When searching for multiple beacons you, or your students, may have come across the following problems with the new generation of digital transceivers:
i Search beacon loses mark and leads you back to the previously found beacon
ii. Distance or direction does not change or leads the searcher in the wrong direction
iii. A Beacon is not detected
In the stressful environment of a live rescue or even an exam these kind of problems can lead to a sense of panic, lost time and bad decisions that can ultimately be fatal for the victim(s).
If you’ve had any of the above problems they could be due to signal overlap of beacons. Backcountry Access (BCA), who make the Tracker beacon, have been investigating this problem. They performed 400 searches on a park of 5 different beacon types using 4 different models of search beacon. All the “victim” beacons were the same, something that is typical in professional fleets. They found a 5% failure rate for 2 beacons, 15-20% with 3 beacons and 30-70% with 4 beacons.
So what is happening? Modern digital avalanche beacons use signal strength and timing analysis to differentiate beacons in multiple burial situations. Once a transceiver is located its signal can be “marked” and it is excluded from the search. This runs into problems when the signal from one beacon masks another so that for a searcher it appears there is only one beacon. In the worst case scenario it may be possible to “mark” two beacons at the same time. Beacons with a long pulse width, such as some older analogue models, are more likely to mask other beacons.
Steve Christie of BCA says it is like a race track “where there are four lanes and four cars travelling at different speeds, at some point during the race all four cars may be lined up alongside each other”, an observer would only see the nearest car, “the race is your search and the cars are beacons under the snow and the speed, their transmission rate”. More cars (beacons) and the greater the chance that one will hide the other at some point. In general, once four beacons are in play you have a high risk of signal overlap.
The Pieps DSP, Mammut Pulse, Ortovox S1 and 3+ can show up to 3 transmitting beacons. They lock on to the strongest signal. With more beacons you will have problems such as being asked to “stand still” while the processor analyses the situation. With four or more beacons you may have to switch to analogue mode and reduce the sensitivity down to around 5 meters. A protocol, such as the 3 Circle technique can then be used. After locating the first beacon, the searcher moves away by 3 meters and walks a circle around the found beacon looking for other signals. This is repeated for 6 and 9 meters to cover the sector. With more searchers the problem can auto-regulate as searchers will be spread out over a wider area which will introduce a sufficient degree of randomness into the search. Outside searchers coming to the scene, such as members of the rescue services, should start the search with their beacon in analogue mode in order to analyze the situation. Groups where some members are wearing analogue beacons should be very careful about using “marking” features. Sometimes you get “fluke” burials, where one victim is lying directly above another. However the biggest factor in survivability remains group organization and technique particularly the available manpower to dig out victims.
Multiple burials are the exception rather than the rule. Research by Frederic Jarry of the ANENA showed that they occurred in 21% of ski touring incidents and 13% of off piste incidents. The difference is down to the exposure and proximity of ski tourers when ascending. Safe travel procedures, such as group spacing, stopping out of the path of avalanches triggered by the group and careful choice of ascent route, remain the best methods to avoid multiple burial situations.
Signal Overlap Explained, Steve Christie, BCA; The Avalanche Review, Vol 30/1 October 2011
Signal Overlap Failures, Sean Metzger, Matt Greenberg, Bruce Edgerly, BCA, The Avalanche Review, Vol 30/1 October 2011
Thomas S. Lund (Signal Strength vs. Signal Timing - The Avalanche Review - Vol 26/2)