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The 121.5 MHz signal is used by SAR to home in on the beacon as they approach it. The 406 MHz signal is detected by multiple satellites and from that information the location of the beacon can be calculated. This data alone is sufficient for SAR to find persons or ships in distress in a reasonable timeframe. 2. Anatomy of your beacon NOTE: When unpacking your beacon, it is recommended that you save the original packaging for re-use when shipping the beacon for battery replacement in the future.