| Analog (121.5 MHz) Beacons |
Digital (406 MHz) Beacons |
| SAR response delay |
SAR response delay |
- SAR response to anonymous beacons can be delayed 4–6 hours, and as much as 12 hours.
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- Resolution or response to registered beacons is very swift. SAR response can be activated within approximately 10 minutes of beacon activation (and GEOSAR detection) if distress is evident.
- Unregistered beacons can usually be responded to after only 1 LEOSAR satellite pass; after two passes, response is immediate.
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| False alerts |
False alerts |
- Fewer than 2 in 1000 alerts and 2 in 100 composite alerts are actual distress.
- The Cospas-Sarsat system has no way of distinguishing between analog beacons and interference (from set top boxes, etc.)
- False alerts may result in a long and fruitless search by costly SAR assets, although rescue co-ordination centres typically analyse the circumstances, considering location, movement of the source and confirmatory reports before launching an operation
- Searches for interference signals and false alerts inhibit SAR assets from being available for real searches
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- All alerts (100%) come from beacons (analog interference is ignored).
- Approximately 7 out of 10 false alerts are resolved by a phone or radio call, therefore,
- SAR resources are not wasted
- SAR assets are more available for actual distress
- Persons responsible for causing false alerts can avoid having to pay fines or paying the costs of operating SAR assets
- Follow-up to false activations allows continuous reductions in the number of false alerts
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| Information transmitted by the beacon |
Information transmitted by the beacon |
| Anonymous siren tone |
- 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:
- 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)
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| Beacon registration information |
Beacon registration information |
| Anonymous beacons cannot be registered |
- There is no charge to register 406 beacons (see below).
- Unless otherwise advised, personal information is used exclusively for SAR distress alert resolution purposes
- Hex Code registration is mandatory in most countries of the world
- Beacons are registered with MCCs who have 24-hour access to registry data, such as:
- Name of the owner of the beacon
- Name or callsign of the ship, aircraft, or other vehicle the beacon is associated with
- Cellular, MMSI, pager numbers, or other contact information
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| Transmission power |
Transmission power |
| 0.1 W continuous - weak signal cannot usually penetrate debris or trees |
5 W pulse mode - strong pulse reaches the satellites |
| Potential to be seen by a satellite |
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
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- 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)
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| 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
- Moving targets (usually false alerts) produce interfering anomalies; calculated positions are inaccurate
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- 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
- Doppler-only accuracy is within 5 km (3 mi) — 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.
- This accuracy can be increased to 99.3% using so-called "combined Leo-Geo processing," and this technique also enables accurate positions to be generated with as little as two or three bursts from the beacon (i.e. less than 4 minutes of transmission) and thus greatly increases the chances of being found even if the beacon is ultimately consumed by fire or is otherwise destroyed
- 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
- Increases in accuracy are a significant improvement over analog beacons - from two possible positions within a 20 km (12 mi) radius, to 15m accuracy at one position; an improvement in accuracy of over 2500.
- The improvements in accuracy are largely responsible for the advent of the phrase "taking the 'search' out of Search and Rescue"
- LEOSAR Doppler triangulation is less affected by beacon movement due to improved frequency stability
- In the future, near-instantaneous detection & position triangulation via MEOSAR
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| Age of technology |
Age of technology |
| 121.5 MHz beacons were developed in the late 1960s, when car phones weighed roughly 20 lb (the first ELT TSO C91 was written in 1971) |
406 MHz beacons use proven, modern technologies reminiscent of those found in modern cell phones. |
See also Cospas-Sarsat - Advantages of 406 Beacons and Canada's National Search and Rescue Secretariat—Advantages of 406 Beacons
Although modern systems are significantly superior to older ones, even the oldest systems provide an immense improvement in safety, compared to not having any beacon whatsoever.