In one sentence: 406 MHz supports alerting, identification and position data; 121.5 MHz supports continuous close-in directional homing.
What 406 MHz does
Registered ELTs, EPIRBs and PLBs transmit brief digital bursts on 406 MHz for the international Cospas-Sarsat system. The encoded message identifies the beacon and may include GNSS position data. Rescue authorities receive alerts through the appropriate national system; in the United States, NOAA operates the U.S. SARSAT program.
Field equipment differs by design. Some 406 MHz systems decode a beacon message. Overwatch406 detects the live intermittent 406 MHz transmission but does not decode the digital message or provide coordinates. Authorized alert and coordinate information comes from the responsible rescue authority and may be entered separately into agency-selected mapping software.
What 121.5 MHz does
Modern distress beacons generally transmit a lower-power analog homing signal on 121.5 MHz. A directional receiver uses changes in signal strength and antenna orientation to guide the operator toward the source. This supports the final phase of the search, including a beacon inside an aircraft, vessel, building, hangar or terrain feature.
VHF propagation is affected by terrain, structures, reflections, antenna position and operator technique. Successful homing requires deliberate movement and confirmation from more than one location.
How the capabilities work together
| Stage | Primary information | Field action |
|---|---|---|
| Alert | 406 MHz message routed through SARSAT | The responsible authority validates the alert and assigns resources. |
| Reported position | Encoded GNSS or system-generated location | Teams plan toward the best available search area. |
| Local detection | Direct 406 MHz detection when available | Equipment performs only the functions documented by its manufacturer. |
| Final approach | Live 121.5 MHz homing signal | A direction finder guides the team to the physical signal source. |
Matching equipment to the job
Guardian provides numerical and audio 121.5 MHz directional homing. Overwatch406 supports field detection of the live 406 MHz transmission. Guardian Pro packages the two separate capabilities in one coordinated field system.
Equipment supports—but does not replace—mission authorization, agency procedures, radio discipline, trained judgment or coordination with the responsible rescue authority.