Search and Rescue via SatelliteThe United States Search and Rescue Satellite Aided Tracking (SARSAT) program is currently managed by NOAA and operated in a partnership with the US Air Force (USAF), US Coast Guard (USCG) and NASA. The SARSAT program has operated within the framework of the International Cospas-Sarsat program and since 1982 and has been credited with saving over 30,000 lives using satellites to detect and relay distress signals to ground stations called Local User Terminals (LUTs). These LUTs track Low Earth Orbiting (LEO) and Geosynchronous Earth Orbiting (GEO) satellites equipped with instrumentation to detect distress signals. Beacons 406 MHz distress beacons are normally designed and constructed either as Emergency Position-Indicating Radio Beacons (EPIRBs) for maritime applications, as Emergency Locator Transmitters (ELTs) for aviation applications, or as Personal Locator Beacons (PLBs) for personal use. Frequencies In the United States, the Federal Communications Commission (FCC) initially authorized EPIRBs and ELTs to operate only on 121.5 MHz and (primarily for military use) on 243 MHz. In 1988, the FCC amended Part 80 rules to permit EPIRBs to operate on the frequency 406.025 MHz as well. In 1993, the FCC likewise authorized the use of 406.025 MHz by ELTs. PLBs have never been authorized to transmit a distress signal on 121.5 MHz, but only on 406.025 MHz. Currently, beacons transmit on one of the following frequencies:
406.025 MHz Emission Designator 16K0G1D 16 kHz wide signal, phase modulation, no modulating signal, data transmission Signal A 406 MHz distress beacon transmits, every 50 seconds, a half-second burst of data. The distress signal starts with 160 milliseconds of unmodulated carrier, followed by the equivalent of 15 bits set to 1, followed by a frame synchronization sequence, followed by the actual data. Data bits are transmitted at 400 bits per second, Manchester encoded, and are phase modulated +/- 1.1 rad. Message The digitally encoded message is transmitted by 406 MHz distress beacons at a nominal data rate of 400 bps. Each message is either a 112 bit (28 hexadecimal digit) short message, or a 144 bit (36 hexadecimal digit) long message. In either case, the first 15 bit positions (1 through 15) are all 1s and constitute the bit synchronization pattern. Bits 16 through 24 constitute the 9-bit frame synchronization pattern, 000101111, for normal operational mode messages, or 011010000 for test-mode messages. Bit 25 is the format flag that contains a zero for short messages and a one for long messages.
The remaining bits (26 through 112 for short messages, and 26 through 144 for long messages) may contain different types of information depending on the setting of the protocol flag (bit 26). This information includes the beacon identification code, country code, supplementary data - that may include encoded location information - and error correcting codes. Beacon Identification The sequence of sixty-one bits that uniquely defines each 406 MHz beacon; these bits occupy bits 25 through 85 of the beacon message. Of these, bit 25 is the message format (long or short) bit, bit 26 is the protocol flag, bits 27-36 the country code, 37-39 the protocol code, 40-83 identification data, and 84-85 auxiliary radio-locating device type(s).
The beacon identification is normally presented as a fifteen character hexadecimal representation of bits 26 to 85 (without the format bit). Note that the 15 HEX ID digit boundaries are not aligned with the hexadecimal digits of the beacon message. Forward Error Correction
PDF-1 (present in both short and long messages) is protected by an (82,61) BCH code capable of correcting up to 3 bit errors.
Reference Beacons
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