Member - Kiwi Kia posted:

MUZBRY, that is for the 'SPOT" beacons, not the 406 MHZ beacons.

Member - Kiwi Kia posted:

Sorry howie - not correct.

The following relates to a 406 MHz EPIRB that does NOT have a built in gps.

The 406 MHz alert signal is transmitted to a geostationary satellite and relayed to the nearest ground station (Canberra) almost instantly. However, to triangulate the signal and work out a very close ground position requires the 406 MHz signal to be picked up by TWO passing low orbit satellites. Depending on the latitude where the beacon is activated this could take up to 10 hours in a worst case scenario.

The first pass of a low orbit satellite will give TWO possible locations, one location either side of the satellite track over the surface of the earth.

The second low orbit satellite pass will also give two possible locations, one of which will be identical to one of the possible locations indicated by the first passing low orbit satellite. This 'matched' location is the position of the EPIRB.

The 406 MHz beacons also contain a 121.5 MHz transmitter and this signal is to aid searching aircraft to 'home in' as they approach the position derived from the two satellites.

So, the rescue centre will be alerted that an EPIRB has been activated and it's unique identifying number almost immediately. IF the beacon has been registered then from the identifying number they will also be able to ascertain the ownership of the beacon and details of the nominated persons who should know where the owner was planning to travel.

Now, back to the TWO possible earth locations of the activated beacon. If one of those possible positions is out in the Indian Ocean and the beacon is registered as being for land use then there is a very good chance that the land location is in fact where the beacon is located so the second pass of a low orbit satellite is only really a confirmation that the owner has not lent the beacon to a boatie mate who has taken out deep sea fishing.

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howie posted:

i did not say your position would be known in seconds, only the signal will be received in seconds.
the final positioning of the non gps beacon(within 5 miles) will be done by successive satellite passes as you say.

my main reason for posting is that there seems to be the idea, by people on this forum, they do not need to get the new 406mhz unit as every passing plane will come to their rescue.

Member - Kiwi Kia posted:

Hi again Howie, I was referring to the following in your posting;

"in answer to your real question, it only takes seconds for the 406mhz to be received. that figure of 90 mins is for the 121.5mhz
analogue signal"

The 121.5 MHz transmission has nothing to do with 90 minutes. The 90 minutes may relate to the satellite passes only.

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howie posted:

we seem to be going down a path that i did not intend.
but, according to the amsa, the old 121.5 takes 90 minutes on average to be detected.
yes, that relates to satellite passes obv.

Member - Kiwi Kia posted:

Howie, please also note that 'detected' does not mean located. It just means that a rescue center has been alerted that a beacon has been switched on somewhere.

Member - Kiwi Kia posted:

Slave, the 121.5 signal will only be heard hundreds of km away if the aircraft is at very high altitude AND they are listening. The signal propagates outwards in an inverted cone shape (or funnel if you like).

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