Distress transmitters, known as BEACONs, are devices with which emergency carriers transmit a digital distress signal to ground stations via satellites. They use the 406 MHz frequency, which transmits a digital signal containing, among other information, the carrier's identification code.

The 121.5 MHz frequency, already used to transmit the distress signal to ground stations, was eliminated from reception by satellite equipment as of 1 February 2009 and will therefore remain a low-power analogue signal used by 406 MHz beacons, which can be radioed by distress equipment and is useful for calibrating and making the action of SAR forces more efficient. At the time of purchase, the 406 MHz beacon must be programmed with the carrier identification parameters. The beacon will be assigned a 15-character hexadecimal alphanumeric code to be entered on the EPIRB, ELT or PLB registration card. The registration of satellite transmitters of Italian nationality must be carried out at the Cospas-Sarsat Satellite Station in Bari, which will take care of the same registration by confirming the database entry operations. In case of transmitter activation, the Cospas-Sarsat Satellite Station, receiving the alarm signal, will send not only the activation signal, but also the registration card of the transmitter in question to the SAR Contact Point for the continuation of rescue operations.

The 121.5 MHz frequency, already used to transmit the distress signal to ground stations, was eliminated from reception by satellite equipment as of 1 February 2009 and will therefore remain a low-power analogue signal used by 406 MHz beacons, which can be radioed by distress equipment and is useful for calibrating and making the action of SAR forces more efficient. At the time of purchase, the 406 MHz beacon must be programmed with the carrier identification parameters. The beacon will be assigned a 15-character hexadecimal alphanumeric code to be entered on the EPIRB, ELT or PLB registration card. The registration of satellite transmitters of Italian nationality must be carried out at the Cospas-Sarsat Satellite Station in Bari, which will take care of the same registration by confirming the database entry operations. In case of transmitter activation, the Cospas-Sarsat Satellite Station, receiving the alarm signal, will send not only the activation signal, but also the registration card of the transmitter in question to the SAR Contact Point for the continuation of rescue operations.

406 MHz rescue frequency characteristics.

Unique digital identifier, registration data information provided by vessel owner/owner, aircraft or personal use. Signal Power Pulse of 5 Watts every 50 seconds Global Coverage Position Accuracy Within 5 km (Doppler), 150 m if GNSS (GPS) the position is encoded within the beacon message. Depending on the use for which they are designed, transmitters are subdivided into:

EPIRB - (Emergency Position Indicating Radio Beacon)

EPIRBs (Emergency Position Indicating Radio Beacons) are devices for maritime use. They transmit a digital identification code on the 406 MHz frequency as well as a low-power 'homing' signal on 121.5 MHz.
EPIRBs at 406 MHz are divided into two categories:

• Category I: they are activated either manually or automatically, with automatic activation occurring when the EPIRB is released from the holder. EPIRBs in this category are equipped with special mounts with hydrostatic release hooks; this mechanism releases, by pressure, the EPIRB at a depth of 1 to 4 metres and, once it is released and reaches the surface, it begins to transmit. For EPIRBs in this category, it is very important that they are mounted on the outside of the vessel's structure where they can be released once the vessel sinks. On large ships, the EPIRB is installed on the bridge fins.
• Category II: are manually activated transmitters. It is therefore advisable to store the transmitter on board the unit in places that are immediately accessible in case of danger.

In both cases, category I or II, signals can be immediately received by geostationary satellites. This means that even a brief, inadvertent activation can generate a false alarm. Therefore, when testing the device, carefully follow the manufacturer's instructions in order to avoid accidental activation.

ELT - (Emergency Locator Transmitter)

The ELT (Emergency Locator Transmitter) are transmitters for Aeronautical use. They transmit a digital identification code on the 406 MHz frequency as well as a low-power 'homing' signal on 121.5 MHz. The ELTs can also be either Category I or Category II, so they can be activated manually or automatically. The automatic activation of an ELT occurs for aircraft crashes or hard landings to which the aircraft may be subjected. In the second case, it is clear that it is a manual activation requested by the person concerned for a possible emergency.

PLB (Personal Locator Beacon)

The PLB(Personal Locator Beacon) are devices for Portable use. They operate in the same manner as EPIRBs or ELTs, but do not replace EPIRBs or ELTs where they are intended for vessels or aircraft. Therefore, vessels or aircraft subject to the relevant regulations will have to carry the relevant EPIRB or ELT and programme it with the identification parameters of the same vessel or aircraft. Unlike ELTs and some EPIRBs, PLBs can only be activated manually and operate exclusively on the 406 MHz frequency. All PLBs have a low-power 'homing' transmitter inside on the 121.5 MHz frequency, which allows rescue forces to locate the transmitter with the help of the radio direction finder.