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The 406Mhz Emergency
Locator Transmitter
In another article we covered the conventional Emergency Locator
Transmitter (ELT), which transmits on the frequencies of 121.50Mhz
and 243.00Mhz. We also discussed some of the problems associated with
this ELT such as containing a transmitter that is prone to drift off
frequency and a false alarm rate of 95.7% along with the whippy 100mW
transmitter. We discussed how 121.50Mhz ELTs can only get their target
aircraft with 450 square miles but a 406Mhz ELT can get the target
aircraft within 12.5 square miles. Unless things change, new laws will
require you to upgrade to the superior 406Mhz ELT by Feb 1, 2009. Before diving into the modern marvels of 406Mhz, let’s
discuss what brought ELTs into existence. The beginning of Sarsat dates
back to 1970 when an aircraft carrying a couple of congressmen crashed
in the remote regions of Alaska. A massive search was started but to
this day, no trace of the congressmen or the aircraft have ever been
found. Since congressmen often fly, they decided to come up with some
laws to protect themselves and that was requiring an aircraft to carry
an ELT. One of the requirements of the ELT is it must automatically
activate and transmit a homing signal if the aircraft hits the ground
somewhat harder than normal. At that time in history satellite
technology was in its infancy, thus the frequency selected for ELT
transmissions was 121.50Mhz. This frequency was set aside as the
international aircraft distress frequency. Basically you flew along in
the aircraft and monitored 121.50Mhz on your VHF com radio and tried to
find the downed aircraft this way. This method was poor at best. You had
to be flying in the area of the crash, and there was no way to verify
where the signal originated. Even today, folks often use 121.50Mhz to
chat on and believe me, that doesn’t help the system any. After
several years, the limitations of ELT’s 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 (406Mhz), it
would have a digital signal that uniquely identified each beacon and
would provide world coverage. Once this system was up and running,
operation and monitoring were turned over to the National Oceanic and
Atmospheric Administration (NOAA). As this system grew, more emergency
beacons found their way into the market. NOAA not only monitors
121.50Mhz but 406Mhz from aircraft, EPIRB’s (boat stuff) and personal
ELTS came into life. A similar system was created by Russia. Four
nations, USA, 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. While
Cospas-Sarsat satellites were primarily designed to for 406Mhz, they
still have provisions to monitor 121.50Mhz. The original four member
nations have now been joined by 25 other nations that operate 28 ground
stations and 15 mission control centers worldwide or serve as search and
rescue points of contact. The keystone of the Cospas-Sarsat System is
the group of polar orbiting satellites from which the system takes its
name. Just what is the difference between a 406Mhz and a 121.50Mhz ELT? It’s much easier to explain what they have in common because there isn’t much. Both transmit a signal in hopes you will be found should your flight come up short of the destination and that’s where the similarities end. Unlike the 121.50Mhz ELT, which transmits a continuous 100mW signal, the 406Mhz ELT transmits a whopping five watts for every 50 seconds for one-half second. Because the system isn’t always transmitting, battery life is fairly long. Transmitting in short bursts allows the satellite to track up to 90 ELTs at once. The 406Mhz ELT was designed specifically for satellite detection and Doppler location and provides the following.
Frequency stabilization of the 406Mhz transmitter is paramount.
406 ELT transmitters have a much tighter tolerance than the older 121.50
ELT, thus Doppler location is far more accurate. Emergency 406 systems
have the ability to transmit information for identification. The 406Mhz sounds great but how do you test it on the ground? Unlike the 121.50 ELT, you can’t just turn the switch on, and listen to the ELT over the radio. The FAA calls out for testing the 121.50Mhz system under FAR 91.207 but I couldn’t find any requirement for testing the 406Mhz system. If you can find one please drop me a note. To ground test this system you would have to either send it to the manufacturer or find an avionics shop with the proper test equipment which may be difficult. 406 ELTs offer navigation interface. Companies such as Artex
sell an interface box that goes between your GPS/Loran and the 406Mhz
ELT. Its function is to receive continous information position updates
for the aircraft GPS/Loran and translate it into the proper format for
use by the ELT. Once you crash, the ELT then transmits digitally the
lat/long of the last known position to the Cospas/Sarsat satellite
system. It doesn’t take a rocket scientist to know how helpful the
lat/long could be if you were lying on frozen Tundra, wounded and a
couple of bears in the neighborhood. With the lat/long position
transmitted to the satellites, Search and Rescue can get your location
down to 100 meters and doesn’t have to wait for a Doppler fix. Most
406Mhz ELT navigation interface boxes want to see an ARINC 429 bus. The question is; if 406Mhz ELTs are so good, why aren’t aircraft owners purchasing them like hot cakes on a cold winter morning? Well, there are several reasons. Most avionics shops don’t push the products because they don’t fully understand the advantages of this ELT system nor do they have the proper equipment to test the product after installation. Another reason is the sad fact that safety is hard to sell. I can sell a farmer cow manure but trying to sell Safety to an aircraft owner is very difficult, they just don’t want to spend money on something that doesn’t flash numbers at them or doesn’t make the machine go any faster. No doubt cost is the biggest factor. A 121.5/243Mhz ELT sell for around $200.00 but a 406Mhz system sells in the $3,500-7,000 range, depending on the options. 406Mhz ELTs for boats sell in the $900.00 range but they don’t have a "G" switch that activates the ELT when the aircraft hits the ground nor are they "FAA Approved" I’m not going to thump you over the head and say go out and purchase one but give it some serious thought. A 406Mhz ELT could be money well spent.
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Studies
show that while the initial survivors of an aircraft crash have less
than a 10% chance of survival is rescue is delayed beyond two days, the
survival rate is over 60% if the rescue can be accomplished within eight
hours. It’s clear that the 406Mhz ELT is far superior to the older
121.50/243.00Mhz units.
These satellites provide the ability to detect and locate 406Mhz alerts
worldwide and 121.50Mhz alerts for about sixty percent of the world. In
addition to the polar orbiting satellites the system is currently
evaluating the use of geo-stationary orbiting satellites. Search and
Rescue will only get better as technology progresses and aircraft owners
start updating to modern ELT equipment.
During
transmission, the 406 ELT will transmit digitally an encoded message
which provides information such as country of origin, tail number of
aircraft, aircraft type, emergency contact, serial number of the ELT and
(with the proper options) position data derived from an internal or
external navigation receiver. Many 406Mhz ELTs also employ a homing
transmitter that transmits on 121.50Mhz, which can be received by just
about any aircraft that has a VHF com installed. When you purchase a
406Mhz ELT, you mail in a form with all of the data and it is entered
into the computer for later reference if it is ever needed. Due to the
fact 406Mhz ELTs are not in the com radio band and they only transmit
for one-half second, you can’t hear them like the old 121.50Mhz
system. But again, the 406 system is designed to work with satellites
and most have a 121.50Mhz transmitter aboard thus you could hear the
distress signal via the 121.50Mhz if you were close enough.
I’d recommend any GPS you purchase today have a 429 data buss output;
a lot of future products will require it. Plan ahead with any avionics
you are thinking of purchasing.