Editing Emergency position-indicating radiobeacon (section)

== Detection and location ==
[[File:Radiogoniométrie_VHF.JPG|thumb|VHF radio direction finding]]
A transmission is typically detected and processed in this manner:
# The transmitter is activated, either automatically in a crash or after sinking, or manually by survivors of an emergency situation.
# At least one satellite picks up the beacon's transmission.
# The satellites transfer the beacon's signal to their respective ground control stations.
# The ground stations process the signals and forward the data, including approximate location, to a national authority.
# The national authority forwards the data to a rescue authority
# The rescue authority uses its own receiving equipment afterwards to locate the beacon and commence its own rescue or recovery operations.

Once the satellite data is received, less than a minute is needed to forward them to any signatory nation. The primary means of detection and location is by the COSPAS-SARSAT satellites. However, additional means of location are frequently used. For example, the FAA requires that all pilots monitor 121.500&nbsp;MHz whenever possible, and the [[USCG]] has a network of direction finder sites along the coastlines.<ref name=":10">{{Cite web|url=http://www.mdcap.org/userfiles/file/MDWG%20Conference%202008%20-%20406%20SARSAT%20(Rev-03).pdf|title=Civil Air Patrol, Maryland Wing Conference, Locating 121.5 & 406 MHz Emergency Beacons}}</ref> The [[National Oceanic and Atmospheric Administration]] maintains a near-real-time map that shows SARSAT U.S. Rescues.<ref>{{Cite web|url=https://www.nesdis.noaa.gov/content/sarsat-us-rescues|title=SARSAT U.S. Rescues}}</ref>

Several systems are in use, with beacons of varying expense, different types of satellites, and varying performance. Carrying even the oldest systems provides an immense improvement in safety over carrying none.

The types of satellites in the network are:
* LEOSAR
** Support Doppler detection and reception of encoded position
** Receivers are payloads on various Low Earth Orbit satellites
* MEOSAR
** Medium Earth Orbiting Search and Rescue
** Receivers are payloads on the U.S. GPS satellites, on the Russian GLONASS satellites, and on the European GALILEO satellites.
* GEOSAR
** Supports only reception of encoded position
** Receivers are payloads on various geosynchronous satellites, including some of the U.S. GOES weather satellites (including [[GOES-16]]).
When one of the COSPAS-SARSAT satellites detects a beacon, the detection is passed to one of the program's roughly 30 [[Mission Control Centre (Cospas-Sarsat)|Mission Control Centers]], such as USMCC (in Suitland, Maryland), where the detected location and beacon details are used to determine to which [[rescue coordination centre]] (for example, the U.S. Coast Guard's PACAREA RCC, in Alameda, California) to pass the alert.<ref>{{Cite web|url=http://www.sarsat.noaa.gov/MEOSAR%20poster%202016%20(June%2023)%20S%20convert%20Reduced%20medium.pdf|title=MEOSAR: Medium Earth Orbiting Search & Rescue|access-date=2018-02-08|archive-date=2017-04-26|archive-url=https://web.archive.org/web/20170426215835/http://www.sarsat.noaa.gov/MEOSAR%20poster%202016%20(June%2023)%20S%20convert%20Reduced%20medium.pdf|url-status=dead}}</ref>

===Beacon operation===
====GPS-based, registered====
The 406-MHz beacons with GPS track with a precision of 100 m in the 70% of the world closest to the equator, and send a serial number so the responsible authority can look up phone numbers to notify the registrant (e.g., next-of-kin) in four minutes.

The GPS system permits stationary, wide-view geosynchronous communications satellites to enhance the Doppler position received by [[low Earth orbit]] satellites. EPIRB beacons with built-in GPS are usually called GPIRBs, for GPS position-indicating radio beacon or global position-indicating radio beacon.

However, rescue cannot begin until a Doppler track is available. The COSPAS-SARSAT specifications say<ref>See COSPAS-SARSAT document A.001, 2005</ref> that a beacon location is not considered "resolved" unless at least two Doppler tracks match or a Doppler track confirms an encoded (GPS) track. One or more GPS tracks are not sufficient.

====High-precision registered====
An intermediate technology 406-MHz beacon (now mostly obsolete in favor of GPS-enabled units) has worldwide coverage, locates within 2&nbsp;km (12.5&nbsp;km<sup>2</sup> search area), notifies kin and rescuers in 2 hours maximum (46 min average), and has a serial number to look up phone numbers, etc. This can take up to two hours because it has to use moving weather satellites to locate the beacon. To help locate the beacon, the beacon's frequency is controlled to 2 parts per billion, and its power is five watts.

Both of the above types of beacons usually include an auxiliary 25-milliwatt beacon at [[distress frequency|121.5 MHz]] to guide rescue aircraft.

====Traditional ELT, unregistered====
The oldest, cheapest beacons are aircraft ELTs that send an anonymous warble on the aviation band [[distress frequency]] at 121.5&nbsp;MHz. The frequency is often routinely monitored by commercial aircraft, but has not been monitored by satellite since Feb. 1, 2009.<ref>{{Cite news|url=https://www.nytimes.com/2007/09/11/world/americas/11iht-fly.4.7466479.html|title=Aircraft beacon has become utterly outmoded, FAA says|first=Steve|last=Friess|newspaper=The New York Times|date=September 11, 2007}}</ref>

These distress signals could be detected by satellite over only 60% of the earth, required up to 6 hours for notification, located within {{Convert|20|km|0|abbr=on}} (search area of 1200&nbsp;km<sup>2</sup>), were anonymous, and could not be located well because their frequency is only accurate to 50 parts per million and the signals were broadcast using only 75–100 milliwatts of power. Coverage was partial because the satellite had to be in view of both the beacon and a ground station at the same time; the satellites did not store and forward the beacon's position. Coverage in polar and Southern Hemisphere areas was poor.

False alarms were common, as the beacon transmitted on the aviation emergency frequency, with interference from other electronic and electrical systems. To reduce false alarms, a beacon was confirmed by a second [[satellite pass]], which could easily slow confirmation of a 'case' of distress to as much as 4 hours (although in rare circumstances, the satellites could be positioned such that immediate detection becomes possible.)

====Location by Doppler (without GPS)====
The Cospas-Sarsat system was made possible by [[Doppler effect|Doppler]] processing. Local-user terminals (LUTs) detecting nongeostationary satellites interpret the Doppler frequency shift heard by LEOSAR and MEOSAR satellites as they pass over a beacon transmitting at a fixed frequency. The interpretation determines both bearing and range. The range and bearing are measured from the rate of change of the heard frequency, which varies both according to the path of the satellite in space and the rotation of the earth. This [[triangulate]]s the position of the beacon. A faster change in the Doppler indicates that the beacon is closer to the satellite's [[orbit]]. If the beacon is moving toward or away from the satellite track due to the Earth's rotation, it is on one side or other of the satellite's path. Doppler shift is zero at the [[closest point of approach]] between the beacon and the orbit.

If the beacon's frequency is more precise, it can be located more precisely, saving search time, so modern 406-MHz beacons are accurate to 2 parts per billion, giving a search area of only 2&nbsp;km<sup>2</sup>, compared to the older beacons accurate to 50 parts per million that had 200&nbsp;km<sup>2</sup> of search area.

To increase the useful power, and handle multiple simultaneous beacons, modern 406-MHz beacons transmit in bursts, and remain silent for about 50 seconds.

Russia developed the original system, and its success drove the desire to develop the improved 406-MHz system. The original system was a brilliant adaptation to the low-quality beacons, originally designed to aid air searches. It used just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listened to each satellite as long as it was above the horizon. Doppler shift was used to locate the beacon(s). Multiple beacons were separated when a computer program analysed the signals with a [[fast Fourier transform]]. Also, two satellite passes per beacon were used. This eliminated false alarms by using two measurements to verify the beacon's location from two different bearings. This prevented false alarms from VHF channels that affected a single satellite. Regrettably, the second satellite pass almost doubled the average time before notification of the rescuing authority. However, the notification time was much less than a day.

===Satellites===
Receivers are auxiliary systems mounted on several types of satellites. This substantially reduces the program's cost. The weather satellites that carry the SARSAT receivers are in "ball of yarn" orbits, inclined at 99 degrees. The longest period that all satellites can be out of line-of-sight of a beacon is about two hours. The first satellite constellation was launched in the early 1970s by the [[Soviet Union]], Canada, France and the United States.

Some geosynchronous satellites have beacon receivers. Since the end of 2003, there are four such geostationary satellites (GEOSAR) that cover more than 80% of the surface of the earth. As with all geosynchronous satellites, they are located above the equator. The GEOSAR satellites do not cover the polar caps. Since they see the Earth as a whole, they see the beacon immediately, but have no motion, and thus no Doppler frequency shift to locate it. However, if the beacon transmits GPS data, the geosynchronous satellites give nearly instantaneous response.