What is the best avalanche beacon for the backcountry? To find out we took 14 top models and put them through rigorous head-to-head tests in the Cascades, Sierras, and Alaska. We had novices test them to see which ones were easiest to learn and use; then we put them in the hands of certified AMGA and ACMG ski guides, avalanche forecasters, and seasoned avalanche course instructors to see which beacons they preferred most. We tested them in both single and multiple burial scenarios and headed out to a football field to gauge their range and other features. Whether you're a brand new backcountry enthusiast or an industry pro looking for a new avalanche beacon, we have a recommendation for you.
The Best Avalanche Beacons
|Price||$500 List||$309.99 at Amazon||$375.96 at Backcountry||$490.00 at Amazon||$350 List|
|Pros||Super fast processor, differentiates between beacons fantastically during multiple burials, best range in this review, best model for multiple and pro-level examinations||Very fast processor, crushes in the fine search, easy to use, light and compact (great for beacon-in-pocket users), low stress sounds||Tons of features, excellent multiple burial features and performance, differentiates close proximity burials very well, long-ish range, analog mode to help with micro-strip searching||Easy to use, long range, may eventually be the wave of the future, performs well in complex rescues and multiple burials||Very fast processor, excellent range, easy-to-use, comfortable to carry, top-notch multiple burial capabilities with excellent signal lock and marking/flagging functions|
|Cons||Expensive, more complicated than other models, okay but not the best for newer or less practiced users||Display screen is just okay, multiple burial function un-suppresses the last marked beacon in only 1 min, leading to confusion and wasting time, can only mark one signal||A little chunky and slightly slightly bulkier for pocket carriers, one of the more complex interfaces, slightly slower than top beacons in the fine search||More experienced users will have to slightly retrain themselves, expensive, not as intuitive as other models||Not as user friendly in the bracketing stage as other models, sometimes tells user to keep the orientation earlier than we would like during the fine search|
|Bottom Line||The new Barryvox S is one of the most capable and highest performing beacons on the market.||This beacon takes previous Trackers top-notch ease-of-use, speed, and intuitiveness, and adds a marking function and a low profile design.||An extremely capable beacon whose features and multiple burial prowess will appease the requirements of any advanced user or pro.||One of the most technically advanced avalanche beacons on the market, but has a few quirks that can take a little longer to get used to.||A great all-around and very capable beacon that works well for the majority of users.|
|Rating Categories||Mammut Barryvox S||Tracker3||Arva Axio||Ortovox S1+||Mammut Barryvox|
|Single Victim Search (20%)|
|Fine Search (15%)|
|Multiple Burials (15%)|
|Specs||Mammut Barryvox S||Tracker3||Arva Axio||Ortovox S1+||Mammut Barryvox|
|Weight||210g / 7.4 oz||215g/ 7.6 oz.||229g / 8.1 oz||260g / 9.2oz||210g / 7.4 oz|
|Number of Antennae||3||3||3||3||3|
|Manufacturer's Range||70 - 95 meters||50 meters||60 meters||30 meters||70 meters|
Best Overall Avalanche Beacon
Mammut Barryvox S
The Mammut Barryvox S is our top overall scorer and winner of our OutdoorGearLab Editors' Choice Award. The Barryvox S is the newer version of our older award winner, the Mammut Pulse Barryvox. The newer Barryvox S shares many of the Pulse's best features while improving on the few drawbacks. The most notable improvement was speed while moving closer than 10 meters and specifically in the fine search while bracketing. With the older Pulse, you had to move slightly slower than other high-end models and would get a stop sign if you moved too fast.
However, there is no such speed limit on this new version. Overall, the new Barryvox scored the best or among the best in every category and is a top-notch all-around avalanche beacon. The Barryvox S is also easier to use for beginners than the older Pulse, but it has many features and functions that a novice user will likely never take advantage of. Instead, the Barryvox S is the best for intermediate and advanced users, and pro's who will take advantage of its many features and its top-notch ability to manage the most complex multiple burial rescue scenarios.
Read review: Mammut Barryvox S
Best Bang for the Buck
Backcountry Access Tracker 2
The Backcountry Access (BCA) Tracker 2 remains among the easiest and fastest option for novice and intermediate users — and it's still wicked fast for advanced users. It's lightning quick at finding a single victim, which is the most important thing an avalanche beacon needs to do. While it does not have any of the more advanced features of some other products, the Tracker 2 serves the vast majority of users extremely well.
If you like the Tracker 2 but wish it had a flagging/marking feature, check out the Tracker 3 or the Pieps DSP Sport, which could move nearly as quickly and were close-to as intuitive, and both at a respectable price. The Arva Evo4 was also a strong contender for this award and sports a flagging feature, but our testers found it to be marginally more challenging during the bracketing stage compared to the Tracker 2.
Read review: Backcountry Access Tracker 2
Top Pick for All-Around Beacon Between $300-$400
Backcountry Access Tracker3
The slick-looking Backcountry Access Tracker3 is the latest of the Tracker family. Like its relatives, it's excellent at finding a single victim and makes bracketing during fine search exceptionally easy, which is nice because this is where the majority of rescuers struggle the most. The Tracker3 takes the ease-of-use and lightning fast processor speed and adds a handful of features aimed at advanced users: a signal suppression/marking function and the best overall pocket-riding triple antenna beacon on the market. We liked nearly all of the Tracker3's features, (especially its BP or Big Picture function) and its control options. We liked the accuracy of the signal suppression/marking function but didn't like that the suppression only lasted for one minute. After that one minute, it reverts to normal, searching where the rescuer is directed to the closest beacon, regardless of which one that is.
While less of an issue in real-world settings, we also didn't love that the Tracker3 can only suppress one signal at a time; if you try to suppress a second, it undoes the first. This takes a little more practice for pro-level examinations and is worth noting as it is different than the rest of the industry. With that said, the Tracker3 remains a competent, lightning fast model that has some of the functionality desired by more advanced users. The Arva Neo and Mammut Barryvox, both at $350, were solid contenders for this award and our review team found it a difficult decision between these three excellent options.
Read review: Backcountry Access Tracker3
Exceptionally Capable for Pros and Advanced Users
The Arva Axio is a top-notch avalanche beacon for more advanced users and is worth consideration by those in the market for this type of transceiver. The Axio's unique, extra long fold-out third antenna helps differentiate signals nearby burials. This, plus an ability to toggle through signals, along with an analog feature that lets its user micro-strip search when the going gets tough, make it one of the best beacons for advanced users and pros to help deal with complex situations or rescue examinations.
Overall, the Axio was one of the toughest beacons to confuse, and miss-marked/miss-flagged signals the least along with the Barryvox S. This model offers several different settings in each of its function to truly cater to the users' needs. The one thing for more novice users who are considering this beacon is that the Axio's directional arrows disappear at 3m instead of 2m. While this is a pretty small difference, we noticed that more novice users typically need a little more practice with it during the bracketing stage than with models whose arrows disappeared at 2m. More experienced users had little trouble with this.
Read Review: Arva Axio
Notable for Being Fast with an Awesome Flagging Feature
Pieps DSP Sport
The Pieps DSP Sport was formerly our Best Buy award winner when it used to cost $275. Now priced at $320, it's still a rad beacon for what is still a good price, but it faces a lot more competition in the mid-$300 price range. With that being said, we still think it's one of the most capable beacons for its price, and it was one of our favorite overall products we tested for its ease-of-use and lightning quick processor.
Read review: Pieps DSP Sport
Analysis and Test Results
Below we break down the criteria we used for evaluation and present a few key pieces of information regarding avalanche beacons. Besides real-world applications, we also performed a series of side-by-side tests to compare range, processor speed, flagging/marking features, battery life and more.
Related: Buying Advice for Avalanche Beacons
If you are heading into the backcountry, check out our Avalanche Airbag Review. Airbag packs aren't as essential as an avalanche beacon, but they significantly increase your chance of surviving an avalanche.
We've used each model in a variety of settings, putting each one to the ultimate test. Our fleet includes the most expensive, and we highlight Top Picks for specific users. We also highlight models that are friendly on your wallet, so you're able to see the highest value for the best price. One beacon that fits this description is the Backcountry Access Tracker 2. At $300, we recommend it if you're on a tight budget.
Speed, along with ease of finding a single victim, is the most important factor you should consider when purchasing a beacon. While all the cool extra features are rad and easy to focus on, speed and ease of finding a buried beacon should be your strongest considerations. Our speed category measured how fast we could find a victim with a given beacon, start to finish. While speed takes into account several other categories we used in our criteria for evaluation, speed has some of its own, the most important being processor speed, dealing with both a single burial and multiple burials, and multiple burials nearby. To test speed, we compared all 13 products side-by-side over several days with well over 200 tests performed. We also let everyone, from relative novices to seasoned ski guides and avalanche professionals, test them to get a broad insight into each contender's performance. In the end, the fastest performers weren't always the most expensive nor the most feature-rich options. We found several of the more complex models to be slightly slower than their more basic counterparts.
The models we found to be the most lightning quick because of their processor speed were the BCA Tracker 2 and Tracker3, the Arva Neo, and Micro along with the Mammut Barryvox S and more basic Barryvox. The Ortovox S1+, Arva Evo4, and Arva Axio were just a touch slower. We were a little disappointed with the original Tracker DTS. This model did okay but was not as quick as other "simple" beacons.
Ease of Finding a Single Victim
Finding a single victim is the most basic, but still, the most important consideration when choosing an avalanche beacon; in our scoring, we weighted this category the most heavily. We put so much emphasis on this category because statistically speaking 85% of the time rescuers are searching for a single burial. The other 15% of the time where multiple people are buried, rescuers won't have the resources to divide and conquer and will likely be focusing all of their effort on one victim at a time. Additionally, we considered the "ease of use," taking into account the simplicity and functionality of the interface, the controls, and the processing speed.
Another significant factor we took into consideration when comparing the different models was how well each beacon dealt with signal spikes. All of the three antennae models performed well, while the older or less expensive double or single antenna designs, like the older Tracker DTS, had their shortcomings.
Interface and Controls
How easy and intuitive the controls and interface were had a direct correlation with how fast rescuers found beacons, both expert, and novice alike. Along with the user interface are the actual controls that help you navigate through menus, go from Send to Search, and flag a buried beacon.
Our favorite controls for simple models were on the BCA Tracker 2 and the Ortovox Zoom+. Both models have intuitive and straightforward controls that are easy to understand and operate. While they don't have any of the more complex features, like a flagging feature, or options on various functions, both are hands down the easiest to use. Of the mid-level complexity models, referring to models that had a search and send feature and flagging/marking function, the Arva Evo4, Arva Neo, Ortovox 3+, and the Pieps DSP Sport, were surprisingly easy to understand and intuitive to use. Among the most complex models, we liked the user interface and controls of both the Ortovox S1+, the Mammut Barryvox S, and the Arva Axio.
Ease of Use in Fine Search
The fine search is the final phase of the total search and involves the final approach and bracketing stages. This is the part of the beacon phase of the search that rescuers struggle with the most. It is essential to slow down during this stage, but this is also the phase of the search where you can see the most considerable difference between products concerning processing power.
When it came to the fine search, we found a lot of variability in the precision; some were slightly better than others at getting the victim in the center of our brackets more efficiently. The Mammut Barryvox S and Mammut Barryvox performed near the top again, along with the BCA Tracker 2, Pieps DSP Sport, and Arva Neo. The Ortovox 3+ was slightly less precise at having our victim in the center of our brackets.
A Note on Range
A manufacturer's given maximum range is always measured with the searching beacon and the victim beacon in perfect orientation or perfect "coupling." This is a standard for manufacturers and is the same format for our OutdoorGearLab Range test. It is important to note that it is unlikely you will get that much range in a real-world setting because the odds of getting that perfect alignment are slim.
Almost all of the products we tested have a manufacturer's maximum range of between 40 and 60 meters. If a perfect scenario range is 50 meters, its worst case range is 25 meters. Because most beacons cite 40-60 meters of maximum range, it means they have 20-30 meters of worst-case range. This is why AIARE and other avalanche educational organizations teach the rescuer to search with a 40-meter wide search strip width; or 20 meters of range on either side of you. That way you could pick up the signal even with the poorest coupling and a beacon with the lowest possible range, including the extremely popular first generation Tracker DTS.
Manufacturers' Maximum Distance Ratings
While the manufacturer's range was reasonably accurate, some of the time it was usually a bit further than we could pick up.
A Note on the Distance Numbers
The units (IE numbers) that describe distance along the flux line that we follow to find our victim are usually not meters. For example, when we picked up a signal with a Tracker 2 while it was reading 47 meters, we were around 42 meters away. In our tests, the only products where the number was accurate were the Mammut Barryvox and the Mammut Barryvox S. Most units displayed a more significant number of meters than the actual distance. The Pieps models were the most notorious for this, though this doesn't have an effect on actual performance, just something to be aware of.
OutdoorGearLab Range Tests
We tested all of the contenders' ranges on a high school football field. We started out of range with as optimal a coupling as we could get with the target beacon 100 meters away. Our test product was at waist level in a typical searching position. We marked the place we first picked up the signal; we would rarely get the same spot with the same beacon, so we either mention the general range we picked up the beacon on or an average of those attempts.
Those with extended ranges were entirely analog or had an analog function. The model with the longest range was the Mammut Barryvox S, which could pick up a signal as far away as nearly 90 meters in analog mode and 72 meters in digital mode. It is worth noting that while we agree analog function is a cool feature, few people know how to accurately use the analog function effectively enough so that it speeds up the rescue. Because it doesn't aid most people's search efficiently, we primarily reported each product's maximum digital range even if they have an analog mode. In digital mode, both the Mammut Barryvox S and the more standard Barryvox offered the longest maximum range, consistently picking up a signal at around 70 meters away and both coming with manufacturers search strip with recommendations of 70 meters.
After countless tests and comparisons, the products with the next longest ranges were the Arva Neo, and DSP Sport. These models would most often pick up their signals around 50-55 meters away, and were our next round of top performers in this category. Just barely behind were the Ortovox S1+ and the Arva Axio, which were consistently in the 45-50 meter range. Next, it was just below a 10-meter gap before the next group, which included the BCA Tracker 2, BCA Tracker4, and the Ortovox 3+, which were often in the 35-45 meter range.
Ease of Use in Multiple Burial Situations
A Note on Multiple Burials
Roughly only 15% of reported avalanches in North America and Europe are instances where multiple people are buried in an avalanche. Of that 15%, 10% of the time there are just two people buried, and the other 5% of the time there are three or more victims.
To even consider using a flagging/marking function on a beacon you need a lot of rescuers, likely more than four. So while flagging features are cool and relied upon for many ski guide and avalanche professional rescue examinations, they aren't nearly as useful in real-world situations as they might seem. If you have multiple people buried and three or four rescuers, all of those rescuers' effort should likely go into finding one person quickly to give that person the best chance of survival. There is no point in finding two people quickly by getting a probe strike and having them die of asphyxiation because you failed to dig them out and subsequently get them an airway slower. It sounds morbid, but it is better to go "all in" and try to find one person faster, rather than walk around flagging people while leaving them buried.
For your beacon to be able to tell other buried beacons apart, it uses a blend of signal strength and cadence. When two buried beacons are close together, the difference in strength becomes minimal, and your beacon can only rely on cadence (or the pulsing "beep" off each beacon). If there are three or more beacons buried even slightly near each other, they will have too much signal overlap and your beacon can't differentiate a specific beacon's cadence and won't be able to accurately mark/flag/suppress a specific beacon.
Universal Multiple Burial Techniques
Knowing and having practiced with your beacons specific masking/marking/flagging function is useful to be familiar with it. Two universal multiple burial techniques will work with any beacon, eliminates the need for flagging, and works fantastically with multiple beacons in close proximity. They are the Concentric Circles method (sometimes referred to as the 3-Circle or expanding circles method) and Micro Strip searching technique. With either of these techniques, a fast processing beacon is crucial to moving quickly, but again no flagging function is necessary.
Our Findings From Side-by-Side Comparisons
Multiple burial situations are where the competition differed the greatest. Again, it is more important to find, probe and dig up a single victim rather than just flagging/marking multiple victims without digging them up. Consequently, we more heavily weighted "speed" and "ease of finding a single victim" in our scoring. But with that said, multiple burials are still a factor to consider. We performed our side-by-side comparisons tests with two, three and four buried beacons to see how well each model resisted getting bogged down. Keep in mind that with any beacon multiple burials are always harder and take even more practice than single burials.
After dozens of tests and comparisons, the undisputed best products for multiple burials are the Mammut Barryvox S and Arva Axio. These two beacons differentiated between close proximity burials exceptionally well could scroll through victims and were extremely difficult to fool the marking/masking function. The Ortovox S1+ also worked in multiple burials situations exceptionally well but weren't quite as solid as the Axio and Barryvox S. All four of these beacons had the option to unmask previously marked beacons. The BCA Tracker3, Arva Neo, and Pieps Micro all did well in our multiple burials test and would likely do quite well in most real-world situations; however, they wouldn't be our first choice for guide and professional level rescue examinations.
This category showed the most variability between the different avalanche beacons that we tested. Some come laden with useful and well-thought-out features, while others have limited options. Here's how we scored the different models on their features, and below we describe the different feature options currently available on the market today.
Group Check Mode
Several beacons in our review feature a "Group Check Mode" designed to be used during the function check that every backcountry traveler should be doing before leaving the trailhead everytime they go out into the backcountry. While a group check mode is hardly necessary for all beacons, it does help models with stronger signal lock, or the design functionality to lock onto the closest beacon and then stay with it for short periods even if they start moving further away. There are advantages and disadvantages to strong signal lock such as the advantage to differentiate close proximity burials to its user. However, for this portion of the review, it's worth noting that a strong signal lock can make it a pain to perform a function check if there are more than just a few other people in your group. We liked the easy access and visual aids of the Barryvox S and Barryvox, which were different despite so many similarities between these beacons, along with the Arva Axio. We liked the S1+ and the Pieps Micro, but they just weren't quite as user-friendly as far as getting to them and using them.Revert to Transmit Mode
Some of the models we tested had a Revert to Transmit feature. This feature has the unit automatically switch from search mode to send mode if there has not been any user interaction like pressing a button or significant movement during a designated period. The idea behind this feature is if the rescuer is searching and their beacon is in search mode, and a second avalanche hits them, it will switch over in hopes that they can be located.
In the majority of models that we tested that have a Revert To Transmit mode, it comes ready to use. The Pieps Sport, has this feature, but you can't set it up in the field. You must set it up at home by plugging the Pieps data cable into its headphone jack. On the flip side, with the Tracker3 if you want the beacon to revert to transmit you need to turn that function on every time you turn on your beacon — otherwise, it won't revert. We liked models that gave the user the option to turn this function on or off. We gave higher scores for beacons that took it one step further by allowing the user to select the amount of time to pass before the beacon reverted to sending. Beacons that had this feature where the Arva Axio, Barryvox S, Barryvox, and Ortovox S1+.
The Plus/Minus of Revert to Transmit Mode
The Revert to Transmit mode is sometimes a controversial feature. Its importance is sometimes overemphasized and some manufacturers, including Pieps, don't recommend it and intentionally manufacture their beacons without it. Why wouldn't you want your beacon to automatically switch back from searching to sending in the event a second avalanche hits you? Because if you are searching for someone your beacon is likely in your hand, and if a second avalanche hits you there is almost zero percent chance that you will be able to hang onto it. The elastic leash attaching the beacon to the wearer's chest harness is also almost sure to snap. So, while we considered this feature when rating all of the products that we tested, we didn't count it as a significant factor.Turn-Around Indicator in the Fine Search
A cool feature that some models have is a built-in compass or turn-around feature that proved especially helpful for newer users in the fine search. These indicate whether you have gone too far and need to turn around, unlike Pieps or Tracker beacons where the arrow still tells you to go straight ahead. The only indication that you need to turn around is the distance numbers. Products that have this compass feature are the Mammut Barryvox S, Arva Axio, and Ortovox S1+.
A note on Features regarding the first of its kind Pieps Micro. This is the first beacon we've tested not to feature a switch or leaver to turn this model from Search-to-Send or back again manually. Instead, it automatically switches to Send when stowed in a pocket or its harness and automatically turns to Search when removed from these places. It can do this by utilizing a proximity sensor on the face of the beacon. This is a cool idea. However, it didn't always work, which isn't ideal in a life-saving device. We never had an issue while stowing it in its included harness but consistently had issues with not being able to put it into Send mode in our pant pocket even when using its included stuff sack which is supposed to solve this problem. While the Micro was super fast and intuitive and scored well in all other categories, this feature (which is cool and could potentially solve common mistakes in theory) is an area that one of our otherwise top scorers didn't do as well and kept it from winning a Top Pick award.Smart Antenna Technology
This is a feature developed by Ortovox that we expect to see other manufacturers use in the future. Any unit using this type of technology figures out which of the antennas is at the best orientation to broadcast from. With most other products, if the primary transmitting antenna is oriented vertically, the range at which other beacons will be able to pick up the buried beacon is dramatically reduced; it could potentially be only 50 percent of maximum range. The Smart Antenna or similar technology uses gravity to determine which is the best antenna from which to transmit. Smart Antenna Technology helps beacons be found more easily by optimizing antenna position, rather than helping a beacon search better.Isotech Technology
With most triple antennas models, only two of the antennas are used to search for a signal, and most of the time one of them is implemented far less than the other. This affects the beacon's maximum range and thus the search strip width that should be used. The Arva Neo is one of the first products to equally power both of its antennas. This means that the Neo has no worst-case orientation and maintains close to its maximum range regardless of orientation or coupling.
Scanning Functions and Big Picture Functions
Several beacons offer a scanning feature that helps advanced rescuers in complex situations asses how many victims there are to locate and how far away they are, rather than just showing the closest signal. The BCA Tracker3 has a BP or Big Picture mode which, when turned on, displays the distance and a direction for every signal it picks up, quickly cycling through all of them in rapid succession. This is similar to an older analog style avalanche beacon or a Mammut Barryvox S in analog mode.
Except for the first generation BCA DTS Trackers, whose antennas are marginally more vulnerable to being cracked because of their location within the beacon, there isn't much difference in durability among beacons. With all of them, it's important to remember that they are fragile pieces of life-saving equipment and owners should do their best to protect them. Avoid impact or unnecessarily leaving them out in the cold (such as in your car) overnight, which can lead the antennas to de-tune. If the antennas become damaged, even a little crack, it can dramatically affect its ability to search, as well as someone else's ability to save you. This is especially obvious under five meters during the fine search. As far as one company's antennas being more long-lasting and resistant to de-tuning compared to others, we couldn't find a major difference.
Though they do not guarantee survival, the products in this review are meant to enhance safety in an avalanche. When it comes to safety, making a selection can be an overwhelming task. This review is designed to help lay out the differences between the avalanche beacons on the market today.
A Note on Wearing an Avalanche Beacon
On average, 38 people die from avalanches in North America every year and around 185 people worldwide. In North America, roughly one-third of those deaths are a result of trauma, and the other two-thirds are a result of asphyxiation. Wearing an avalanche beacon doesn't guarantee survival in an avalanche and the fact that you are wearing one should not persuade you to ski, snowboard, or snowmobile in a way you otherwise wouldn't.
Practice Makes Perfect
No matter how fancy a product you decide to purchase, training and practice are essential. The above graph emphasizes the importance of proficiency with a rescue beacon. Experienced backcountry enthusiasts and avalanche professionals can find multiple beacons in under six minutes, while the unpracticed novice can easily take 25 minutes or more. While the average rescue time is heavily debated, it is thought that from the moment the person is caught in the slide to the time the victim's airway is exposed on the surface is around 20 minutes. Wouldn't you and your partners like to be on the faster side of that average?We recommend taking an AIARE (American Institute for Avalanche Research and Education), American Avalanche Institute (AAI) or other American Avalanche Association (AAA) recommended avalanche course. Additionally, if you haven't taken one in the last 5-10 years, you should consider retaking one since a lot has changed with recent studies. It would be extremely rare for a person not to find retaking a Level 1 useful, even if they have taken a course several years prior.
Wearing an Avalanche Beacon
There are two locations on your body that are considered acceptable to wear an avalanche beacon. The first is in your beacon's harness not exposed to the outside (i.e., covered up); this puts it in the most significant "target area" on your body, and it's slightly more protected from impact. While wearing the beacon in its harness, it is not acceptable to have it exposed to the outside. It needs to be underneath at least one layer of clothing because the odds of it getting ripped off your body are too high otherwise.
It is not okay to have the beacon uncovered no matter how hot it is, or how cool you think it makes you look. The other acceptable location is in a zippered pants pocket. The pants pocket needs to be an internal (i.e., inside hanging) zippered sewn-in pocket. Laminated pockets are not okay because these types of pockets have been torn off during an avalanche. Despite worry from some readers, there has never been a reported case of someone having their pants ripped off in an avalanche with their beacon inside.
Don't keep your cell phone, radio, GoPro, or any other device that broadcasts a signal within 30 cm of your avalanche beacon. There have been at least two high-profile deaths in the last three years because a cell phone interfered with the wearer's beacon. At least one of these cell phones was off. Recently there have been reports of heated gloves and boots causing interference.
— Ian Nicholson and Chris McNamara