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Distress radiobeacon - Wikipedia, the free encyclopedia
ELT Requirements
EMERGENCY BEACONS
Balises de détresse aviation - ELT
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Digital mode – 406 MHz beacons
406 MHz beacons transmit bursts of digital distress information to orbiting satellites, and may also contain a small integrated analog (121.5 MHz) homing beacon. Advanced 406 MHz beacons are capable of transmitting a highly-accurate GPS location within their distress message, thus, the process of distress relief is reduced from "search and rescue" to "get to and rescue". Getting to the location nonetheless may be very difficult.
The distress message transmitted by a 406 beacon contains the information such as:
which country the beacon originates from
a unique 15-digit hexadecimal beacon identification code (a "15-hex ID")
The encoded identification of the vessel or aircraft in distress, either as an MMSI value, or as, in the case of an ELT, either the aircraft's registration or its ICAO 24-bit address (from its Mode-S transponder)
When equipped, a GPS position
Whether or not the beacon contains a 121.5 MHz "homer"
The digital distress message generated by the beacon varies according to the above factors and is encoded in 30 hexadecimal characters. The unique 15-character digital identity (the 15-hex ID) is hard-coded in the firmware of the beacon.
[edit]406 MHz beacon facts
406 MHz beacons transmit for a quarter of a second immediately when turned on, and then transmit a digital burst once every 50 seconds thereafter. Both GEOSAR and LEOSAR satellites monitor these signals.
[edit]Hex codes
Example hex codes look like the following: 90127B92922BC022FF103504422535 [8]
A bit telling whether the message is short (15 hex digits) or long (30 hex digits) format.
A country code, which lets the worldwide COSPAS/SARSAT central authority identify the national authority responsible for the beacon.
Embedded 15-Hex ID or 15-hex transmitted distress message, for example, 2024F72524FFBFF The hex ID is printed or stamped on the outside of the beacon and is hard-coded into its firmware. The 15-hex ID can only be reprogrammed by certified distress radiobeacon technicians. The national authority uses this number to look up phone numbers and other contact information for the beacon. This is crucial to handle the large number of false alarms generated by beacons.
A location protocol number, and type of location protocol: EPIRB or MMSI, as well as all the data fields of that location protocol. If the beacon is equipped with GPS or GLONASS, a rough (rounded) latitude and longitude giving the beacon's current position. In some aircraft beacons, this data is taken from the aircraft's navigation system.
When a beacon is sold to another country, the purchaser is responsible to have the beacon reprogrammed with a new country code and to register it with his/her nation's beacon registry, and the seller is responsible to de-register the deprecated beacon ID with his/her national beacon registry.
One can use the beacon decoder web page[9] at Cospas-Sarsat to decrypt/extract the 15-hex ID from the 30-hex distress message.
Information transmitted by the beacon
A unique 15, 22, or 30 digit serial number called a Hex Code is transmitted
The Hex Code can contain a plethora of information, such as:[17]
the Country of beacon registration
the identification of the vessel or aircraft in distress, and
Identification for aircraft ELTs can be in the form of the aircraft's callsign or its ICAO 24-bit address (from its Mode-S transponder)
optionally, position data from onboard navigation equipment (GPS)
Potential to be seen by a satellite
To hear and process an analogue signal, both a LEOSAR and a LEOLUT must be within sight of each other continuously for a sufficient duration (several minutes). The LEOLUT does signal analysis via a dotplot of the signals heard, and the computer looks for a Doppler shift in the signal to triangulate a position.
Second detection is necessary due to false alerts and to resolve position—takes an additional 45–100 minutes before SAR assets can be called
GEOSAR provides nearly-instantaneous coverage 70 degrees north and south of the equator
Worldwide coverage via LEOSAR — 6 satellites
For 406 signals, LEOSARs do not have to be in sight of a LUT to relay a distress message to Cospas-Sarsat. Once a 406 signal is detected by a satellite, the satellite will "dump" this data towards Earth (thus to all LUTs) for 24 hours.
Future use of GNSS satellites will allow worldwide real-time coverage (MEOSAR)
Location detection Location detection
Two (roughly) "50% chance" mirror-positions (called the "A-side" for the most likely and the "B-side" for the least likely to be accurate) are generated by Doppler triangulation after the first pass of a LEOSAR
Due to false alerts, no reaction can occur based on first pass alert
A second pass resolves the ambiguity and resolves the search location to a radius of 20 km[18]
Moving targets (usually false alerts) produce interfering anomalies; calculated positions are inaccurate[citation needed]
LEOSAR uses same technique as for analog beacons, but, since beacons are uniquely identified as beacons and have improved frequency stability, response can occur based on first-pass information[18]
Doppler-only accuracy is within 5 km (3 mi) (3.1 statute miles or 2.6 nautical miles)— that is, the position is sufficiently accurate for SAR purposes even after only one pass. What's more, the "A-position" (the most likely of the two 'mirror' positions) can be determined valid with 98.5% accuracy after only one satellite pass.
GPS Position can be encoded into the Hex Code and can be updated real-time via GEOSAR
Encoded GPS position accuracy is about 15 m (45 ft), however, the space in the hex message protocol for position information is limited, so transmitted accuracy is approximately +/- 125 metres
GEOSAR
The GEOSAR satellites are monitored by 16 GEOLUTs (Geostationary Earth Orbit Local User Terminals.) [5] The GEOSAR satellites provide continuous coverage of the entire earth below 70 degrees latitude with a view toward the equatorial sky. Some locations have poor radio reception toward the GEOSAR satellites and polar regions are not well covered.
SARP are installed on the following geostationary satellites: [6]
The GOES geostationary satellites GOES-East at 75° W and GOES-West at 135° W
The INSAT-3A geostationary satellite at 93.5° E
The Meteosat Second Generation (MSG) geostationary satellites MSG-1 at 9.5° E and MSG-2 fixed over the Prime Meridian
So .. what happened with the "EPIRB thingy" of the AF447 ??
