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INTRODUCTION TO 406 MHz BEACONS
Currently there are several types of Emergency locating transmitters or beacons. One of which
is the earlier 121.5 MHz type and another, the newer 406 MHz to 406.1 MHz type. The earlier
type transmits a continuous amplitude modulated signal without any information and relied on
persons who heard it to use direction finders to locate the source. While better than nothing,
the system was prone to many false alarms, and significant delays until the signal was first
discovered. The newer system resolved these problems by incorporating an identifying
message in the transmission along with optional location information and a system of orbiting
satellites that continuously monitor the 406 to 406.1 MHz band and relay the signals back to
Earth command centers.
There are two basic messages each having many variations tailored to suit the application using
the beacon. One is the short message (440 mS) which has no location information. The other is
the long message (520 mS) which can include location information. A portion of both messages
contain the following basic information; the protocol being used, the identity of the beacon, its
country of registration, and if it is a normal actual alarm or a test signal. If it is the long
message, also included is the last location, within a limited time frame, when the beacon was
activated.
Some additional requirements are to be met. When the beacon is activated for an actual alarm,
there is a brief delay prior to the first actual transmission. This is to allow the beacon to be
deactivated if the activation was inadvertent. Then once the transmission starts, the first 160
mS are to be un-modulated, in other words no data is sent. This is followed by a short
synchronizing signal which is then followed by the message sent in a specialized digital format
that contains the information needed to help identify and locate the beacon. This transmission
is supposed to be made at approximately 50 second intervals which are to be random in nature
so as to minimize any two beacons from continuously interfering with each other. There are
other features of the system which have been well thought out and implemented; the details of
which can be found in the specifications for the system from the Cospas Sarsat web site.
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