms.

In contrast to the time lag inherent in the Leosar system, ELT signals picked up by the three satellites known as the geostationary array are detected instantly. This is the geostationary earth orbit search and rescue (Geosar) satellite system. This array is locked in geosynchronous orbit — as the Earth rotates, the satellites are always over the same position relative to the Earth. These three satellites are located much farther away from Earth's surface than the Leosar array and provide full coverage of every spot on the Earth from 70 degrees north to 70 degrees south latitudes. This system detects any ELT signal in an instant. That's a good thing, but unless the ELT is capable of sending its GPS location (only 406-MHz ELTs have this capability) the Geosar satellite must wait for the location-determining skills of the Leosar satellite array for a position location. It's true that the 406-MHz ELTs transmit sharper and stronger signals that enable a tighter search area determination, but there's still the time-lag factor because of the multiple pass requirements for all except GPS-encoded 406-MHz ELT signals.

The location of a GPS-enhanced 406-MHz signal, meanwhile, is almost instantaneous since it's not necessary for the multiple passes of one or more Geosar satellites for position determination.

Each 406-MHz ELT transmits a burst of data every 50 seconds that includes a key code identifying the ELT. This code identifies an information file that contains important contact information related to the owner. Installation of a 406-MHz ELT requires that the owner register the ELT ( to provide information for the contact information file. With this information on file, search-and-rescue personnel can, by placing a phone call or two, determine if the signal is valid and set the search-and-rescue machine in motion — or not.

There are a few drawbacks to the GPS-derived Geosar system — since the Geosar satellites are stationary, obstructions can block ELT signals. The moving Leosar satellites can almost always pick up a signal at some time during each pass. And the Geosar satellites don't provide coverage for Polar Regions. Today the combination of Leosar and Geosar satellites is a solid system.

February 2009

There's no requirement in the United States to replace the first- and second-generation 121.5-MHz ELTs, but the international federation that supports the Cospas-Sarsat system has announced that after February 1, 2009, the system will no longer detect 121.5- or 243.0-MHz distress beacon signals (a move AOPA is working against as the association believes that pilots should at least have the option of retaining their existing ELTs).

A pilot who has already invested in the latest in moving-map, Nexrad-equipped, paperless-cockpit technology in his or her panel — will likely have no problem justifying the value of a GPS-enhanced 406-MHz ELT.

Right now there's not a big surge toward installing these units. Take a look at the display ads by the big aviation supply houses in the latest issue of Trade-A-Plane. Hardly anyone even mentions a 406-MHz ELT. According to Wendell Neumeyer, director of sales and marketing at Artex Aircraft Supplies Inc., of Aurora, Oregon, Artex is the only U.S.-based company that currently sells a 406-MHz unit for installation. Artex chose AOPA EXPO 2004 to introduce its new low-cost (retails for $1,000 with antenna) 406-MHz model. This unit does not have nav capabilities, nor does it have an internal GPS receiver, but even at that, it's still capable of tightening the search area by a factor of up to 10. Artex's GPS-enabled 406-MHz units cost just more than $1,500.

A 406-MHz ELT is a lot like that little bit of money AOPA members pay for enrollment in the AOPA Legal Services Plan. No one plans to need legal advice or to get in an airplane crash, but these events take place every day.

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Maximize that ELT

OK, the unconscionable has happened — your airplane's down, but you have survived the crash and can still take action. Your ELT is your best tool to get help. How do you maximize that ELT's effectiveness?

  1. If you are able to access the ELT — you do know where your ELT is mounted, don't you? — move the switch to On and leave it there. Don't turn it off to "save" the batteries. The battery packs in all common ELTs have sufficient capacity to send out full-strength signals for 48 hours at minus 20 degrees Celsius.
  2. Visually inspect the ELT's antenna-to-ELT coaxial cable. It must be intact and connected at both ends.
  3. If your ELT has voice transmission capabilities — some 121.5-MHz ELTs do — locate a microphone and be ready to give voice instructions to search aircraft.
  4. If you're capable of moving to exposed or high ground, and the wreckage (and ELT) is located in a valley or canyon, consider removing the ELT from the airframe and moving it so the ELT signal won't be shielded by surrounding terrain or foliage.

And before you find yourself in this situation, take the time now to learn how to extend the antenna and how to operate the controls of your ELT. — SWE