* Mass. Conference

Plymouth Composite Squadron

Serving southeastern Massachusetts

ELT searches

In general:

ELTs (Emergency Locator Transmitters) are supposed to be carried by all aircraft, marine craft greater than a certain displacement, and may be carried by anyone including individuals. Hunters, hikers, and campers could benefit by carrying them in case the outdoorsman became lost.

When activated, the transmitter sends a signal that can be picked up by orbiting satellites, monitoring aircraft, or hand-held stations. The receiver alerts the Air Force at Langley, Va., which then alerts the C.A.P., if the signal comes from land, or the Coast Guard, if over water. The initial receiver can only give a general location.

The C.A.P. sends in slow-flying aircraft and ground teams to pinpoint the location. C.A.P. volunteers are responsible for saving, on average, more than 100 lives every year.

The information below was copied from NASA Goddard Space Flight Center

The Cospas-Sarsat System, operational 24 hours a day/365 days a year, automatically detects and locates emergency radio beacons globally by satellites and alerts appropriate rescue authorities.

Emergency Beacons:

There are three different types of beacons:

- Emergency Position Indicating Radio Beacons (EPIRBs) are used in a maritime application.

There are two types of EPIRB. The oldest type transmits an analog signal encoded information on 121.5 MHz. The newest type transmits encoded digital information on 406 MHz and has a low-power "homing" signal on 121.5 MHz.

The 406 MHz EPIRBs are divided into two categories: Category I EPIRBs are activated either manually or automatically at (float-free) deployment. The automatic activation is triggered when the EPIRB is released from its bracket. Category I EPIRBs are housed in a special bracket equipped with a hydrostatic release. This mechanism releases the EPIRB at a water depth of 3-10 feet. The buoyant EPIRB then floats to the surface and begins transmitting. Category II EPIRBs are manual activation only units.

EPIRBs are mandated by government regulations to be carried on commercial fishing vessels, vessels over 300 gross tons, and vessels for hire that operate greater than three nautical miles from shore and carry six or more passengers.

- Emergency Locator Transmitters (ELTs) are used on airplanes. These beacons are activated either automatically by G-switches or by manual operation.

- Personal Locator Beacons (PLBs) are used by individuals in distress in remote areas. PLBs are activated manually by the user.

Beacons transmit on three dedicated emergency frequencies:

121.5 MHz: This frequency is used by older beacons which do not transmit any encoded information. The power output of these beacons is specified as 50, 75, or 100 mWatts. 121.5 MHz is also the transmission frequency of low power (25 mWatts) final homing transmitters included in many newer 406 MHz beacons.

- Presently there are approximately 590,000 beacons which operate on 121.5 MHz in use worldwide.

Worldwide discussions are currently taking place regarding the discontinuation of Cospas-Sarsat satellite coverage for 121.5 MHz and 243 MHz beacons.

243.0 MHz: This frequency is used in some older beacons deployed by the U.S. military and NATO forces. This frequency is not monitored by the Russian Cospas satellites.

406.025 MHz: Modern beacons which transmit digitally encoded information within the message use this frequency. Encoded information can include beacon ID to allow use of a user reference data base and beacon location determined by a sophisticated navigation device such as GPS. The RF power output of 406 MHz beacons is specified as 5 Watts. This higher output power allows these beacons to be received reliably by geostationary orbiting satellites.

- There are approximately 156,000 beacons presently in use worldwide which operate on 406.025 MHz.

The information below was copied from: NASA Goddard Space Flight Center, Emergency Beacon Development

NASA Goddard has developed prototype self-locating EPIRBs, ELTs, and PLBs by modifying existing designs. The EPIRB and ELT modification work was performed by the Naval Research Laboratory. The PLB is described in more detail in its own section.

These beacons use internal or external GPS receivers for their navigation data. External GPS receivers are coupled to the beacon via a serial interface. They function through both the LEO and GEO satellites and can also be located by conventional Doppler processing on the ground.

Through the GOES satellites, they can provide immediate, precise location. The self-location feature improves system reliability by providing redundancy. All of the prototypes have been successfully tested through the satellite system.


An experiment was performed in Simi Valley, California with a self-locating PLB coupled to a GPS receiver. The beacon was located from Maryland via the GOES 8 satellite. The location was plotted on an electronic map with an accuracy within 200 feet.

In another experiment performed in California, a beacon (and GPS) with a fast update rate was placed in an automobile moving on the highway. The position information received in Maryland was plotted automatically on a map. This same experiment has since been performed in an aircraft in-flight.

In another California experiment, a beacon (and GPS) with a fast update rate was installed in a passenger airplane. The aircraft was tracked as it took off, flew a pattern, and landed. The position information received in Maryland was received and automatically plotted on a map.

Personal Locator Beacons:

A state-of-the-art miniature Personal Locator Beacon (PLB) is being developed through Small Business Innovative Research (SBIR) contracts funded by NASA and DOD. This small, lightweight, easily carried unit is made by Microwave Monolithics, Inc. in Simi Valley, California.

This PLB meets the complete Cospas-Sarsat beacon specifications and has successfully completed Cospas-Sarsat certification testing at the Fort Huachuca Electronic Proving Ground Certification Lab in February 1999.

It contains a serial interface to allow attachment to a GPS receiver in order to download navigation data. Once the download process is complete in a few seconds, the GPS may be disconnected. The PLB will then function as a self-locating beacon.

It can function through both the LEO and GEO satellites. It is currently the smallest PLB in the world. The RF circuitry utilizes state-of-the-art GaAs technology which allows very high efficiency. Transmitter power efficiency on the order of 75% is obtainable as compared to 40% for conventional RF technology.

Alaska PLB Experiment:

NASA Goddard supported the first experimental PLB program in the United States on the North Slope in Alaska. This highly successful program has saved over 75 lives to date since its inception in 1993.

Questions or comments? Please e-mail your inquiries to PilgrimCAP@aol.com and we'll get back to you as soon as possible.

Copyright © 1999 Pilgrim Composite Squadron, Civil Air Patrol
P.O. Box 000; Plymouth, MA 02360  (508) 747-0000