Emergency Transmitter

eIn this guide we use the phrase Emergency Transmitter to mean ELT/EPIRB/PLB transmitters collectively. All modern models of all three types transmit a digital radio signal on one of a number of frequencies near 406 MHz. Because of this they are often called 406 beacons. All ELTs, regardless of date of manufacture, and many models of EPIRB and PLB  also transmit on 121.5 MHz. The reason for transmitting on two frequencies is simple, but requires some knowledge of the 406 MHz signal to understand.

406 MHz Digital Signal

Emergency Transmitters that broadcast on 406 MHz send a 5 Watt digital signal of 500 ms (half a second) duration approximately ever 50 seconds. The transmission interval is slightly randomized so that if two or more beacons transmit at the same time in one 50 second interval, they will transmit at different times during the next interval. This helps to reduce interference between multiple active beacons in an area. Each beacon is identified by a unique number which is encoded into each 406 MHz transmission. This allows the automated systems to keep track of which transmission is from what transmitter. This does make traditional simple electronic search methods all but impossible. The designers of the system were aware of these difficulties and so included the old 121.5 MHz continuous 0.1 Watt (or in some cases 0.05 Watt) amplitude modulated broadcast beacon as a way for search assets without sophisticated equipment to locate the beacon. 

A Few Words on Transmit Power

We have mentioned a number of transmitter power levels: 5 Watts; 0.1 Watts; 0.05 Watts. It is tempting to leap to the conclusion that more is always better, and that when a 5 Watt 406 MHz signal is available searchers should only use that signal since it will be able to penetrate foliage and travel further being more powerful. This is not necessarily the case. But before we try to convince you of that, let's consider a couple of examples that are more common in every day experience.

In our first example we will consider how efficiently the power is used. Until recently domestic lighting was provided almost exclusively by incandescent light bulbs that would be very familiar to Edison. When selecting a bulb for a particular application it was quite easy since if we wanted a lot of light we would pick a 100 Watt or maybe even a 150 Watt bulb. For less demanding tasks we may select a 60 Watt bulb, and for a bedroom night stand maybe a 45 Watt or 30 Watt bulb. Now we have compact fluorescent lamps which produce the same amount of light using less power. For example the compact fluorescent equivalent to a 150 Watt incandescent bulb consumes between 30 and 52 Watts. In fact, comparing light bulbs on the basis of power consumed has been incorrect. If our desire is to select a bulb that produces a specific amount of light, we should be comparing them based on the light output in lumens

As a second example consider an auto mobile and an eighteen wheeled tractor trailer. Which has more power? Unless the car is quite exotic and expensive it is very likely the tractor will have a more powerful engine. However, even if this is the case which vehicle will be able to climb a mountain pass more quickly? Highway engineers don't, in general, build passing lanes so that trucks may pass cars (though that does happen occasionally). The truck may have more power, but it is probably also carrying more weight. This allows the car to quickly out distance a truck on a mountain road. 

Returning to the emergency transmitters, we hope that now you can see that it isn't a simple matter of more power is better. One must also consider how efficiently that power is used, and what it is used for. In motor vehicles, as we increase weight we must also increase power to maintain comparable acceleration, climb and cruising speed. In radio transmission as we increase the amount of information sent per unit of time, we must also increase power to stay comparable with other specifications like transmission range. A 406 MHz beacon does not transmit with 5 Watts because it is 50 times better than the 0.1 Watt of a 121.5 MHz beacon. It must increase power because it is transmitting much more information in a much shorter time. A 406 MHz beacon may out perform a 121.5 MHz beacon in some situations, but the short duration of the signal, and irregular transmission schedule can easily reduce or eliminate that advantage unless sophisticated receiving equipment is available. 

One final comment. Prior to February 1, 2009 when COSPAS-SARSAT discontinued monitoring 121.5 MHz, it was quite routine for the satellites to be able to detect these beacons even when the beacons had their antennas removed, packed in a box and placed on a truck with many other packages to be shipped to a servicing company for annual testing. It was also routine to detect activated ELTs mounted in aircraft stored in all metal hangars, or lying horizontal on a table in a home. All this from 800 km in space. 

Off Tuning

As emergency equipment Emergency Transmitters are not required to perform to all the same requirements as other transmitters. One requirement that has been significantly relaxed for ELT 121.5 MHz broadcasts is the attenuation of spurious emissions. One effect is that transmissions from beacons which take advantage of these relaxed requirements can be detected on frequencies adjacent to 121.5 MHz. These spurious emissions usually have less power than the intended transmission. Ground teams can take advantage of this when close proximity to the transmitter, within a few hundred meters, overloads their homing equipment. Airborne teams may be tempted to use this technique in the assumption that if they can receive the weaker spurious transmission they must be close. This may sometimes be true in very simple circumstances, such as training exercises (depending on how the training beacon is placed). In the case of an aircraft crash there is a very good chance that this assumption will not be true as we shall see in the next section on Radio Propagation

The standard that allows this is a permissive standard. Manufacturers will only allow spurious emissions as long as eliminating them will make the transmitter more complex, heavier or consume more power. There is no guarantee that any particular make, model or example of emergency transmitter will produce spurious emissions with the amount of power allowed. Any technique that depends on spurious power output at the levels allow, must be expected to fail when beacon manufacturers find efficient methods to reduce the power lost to these emissions.
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