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Old 30 March 2004, 09:51   #21
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121.5 MHz (EPIRBs) info

Hi there in anwser to your query yes I do have a lot of experience with Sarts and Epirbs and have just done a refresher course not less than 3 weeks ago.I hope I do not bore you with the chapter listed below though !! .They do have severe limitations though and the success rate of recovery can be hit or miss .Attached a Jpeg of the overview of how it all works.The frequency 121.5 MHz is an aeronautical emergency frequency. 121.5 MHz radiobeacons were developed in the mid-seventies for installation on aircraft, as Emergency Locator Transmitters (ELTs). However, they can also be used on board ship as Emergency Position-Indicating Radio Beacons (EPIRBs) or as Personal Locator Beacons (PLBs). These beacons transmit signals that are relayed by Cospas-Sarsat LEOSAR satellites to Cospas-Sarsat LEOLUTs which process the signals to determine the beacon location. The alert which consists of the computed position of the transmitter is then relayed, via a Mission Control Centre (MCC), to the appropriate Search and Rescue Point Of Contact (SPOC) or Rescue Coordination Centre (RCC).Doppler location (using the relative movement between the spacecraft and the beacon) is the means used to locate these very simple devices. To optimise Doppler performance, satellites in a low-altitude near-polar orbit are used, however, the location accuracy of 121.5 MHz beacons is not as good as the accuracy achieved with 406 MHz beacons because of the relatively poorer frequency stability performance of these older generation beacons. The low satellite altitude results in a low uplink power requirement, a pronounced Doppler shift, and short intervals between successive satellite passes. The near-polar orbit could provide full global coverage, but 121.5 MHz alerts are produced only if the satellite "can see" both the beacon and the LEOLUT simultaneously for a period of time. This constraint of the 121.5 MHz system limits the useful coverage to a geographic area of about 3,000 km radius around each LUT. Furthermore, the Doppler processing technique provides two positions for each beacon: the true position and its mirror image relative to the satellite ground track. In the case of 121.5 MHz beacons, normally, a second satellite pass is required to resolve this ambiguity.The local coverage mode at 121.5 MHz has proven very effective in providing quick alert and location data for the large number of existing 121.5 MHz distress beacons. However, the transmission characteristics of these beacons do not permit satellite on-board processing of the signal, therefore, global coverage is not possible. Furthermore, the system cannot generally distinguish between a 121.5 MHz distress beacon transmission and any other 121.5 MHz signal. This limitation results in a large number of false alerts in the form of Doppler locations which do not correspond to distress beacons.The bad news is its due to be phased out in 2008 ? i think.Personal experience says as a rule of thumb getting down to basics on a good day we would get your posistion down to maybe a couple of miles however they should always be used in addition to cosatl flre back heliographs etc etc etc initial ICAO standards were not established with the aim of satellite reception of 121.5 MHz signals. Therefore, the 121.5 MHz Cospas-Sarsat system was designed to serve the existing type of beacons, even though system performance is constrained by their characteristics. Parameters such as system capacity (number of simultaneous transmissions in the field of view of the satellite which can be processed by LEOLUTs) and location accuracy are limited. No information is usually provided about the operator's identity, although a morse coding of the signal is included in some models; however, these data are not processed automatically by Cospas-Sarsat LEOLUTs. The efficiency of 121.5 MHz beacons has been greatly enhanced by the use of satellite detection and Doppler location techniques.Typical 121.5 MHz Beacon Characteristics.




RF Signal

Transmitted power : 50 - 100 mW PERP*
Transmission life : 48 hours
Frequency : 121.5 MHz 6 kHz
Polarization : Linear
Modulation

Sweep rate: 2 - 4 Hz
Range : 300-1600 Hz (swept at least 700 Hz)
Modulation type : AM
Modulation depth : > 85%
Duty Cycle : 40%
* Peak Effective Radiated Power relative to a 1/4 wavelength monopole mounted on a ground plane

Hope this helped a bit ..............My strong point is Engines
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Old 30 March 2004, 12:15   #22
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Wow,

Now that is what I call an answer!!! Thank you very much. From what I can deduce is that it is better than nothing at all and not quite up to EPIRB standard. But for the price a good additional piece of safety kit.

Many thanks
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Old 31 March 2004, 05:34   #23
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Quote:
Originally posted by Aging Youth
Wow,

Now that is what I call an answer!!! Thank you very much. From what I can deduce is that it is better than nothing at all and not quite up to EPIRB standard. But for the price a good additional piece of safety kit.

Many thanks
yes it is a very useful item to have on you along with DSC VHFs etc it will hoepfully never be used though !
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