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Aviation distress beacons

  
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This document was written in 1999 by David McBrien of AusSAR. It appeared in both the AOPA and AUF magazines. There are some parts that are not current, but nothing of any real significance. The page was last reviewed 30 November 2009.



Distress beacons have been used in aviation for many years and, with some flights now being conducted without the lodgement of flight plans or flight notes or reporting progress, there is increasing importance on having an effective distress beacon as a means of last resort to alert the SAR system that you are in grave and imminent danger. The carriage of aviation distress beacons has been the subject of much debate in the past and this article is designed to bring readers up to date on some of the related issues.
The Cospas-Sarsat System
The Cospas-Sarsat satellite based system provides distress alerting and location information to search and rescue (SAR) authorities in the aviation, maritime and land environments. The system, which has been in operation since 1982, was originally designed to service a discrete distress frequency on 406.025 (generically stated as 406) MHz but the requirement was expanded to include a service on the aviation distress frequency of 121.5 MHz. In the case of the latter, the physical characteristics of the radio frequency and the output signal mean that there is coarser resolution with beacons operating on this frequency compared to those operating on the higher frequency. There has been major penetration of the 121.5 MHz beacons into non-aviation environments because of their relatively low cost.

The Cospas-Sarsat space component comprises a minimum of four Low Earth Orbit SAR (LEOSAR) satellites in polar orbit (two Russian Cospas satellites and two US SARSAT satellites with some reserve units) which monitor 121.5 and 406 MHz. Additionally, the Sarsat satellites monitor 243 MHz which is the military aviation distress frequency. More recently, a number of 406 MHz repeaters have been added to satellites in an equatorial geostationary orbit (termed 'GEOSAR') which provide a supplementary source for near instantaneous alerting of a distress alerting signal should the LEOSAR satellites not have the source in view. A more detailed explanation of the Cospas-Sarsat System can be obtained from the Cospas-Sarsat website.

Australia, through Australian Search and Rescue (AusSAR), is responsible for operating the nodal Cospas-Sarsat ground segment in the South West Pacific region. This is done by monitoring satellite intercepted signals from three ground stations, termed Local User Terminals (LUTs), in Albany, Bundaberg, and Wellington (NZ). With 121.5 MHz signals, the three elements in the process (ie the beacon, the satellite and the ground station) must be in view of each other. This is often termed 'local' coverage. With the 406 MHz signal, the satellite has the capacity to time tag the digital information and repeat it when it is next interrogated by a LUT. Through this means, 406 MHz beacons provide 'global' coverage.
Beacon Terminology
There have been a number of conventions used in the past to describe the various types of distress beacons that have been available in the market place. The current [1999] practice is to use Emergency Locator Transmitter (ELT) to describe those that are fitted to an aircraft, Emergency Position Indicating Radio Beacon (EPIRB) to describe those that are designed to float when immersed in water, and Personal Locator Beacon (PLB) to describe the portable units that are designed for personal use.
Compatibility of Older Technology Beacons
The 1960s saw the emergence of aviation distress beacons that operated on 121.5 MHz. These beacons met the FAA TSO C91 standard and provided an audible tone on the frequency with the likelihood that other aircraft or air traffic services in the area would intercept it and become aware that an aircraft was in distress. A large number of aircraft were fitted with crash activated fixed ELTs during this period and many commercial operators carried the man-portable Electronic Locator Beacons such as the Garret Rescue 99. These systems are not covered by the Cospas-Sarsat system and continue to rely on the aviation sector for SAR alerting purposes.

When a decision was taken to extend the Cospas-Sarsat system to include 121.5 MHz, the standard pertaining to aviation beacons was revisited and a new standard (FAA TSO C91A) was set making the beacon emission suitable for intercept by satellite. The new standard was not made retrospective and many aircraft in Australia still have non-Cospas-Sarsat compatible units fitted.
Benefits of Later Technology Beacons
The 121.5 MHz beacons in current production are relatively lightweight and inexpensive (with lower end of the market PLBs costing in the vicinity of $A200). They provide an affordable alternative to the more expensive 406 MHz beacons, (which currently [1999] cost from $A1600 but expected to get cheaper) but at an operational cost. There are also 406 MHz beacons being released on the market that have an embedded GPS and automatically report the beacon position in digital form via the satellite system when activated.

A comparison of the 121.5 MHz versus 406 MHz beacon technologies is shown below:

 121.5 MHz406 MHz
Location Accuracy15 – 20 km [design specification]2 - 3 km [design specification]
CoverageLocal – the beacon, the satellite and the LUT must be in sight of each otherGlobal – the satellite has the capacity to store the information and repeat it for subsequent processing
Signal Power0.1 Watt5 Watts
Signal TypeAnalog audio signal with no identification feature and subject to high false alert rate due to interference signalsDigital with encoded identification of beacon registered owner and capacity to overlay externally provided or embedded GPS position
Alert TimeDepends on location and varies from 2 hours to the system being ineffective outside coverage areas with ambiguous fix positions often being provided on the first passNear instantaneous with GEOSAR assisting to provide alerting data if a LEOSAR is not in range. The exception is polar regions where very short delays can be expected.
Doppler LocationOne satellite pass but an ambiguous fix position until resolved by other means or another satellite passSingle satellite pass
GPS LocationFunctionality not available160 m accuracy (if fitted)
HomingAircraft and vessels use the 121.5 MHz audio signal for homingThese types of beacons simultaneously transmit on 121.5 MHz for homing purposes


As a result of the location of the three LUTs servicing the Australian region, there are approximately fifty satellite passes serviced per day by AusSAR which results in a typical coverage area and average times for detection of a 121.5 MHz beacon.

It should be noted that there are areas, mainly in open ocean areas, around the world where there are gaps in 121.5 MHz coverage. Specific areas not covered of interest to Australia include the Antarctic area, the western Indian Ocean, the southern two thirds of Africa, the mid-southern Pacific Ocean and a gap on the regular Australia to United States air route between the Wellington and Hawaii ground sites. The gap in the southern two thirds of Africa is being addressed in two ways. The first is through ICAO which has mandated that international carriers are to be equipped with 406 MHz beacons when operating in Africa and the second is the planned location of a new LUT site in South Africa which is expected to be operational by late 1999.

The major implications for general aviation aircraft operating in Australia using 121.5 MHz beacons is that if the beacon is of the older type, then there is a reliance on other aircraft to detect the 121.5 MHz signal and raise the alarm. This may be problematic in many parts of Australia as only the larger commercial aircraft regularly monitor this frequency. If the beacon is Cospas-Sarsat compatible, the system will generally detect the signal but produce an ambiguous fix position either side of the satellite pass. Follow-on passes, collateral information, or the use of aircraft to investigate both possible positions are used to refine the correct distress beacon position.

This evolution takes time and the accuracy of the Cospas-Sarsat derived position is less accurate than with the more technically advanced 406 MHz beacon which usually provides an accurate position on the first pass. These beacons are also encoded with the details of the registered owner and, through the GEOSAR supplementary repeaters, provide near instantaneous advice that an emergency situation exists prior to a Cospas-Sarsat satellite pass. If an embedded GPS is fitted, a position will be passed along with this initial alert advice. The time critical nature of an adequate response is a major consideration when considering the safety of life.
Recent ICAO Decisions
The ICAO Council agreed in March 1999 that new aircraft operated on extended flights over water or flights over designated land areas shall be equipped with a 406 MHz beacon from 1 January 2002 and existing aircraft will be required to carry them from 1 January 2005. The Council also agreed to write to the Cospas-Sarsat governing body recommending that satellite processing of 121.5 MHz signals cease from 1 January 2009. [This came into effect February 2009] There is an expectation that the international maritime community will follow this lead.

A decision regarding the carriage of distress beacons by domestic aircraft in Australia rests with CASA. The FAA has announced that, at this stage, it plans not to mandate the carriage of 406 MHz beacons for general aviation aircraft. One of the major reasons for this position is that emerging technologies may have the potential to offer better and more cost effective solutions for this sector of the industry. There has also been some discussion that, given the dense aviation environment experienced in the US, that 121.5 MHz beacons may remain an effective option without the need for satellite monitoring given that all large commercial operators guard this frequency.

However, this is not the case in Australia where many remote operations are conducted without SAR details being lodged and where there is a reliance on distress beacons to act as the primary SAR alerting method. For remote area operations, the low frequency of other traffic in the area must be a consideration in the Australian context when selecting a replacement distress beacon.
Conclusion
Understanding the division of responsibilities between agencies, informing the system when you are in difficulties and understanding the limitations of distress beacon technology are all important aspects of airmanship. The professional response by the aviation community to the numerous SAR incidents that occur around Australia is publicly acknowledged for, without your assistance, the coordination role of AusSAR would be impossible.

The next section of the Coping with Emergencies Guide is 'Understanding SAR services' also written by David McBrien of AusSAR.

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Groundschool – Coping with emergencies

| Guide contents | Knowing the aircraft | Deceleration forces | Forced landing procedures |

| Overcoming aircraft control failures | Procedure when lost | Safety and emergency communication procedures |

| Aviation distress beacons | Understanding SAR services |

| Comfort and survival in a remote environment | ERSA emergency and survival procedures |