Avalanche Beacons, Getting The Drift By Bob Lee

These days just about every backcountry snow traveler who is serious, savvy, and responsible has an avalanche beacon, those small radio transceivers that give a buried avalanche victim a chance of being recovered alive by sending out a signal that can be tracked and pinpointed by the other members of the group.  Everyone that ventures into snow-covered terrain that’s steeper than a meadow should own a beacon, as well as a shovel and probe.

Using an avalanche beacon to find another one is a combination of science and art, as is snow analysis and winter backcountry travel in general.  Avalanche awareness and training classes are invaluable information sources for people that want to get out into the backcountry during winter and there are a number of good primers and tutorials online.  Links to some of those can be found at the end of this piece.

But what I want to write about here is a problem with older or traumatized beacons that many users may not be aware of – frequency drift.  First, a little background: The earliest avalanche beacons came on the market in 1973 and transmitted a signal at 2.275kHz.  For a while 2.275 kHz was the North American standard and 477 kHz was the standard in Europe.  In 1997, the 2.275 kHz frequency was totally abandoned in favor of the current 457 kHz standard,  The two frequencies are not compatible and any 2.275 kHz beacons still in use are obsolete and need to be retired immediately.

Currently there are two modes of beacons.  Analog beacons use an audible beep and (usually) a visual display that get stronger as the receiving beacon approaches the transmitting beacon.  Digital beacons use multiple antennas and digital processing for a display that shows the direction and distance to a buried beacon.  Some beacons combine both analog and digital modes to make use of the benefits of each – analog signals have more range, digital processing helps to locate victims more quickly.

Back on the issue of frequency drift, a fairly recent study by Bruce Edgerly and John Hereford – linked below – found that modern digital beacons may not be able to receive signals from older and heavily used beacons because 1) the older beacons may be transmitting signals that have “drifted” off from the designated frequency and 2) the demands of digital signal processing require some “narrowing” of the range of frequency a digital beacon can receive.  The study can be found at:
http://beaconreviews.com/transceivers/pdfs/Frequency_BCA.pdf

A somewhat less scholarly (and easier to read) version of the same study can be found at:
http://www.backcountryaccess.com/english/research/documents/FreqDrift.pdf

The important message to winter backcountry travelers is that older avalanche beacons, especially analog ones, may not be capable of having their signals received by the newer digital units.  Older beacons can be returned to the manufacturer to be checked, or if you have access to a newer Pieps DSP beacon it has a feature that can check other beacons for frequency drift:
http://beaconreviews.com/transceivers/Specs_PiepsDSP.asp

Bottom line: if you have an older beacon – such as the Ortovox F1 or M1/M2, the SOS F1-ND, or the Pieps 457 Opti 4 – I strongly urge you to have it checked or consider replacing it.  Extra strong urging if the older beacon has ever been dropped or tumbled.  New beacons aren’t particularly inexpensive, but peace of mind, as they say, is priceless.

Links:
Northern New Mexico avalanche training:
http://www.nnmae.org/edu.html

Backcountryaccess.com learning and education:
http://www.backcountryaccess.com/english/education/learning.php

Backcountryaccess.com research:
http://www.backcountryaccess.com/english/research/index.php

Beacon reviews and information:
http://beaconreviews.com/transceivers/

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