When I'm skiing, I have a cell phone with me, if only for safety reasons. Of course, taking pictures and listening to music on your smartphone isn't bad either. Sometimes I take a helmet camera with me, or maybe a real camera, or both, or an MP3 player, or a GPS, or a radio, or something else from the electronic fleet that many of us have at home. Unfortunately, none of this really works well with the avalanche transceiver. Why is that and what should I pay attention to?
What does the avalanche transceiver do?
Avalanche transceivers emit electromagnetic signals and can also receive them in search mode. According to standard EN 300718-1, the avalanche transceiver signal must have a frequency of 457 kHz, with a tolerance range of +/- 80 Hz. The signal characteristics are also standardized: The signal is not sent continuously, but as individual pulses at intervals of around one second. The period duration must be 1000+/-300 ms (1 second, tolerance range 300 milliseconds), whereby the pulse duration must be at least 70ms and the pulse pause at least 400ms long. The signal strength or transmission power is also standardized and must not exceed a certain maximum value or fall below a minimum value.
What can interfere with the avalanche transceiver
Transmission mode:
Metallic objects and electronic devices that are close to the transmitter can "shield" the signal and reduce the transmission power. This includes avalanche shovels, metal belt buckles, carabiners and the like, but also things like chewing gum paper with metal content and jam sandwiches wrapped in aluminum foil. As a rule, negative effects only occur if the interfering objects are very close (less than 30 mm) to the transmitter. However, as anything can slip in the event of an avalanche (and anything else), a minimum distance of 20 cm from the transmitter to any electronic devices and metal objects is recommended. Although the test in the video is not particularly realistic, it shows the shielding effect. If the transmitting device is not only wrapped in aluminum foil, but the aluminum foil is also pressed against the device by hand, the signal deteriorates further. In this case, the transmitting device recognizes that a source of interference is present and displays an error message, which disappears when the aluminium foil is removed.
Search mode:
If the radio suddenly hisses because the cell phone next to it is ringing, this is due to electromagnetic interference. In addition to the desired signal, the radio also receives a signal from the cell phone and can no longer distinguish the correct signal from the wrong one. The same effect also occurs between search avalanche transceivers and other objects that produce electromagnetic fields. Avalanche transceivers are designed to receive weak signals from a great distance. Devices with a long range must be correspondingly sensitive. However, this also makes them susceptible to interference signals: if the device has to identify and isolate the already distant, weak signal from the transmitter during the signal and coarse search, it has a correspondingly harder time with increased background noise.
The distance to the avalanche transceiver is decisive for the extent of the interference caused by electrical gadgets or metal parts. According to Genswein et al. (2013), the strength of the interference changes approximately to the third power of the distance between the avalanche transceiver and the source of interference. If I halve the distance between the cell phone or similar and the avalanche transceiver, the interference potential is not doubled, but increased eightfold. Conversely, the interference can be reduced accordingly by keeping a greater distance.
Different devices cause different levels of interference. Smartphones with an active display are much more critical than when switched off. Operating the display, whether on a cell phone or another device, generates additional magnetic fields that are constantly changing. There are not too many studies on this topic and those that do exist use different methods to investigate the influence of certain electrical devices. The figures above and below show the results of laboratory tests (Meister & Dammert, 2014). Barkhausen (2012, see link below) investigated the influence of interference sources with range tests in the snow. The results differ in details and presentation, but the conclusion is the same: You should be careful which pocket you put your cell phone (and all that other stuff) in.
Magnets
Magnets are another potential problem. They can be found, for example, in the form of magnetic buttons on clothing or built into loudspeakers (radios, cell phones, etc.). As avalanche transceivers must be waterproof, external on/off/transmit/search switches or levers on some models are coupled with magnetic switches inside the device, which can also be activated by external magnets in unfavorable cases. Various avalanche transceiver models also have a magnetic compass, which is primarily used to optimize the directional display. Here, too, there is potential for interference, especially in search mode.
Practical recommendations:
The general recommendation from ICAR, the Alpine Club, mountain rescue services etc. is:
In transmit mode, keep a minimum distance of at least 20 cm between the avalanche transceiver and all electronic devices and metal objects; in search mode, keep a minimum distance of at least 50 cm between the avalanche transceiver and all electronic devices and metal objects. Switch off electronic devices. In search mode, electronic devices must be switched off (flight mode on the cell phone is not sufficient). If you have to make a phone call during a search operation, it should ideally only be short and at least 25m away from the person searching. If there is no other option than to carry the cell phone during the search (single person searching, contact with mountain rescue must be maintained), it must be kept as far away as possible from the avalanche transceiver (backpack, back trouser pocket, etc.). This arrangement can lead to problems during the search. Ideally, shovel, probe and ski poles should not be held in the same hand as the avalanche transceiver being searched for, or lie next to the avalanche transceiver on the snow surface when crossing (influence on the search in this case according to the DAV "acceptable", if it cannot be avoided). Multifunction wristwatches (large display, GPS function, etc.) should also not be worn on the avalanche transceiver hand. With GoPros chest mounts or other cameras, you should bear in mind that the camera may be 20 cm away from the avalanche transceiver in transmit mode (e.g. if the avalanche transceiver is in your trouser pocket), but the recommended distance (at least 50 cm) can hardly be reliably maintained in search mode (avalanche transceiver in hand). High-voltage power lines, lift masts and other large, immovable sources of interference can significantly impair the search in a relatively large area. Only a search in analog mode can help here (the human ear can identify the "correct" signal better than a processor), depending on the very narrow search strip width.
Sources:Barkhausen, 2012. The effect of external interference on avalanche transceiver functionality. Proceedings ISSW, 2012.Genswein et al, 2013. Recommendations on how to avoid interference issues in companion and organized avalanche rescue. Proceedings ISSW 2013.Meister & Dammert, 2014. The effect of consumer electronics on avalanche transceivers. Proceedings ISSW 2014.
