Editing Emergency position-indicating radiobeacon (section)

==Beacon content==
The most important aspect of a beacon in classification is the mode of transmission. There are two valid transmission modes: digital and analog. Where digital usually has a longer range, analog is more reliable. Analog beacons are useful to search parties and SAR aircraft, though they are no longer monitored by satellite.

===Analog 121.500 MHz homing signal===
[[File:COSPAS-SARSAT Locator Beacon Sweep Down.ogg|thumb|The radiotelephony locator beacon sound made by ELTs and some EPIRBs]]All ELTs, all PLBs, and most EPIRBs are required to have a low-power homing signal, that is identical to the original 121.500&nbsp;MHz VHF beacon signal. However, due to the extremely large number of false alarms that the old beacons generated, the transmit power was greatly reduced, and because the VHF transmitter typically uses the same antenna as the UHF beacon, the radiated signal is further reduced by the inherent inefficiencies of transmitting with an antenna not tuned to the transmitted signal.

===Digital 406&nbsp;MHz beacons===
406&nbsp;MHz UHF beacons transmit bursts of digital information to orbiting satellites, and may also contain a low-power integrated analog (121.500&nbsp;MHz) [[homing beacon]]. They can be uniquely identified (via [[GEOSAR]]). Advanced beacons encode a [[GPS]] or [[GLONASS]] position into the signal. All beacons are located by Doppler triangulation to confirm the location. The digital data identifies the registered user. A phone call by authorities to the registered phone number often eliminates false alarms (false alarms are the typical case). If there is a problem, the beacon location data guides search and rescue efforts. No beacon is ignored. Anonymous beacons are confirmed by two Doppler tracks before beginning beacon location efforts.

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 [[aircraft registration|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&nbsp;MHz homing transmitter.

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. The 406.025&nbsp;MHz carrier signal is modulated plus or minus 1.1 radians with the data encoded using [[Manchester code|Manchester encoding]], which ensures a net zero phase shift aiding Doppler location<ref>Albert Helfrick, Principles of Avionics, 5th Edition,
Avionics Communications, 2009
{{ISBN|1885544278}}, p 287</ref>

====406 MHz beacon facts and transmission schedule====
* 406&nbsp;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.
* The repetition period shall not be so stable that any two transmitters appear to be synchronized closer than a few seconds over a 5-minute period. The intent is that no two beacons will have all of their bursts coincident. The period shall be randomised around a mean value of 50 seconds, so that time intervals between transmission are randomly distributed on the interval 47.5 to 52.5 seconds. (specification for first-generation beacons)<ref>{{cite web |url=http://www.cospas-sarsat.int/images/stories/SystemDocs/Current/CS-T-001-Oct2014.pdf |title=SPECIFICATION FOR COSPAS-SARSAT 406 MHz DISTRESS BEACONS C/S T.001 Issue 3 – Revision 15 |date=October 2014  |website=[[International Cospas-Sarsat Programme]]}}</ref>
* Preliminary specification for second-generation beacons. From beacon activation a total of [6] initial transmissions shall be made separated by fixed [5s ± 0.1s] intervals. The first transmission shall commence within [3] seconds of beacon activation. Transmissions shall then occur at nominally [30] second intervals until [30 ± 1] minutes after beacon activation. The repetition period between the start of two successive transmissions shall be randomised around the stated nominal value, so that intervals between successive transmissions are randomly distributed over ± [5] seconds. Subsequent transmissions [TBD].<ref>{{cite web |url=http://www.cospas-sarsat.int/images/stories/SystemDocs/Current/T-018-OCT-2014.pdf |title=SPECIFICATION FOR SECOND-GENERATION COSPAS-SARSAT 406-MHz DISTRESS BEACONS C/S T.018 Preliminary Issue A |date=October 2014 |website=[[International Cospas-Sarsat Programme]]}}</ref>
* 406&nbsp;MHz beacons will be the only beacons compatible with the [[MEOSAR]] (DASS) system.<ref>{{Cite web |url=http://searchandrescue.gsfc.nasa.gov/dass/index.html |archive-url=https://web.archive.org/web/20160304072004/http://searchandrescue.gsfc.nasa.gov/dass/index.html |url-status=dead |title=NASA Search and Rescue Mission Office : Distress Alerting Satellite System (DASS)<!-- Bot generated title --> |archive-date=March 4, 2016}}</ref>
* 406&nbsp;MHz beacons must be registered ([[#Registration|see below]]).

====Hex codes====
Example hex codes look like the following: 90127B92922BC022FF103504422535<ref>[https://web.archive.org/web/20090318050833/http://www.cospas-sarsat.org/BeaconsCodingGuideExample/sdlnmms.htm Example of 406 MHz Beacon Coding<!-- Bot generated title -->]</ref>
* 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 for having the beacon reprogrammed with a new country code and to [[Emergency Locator Transmitter#Registration|register]] it with their nation's beacon registry, and the seller is responsible to [[Emergency Locator Transmitter#Registration|de-register]] the deprecated beacon ID with their national beacon registry.
* One can use the beacon decoder web page<ref>[http://www.cospas-sarsat.int/en/seeing-information-programmed-in-your-beacon beacon decoder webpage], When one enters the transmitted (i.e. GPS-location-included) 15-hex into the decoder, the unmodified 15-hex ID is printed at the bottom of the output of the Beacon Decoder page. This method can be used to confirm that a beacon is encoding the correct 15-hex ID (as printed on the side of the beacon) into its distress messages. URL updated 26 March 2021.</ref> at Cospas-Sarsat to extract the 15-hex ID from the 30-hex distress message.