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Author Topic:   EPIRB Primer
jimh posted 02-20-2013 02:16 PM ET (US)   Profile for jimh   Send Email to jimh  
A Primer on EPIRB

An EPIRB is a emergency position-indicating radio beacon used as a maritime distress alerting and locating device. There is a long history of radio devices used for maritime distress alerting and locating, but in February 2009 a significant change occurred in the global systems which monitor for these radio signals. Rather than dwell on the pre-2009 historical devices, only post-2009 EPIRB devices and systems are described. As implied by the description as a beacon, these devices are transmit-only devices. They send a radio signal alerting others of a distress or emergency situation. They do not receive signals or provide two-way communication.

EPIRB devices for marine use are covered by the Global Marine Distress and Safety System (GMDSS). International regulations for ships require compliance with GMDSS, including mandatory carriage of EPIRB devices. Recreational vessels are typically not required to conform to GMDSS regulations, but carriage of digital selective calling (DSC) radios, another segment of GMDSS, is becomming more common in the USA. EPIRB carriage on recreational vessels is not required in GMDSS or in the USA.

EPIRB devices transmit on two frequencies at two power levels: a digital signal with encoded data is sent on 406.025-MHz (called "406" or "406-MHz") at a power of about 5-Watts in 0.25-second bursts that occur on initial activation and repeat every 50-seconds; in some EPIRB devices, upon activation a 0.05 to 0.1-watt amplitude modulated signal at 121.5-MHz is continuously transmitted in addition to the 406 signal.

406 Signal

The data encoded in the 406 signal includes information about the registered user of the beacon, and in most modern devices, information about the location of the device obtained from its internal GNSS receiver is also sent.

The usefulness of any emergency beacon depends on who is listening for it. In the case of the 406-MHz signal, a system of orbiting satellites called the COSPAS-SARSAT system is monitoring. The COSPAS-SARSAT system is a cooperative effort of more than 25 countries. There are five geo-synchronous orbit satellites (called GEOSAR's) and six low-earth polar-orbit satellites (called LEOSAR's) presently in use and monitoring for the 406 signal.

Coverage in North American is provided by GEOSAR satellites at 75-degree-West longitude (GOES 13) and 135-degree-West longitude (GOES 15). These satellites on the eqautor give continuous coverage up to 70-degree latitude. At northern-USA or Great Lake latitude, the look angle to GOES 13 is typically below 40-degree elevation.

The six LEOSAR satellites are in orbits with approximately 100-minute periods. They provide coverage of the polar regions and higher lattitudes. If the orbits were optimally staggered, the six satellites in 100-minute orbits would suggest overflight every 17-minutes at the poles. The LEOSAR satellites have store-and-forward capabilities so they can relay information to ground stations when they are in range.

The GEOSAR satellites over North America are in geo-stationary orbit. This means their position relative to the Earth does not change, and they cannot use Doppler Shift methods to deduce a beacon transmitter location. GEOSAR satellites depend on the EPIRB device to transmit its position.

The LEOSAR satellites use sophisticated methods based on Doppler Shift of the received signal, and the position of the beacon can be deduced, even if the beacon is not transmitting position data in its encoded digital data payload. Because Doppler Shift methods rely on knowing the exact frequency of the original transmission, the 406 signal is sent with very high precision at the 406.025-MHz carrier frequency. The precision of the deduced position using Doppler Shift is about two-miles.

All EPIRB devices must meet many minimal requirements:

--they must float in an upright position so as to permit their transmitters to function correctly;

--they must have a battery with sufficient power to provide 48-hours of operation at a temperature of -4-degrees-F (-20-degrees-C);

--they must have a strobe light;

--they must specify battery shelf life, including provision for activation for self-testing;

There are two principal categories for EPIRB devices, Category I or II. Category I devices are intended for automatic deployment. They are typically mounted on a vessel in a location where they can be automatically released and float away without interference in a specialized mounting-release bracket. They must release when hydrostatic pressure increases to the equivalent of a depth of 4 to 14 feet. The mounting bracket of a Category I device must also provide protection against ultraviolet radiation (so that the integrity of the unit is not degraded by exposure to sunlight).

Category II devices are intended for manual deployment. They are provided with a mounting bracket, but the bracket does not have an automatic hydrostatic release system.

Both Category I and II devices must activate automatically in water, but not if still retained in their mounting bracket (to prevent false activation from rain or spray). And both must be able to be manually activated while still in their mounting bracket. Note that if a Category II device is not stored in its mounting bracket, for example, if stored in a ditch bag, it must be stored in a dry place.

When activated, all EPIRB devices transmit a string of digitally encoded data known as the hex-code. The hex code contains the following data:

--a country code, identifying the national authority responsible for the beacon device and its registration;

--a beacon ID, identifying the individual beacon sending the transmission; because all beacons must be registered, this leads to the identity of the registrant, usually the boat owner, as well as information about the vessel;

--position data, with information about the location of the beacon transmitter (from internal or external devices), and to varying degrees of resolution; the best resolution seems to be to 4-seconds of resolution (0.067-minutes). This is a merdional distance of about 0.077-miles or about 400-feet. The position data typically comes from an internal GNSS receiver. Most modern EPIRB devices now include their own internal GNSS receiver.

There are a number of methods for establishing the identity of the beacon. The vessel MMSI can be used. There are several methods for sending position data. For the technically curious (and you need to be extremely curious and extremely technical), see the guide at 286-standard-location-protocol

When an EPIRB is activated, it beacon transmitter begins to operate, and a global network of receivers and watch-standers is ready to pick-up the signal. The details of this process are well explained in other resources, and will be omitted here.

Because an EPIRB device is only intended to be operated and activated in emergency or distress situations, there is no intrinsic method to test the device. Most modern EPIRB devices now provide a method of self testing to permit the operator to verify that the device is functioning properly and, if activated, would work as intended. The extent of the self-test functions vary greatly with individual devices.

Modern EPIRB devices can actually send a specially encoded self-test transmission to the COPAS-SARSAT system. The COPAS-SARSAT system ignores these self-test messages, that is, it does not react to them. It simply passes the message through the system. In order that an EPIRB owner might further test their device, at least one manufacturer (ACR-ARTEX) is offering an additional service. The manufacturer monitors the COPAS-SARSAT for messages, allowing the manufacturer to receive all self-test messages. When a self-test message is received from one of their devices, and if the registrant of the device has subscribed to their service, the manufacturer will send an email to the registrant confirming that their device successfully transmitted a self-test message through the entire COPAS-SARSAT system. The message will also include the position information sent in the self-test transmission. For more information on this service, see

Because the EPIRB is a battery-operated device, manufacturer specifications limit the number of self-test operations, presumably to avoid discharging the battery below a charge that would meet the specified operational life in actual distress activation. Some self-test procedures are limited to only one use within the specified battery shelf life period.

EPIRB devices with internal GNSS receivers are now common. Upon activation, the GNSS receiver may take several minutes (and perhaps up to 20-minutes or longer) to acquire a position solution. Once an internal GNSS receiver in an activated EPIRB has acquired a valid position, the GNSS receiver may go into a sleep or rest mode to conserve battery power, and it will return to an active state periodically to provide a fresh position solution for transmission.

Some manufacturers offer enhanced EPIRB devices which can be interfaced to a vessel's GNSS receiver. In this way, while in its mounting bracket the EPIRB will already know its position, and upon activation, the vessel position can be sent immediately, with the first burst transmission, instead of waiting several minutes for the internal GNSS to acquire a position solution. Because the EPRIB must be completely waterproof, the connection of the external GNSS receiver to the EPIRB is accomplished by using an optically-encoded link. (I found this to be quite an elegant solution.)

To communicate with an external GNSS device requires some electrical power. Accordingly, the typical arrangement is for the EPIRB to check for an external GNSS only when the specialized optical connector is physically in place. Once data has been obtained from the GNSS, the EPIRB only reactivates at 20-minutes intervals to update the data. In a worst case, the position data might be 20-minutes old at the moment the EPIRB is removed from its mounting bracket and activated. If GNSS data from the external source becomes unavailable, the last data obtained is retained for four hours, then discarded. In this way, if a vessel lost power and then sank, the EPIRB would still have a recent position from the vessel's GNSS receiver.

121 Signal

The 121.5-MHz signal is a 0.05 to 0.10-watt carrier signal with amplitude modulation of a continuous audio sweep tone. The direction of the sweep frequency upward or downward conveys the type of beacon sending the signal, with downward indicating ELT devices, upward indicating PLB devices, and EPIRB devices using either.

The 121.5-MHz beacon is a legacy system used originally in aviation. This signal provides a beacon for direction-finding receivers which can be used by near-by rescuers to home-in on the EPIRB. The range of these signals is limited, although aircraft may be able to receive them at much greater distance than searchers at sea level. A further problem with 121.5-MHz is the sharing of the frequency with voice transmissions. Despite the low power and potential for interference, the 121.5-MHz beacon can be useful because many responders have 121.5-MHz direction-finding equipment. A real-time, continuously-available, radio-direction bearing obtained on-scene may be more helpful than a 20-minute-old GNSS position with three-quarter-mile resolution relayed via satellites or an approximate position deduced by triangulation from LEOSAR satellites. Many search and rescue agencies have 121-MHz direction-finding devices, but do not have 406-MHz direction-finding. Also, the 406-MHz signal is sent only as a short burst every 50-seconds; this reduces its value for direction finding purposes.


The cost of a top-of-the-line EPIRB of Category II type with mounting bracket for manual deployment is not cheap. The ACR-ARTEX iPRO 406-MHz EPIRB with internal GNSS receiver, optical interface for external GNSS receiver input and an LCD display screen for showing status information is about $820--on a promotional deal. A subscription to the enhanced self-testing service is $40 per year. Battery replacment every five years (or after any activation) is necessary. Battery replacement must be performed by an authorized service center and costs about $350 for an ACR iPRO 406-MHz EPIRB at present.

Accounts of Actual Rescues Facilitated by EPIRB Devices

There are many narratives of amazing rescues facilitated by EPIRB. One of the best is an incident in which a teenaged girl was sailing alone around the world in a 40-foot sailboat. In a storm in the Southern Ocean her boat capsized, leaving her in distress and about 2,000-miles from land. She activated her EPIRB. Within 24-hours an airplane guided by the EPIRB position overflew her boat, and established she was in distress. A day to two later a commercial fishing vessel diverted from its route and rescued her. Accounts such as these are good examples of the success of the 406-MHz EPIRB system.


Direction Finding of EPIRB Signals

One aspect of the EPIRB radio signals should be clear to boaters: it is almost a certainty that no other recreational boaters will have any sort of equipment on their boats to receive a signal from an EPIRB. Neither the 406 signal or the 121 signal will be received by other recreational boater. A typical recreational boat could be a mile away from someone in distress whose EPIRB was activated, and the boater would have no idea there was any distress situation as a result of detection or reception of the EPIRB signals. There is a very minute probability that a recreational boat might be carrying a 121.5-MHz direction finding receiver, but it would extraordinarily unlikely any recreational boat was maintaining a radio watch on the 121.5-MHz distress frequency.

I believe that some aircraft maintain a radio watch on 121.5-MHz. Perhaps a pilot could comment on that probability.

Regarding the 121.5-MHz distress beacon signal, its legacy is from aviation. In aviation radio communication there is a long history of using amplitude modulation. I believe that even present-day aviation radios continue to use amplitude modulation methods. In contrast, VHF Marine Band radios use frequency modulation methods. The use of amplitude modulation in aviation apparently is a carry over from the earliest days of in-flight radio use.

It is a characteristic of amplitude modulation that a receiver can demodulate the signal with the simplest of methods. Only an envelope detector is needed, and this can be accomplished with a single component: a simple diode will suffice. As a result, a 121.5-MHz beacon receiver could be of extremely simple design.

The use of a continuously sweeping audio tone modulation for the 121.5-MHz beacon is also interesting. The varying tone modulation tends to stand out from other signals--much like a chirping SONAR signal. Interfering signals tend to produce steady tone beats. A sweeping tone modulation makes the 121.5-MHz beacon more easily distinguished by ear in a receiver.

The choice of 121.5-MHz is probably related to the original allocation of frequencies for aviation band use. I have not researched this history. In any case, at 121.5-MHz it is not too cumbrous to hold a small Yagi antenna by hand and to rotate it, looking for signal peaks and nulls. At one time a few manufacturers were making 121.5-MHz beacon direction finder receiver kits with a two-element yagi antenna, like this:

but these products now seem to be mainly discontinued.

You may find some Coast Guard search and rescue boats will have a permanently-installed radio-direction-finding receiver. These typically use an antenna known as the Adcock Antenna. The Adcock Antenna usually has at least four or more vertical dipole antennas arrayed around a central support. They look like this:

The theory of their operation is explained in

and dates back to World War II.

A number of manufacturers continue to make VHF direction finding equipment for search and rescue that can use the 121.5-MHz frequency. A few examples: _sessid=3626ac108f47d8a77ddd05181db68514&action=sku&sku=dOM-HAM_kit01

In theory, a four-dipole Adcock Antenna can provide a bearing to the transmitter source with an accuracy of around two-degrees. You can understand how useful this sort of locating information could be. Imagine a situation where a rescue helicopter is hovering above the water in a storm, looking for someone in the water. The 406-MHz beacon digital data has brought the helicopter on the scene. The 121.5-MHz beacon signal can now direct the search toward the beacon transmitter with two-degree resolution. This should be a powerful combination of technologies.

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