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Sunday, 6 November 2011
Re Post - Signal Overlap in Multiple Burials
If you have a beacon with a "mark" feature beware of signal overlap. Listen to the audio signals from this old analog beacon. Analog is good for demonstrating signal overlap because of the audio picking up and letting you hear all signals. This short home experiment I did illustrates overlap, as two pulses merge to become one for a time. Imagine you have four vehicles on a track. One travelling at 20mph, one at 30, one at 40 and one at 50mph. At some point all 4 would be in line and seem as one. So like the vehicles the more beacons on transmit the more chance of signal overlap. There is a risk when you "mark", that you mark two as one therefore missing one victim, or more if its a really big scenario. Manufacturers do try and address this by varying the pulse rate. Imagine what would happen in a large group with the same Beacon pulsing at the same rate! But, there is a limit to how much they can do this before performance is affected. Therefore there is still very realistic possibility of synchronization of the signal and marking two, with one hidden.
Signal overlap is more frequent with 3 and even worse with 4 beacons. For that reason on training session limit your transmitting beacons to less than 4 as you might confuse a student, or in real time miss a victim. At least if you know of signal overlap you can be aware of it. Establishing as soon as possible how many are in a party, how many have beacons, and how many victims are left out of the tip can help the rescue leader establish if the search is compromised and the searchers can try and separate any overlapping signals.