For years we've been aware that electronic devices, metallic objects—and even foil energy bar wrappers--may interfere with an avalanche transceiver search. In this study recently published by the Alpine Club of Canada and the Canadian Avalanche Association, electrical engineer Ivar Finvers and mountain guide Doug Latimer explore avalanche transceiver interference sources and propose some practical search strategies to address them. While their experiments used Mammut and Pieps transceivers for searching (and Tracker2 for transmitting), their findings are consistent with our findings using Tracker transceivers for searching.
Finvars and Latimer evaluated a number of potential sources of interference in the field and propose some practical search strategies to address them. The purpose of this article is to:
- Identify and quantify potential threats to a clean signal search.
- Begin a meaningful discussion on how to mitigate signal interference.
- Introduce possible search strategies for a buried victim when interference is present.
THE SOURCES OF AVALANCHE TRANSCEIVER INTERFERENCE
We gathered three transceivers and an arrangement of electronic devices to test. The transmitting transceiver was a BCA Tracker2. It was placed on the ground 20m away from the receiving transceiver, which was on a wooden platform about a meter off the ground. The poles of the antennae were aligned pole to pole for the first test. The interference was measured indirectly by recording the distance reported by the receiver (actually the range of readings, since they always fluctuated) and comparing it to the baseline with no interference. Once interference from all devices was measured, the transmitter's antenna was re-oriented to be perpendicular to the receiving antenna, and the test was repeated.
Each potential source of interference was placed at a right angle from the receiving antenna at 30, 20, and 10 centimeters away and then in contact with the receiver. Two different receivers were tested, the Mammut Barryvox and the Pieps DSP. Figures 2 and 3 provide a graphical summary of the interference recorded by each device. Each sub-plot indicates the distance reading indicated on the receivers when the source of interference was placed at each distance from the receiver. The amount of interference was inferred by comparing the distance readings to the baseline for the test. Any change in distance, direction, or variability was taken to imply interference.
The test was conducted at the Banff airfield; no other potential sources of interference could be found in the area. First, we recorded the baseline distance for each receiving transceiver. At 20m, with the antennae aligned, the baseline for the Mammut Barryvox varied slightly with a signal of 24–25m, while the DSP signal fluctuated between 20–24m. When the transmitting antenna was aligned perpendicular to the receiver, the Mammut Barryvox recorded a distance of 31–33m, and the DSP reported distances of 36–39m.
When devices of potential interference were introduced, they either had no immediate effect, or the display indicated an altered distance to the transmitter. When interference became more pronounced, the arrow indicating the location of the transmitter began to shift. Additional interference would produce erratic signal locations and variable distances. At this point, it was not possible to use the receiver to ascertain the distance or direction from which to search. Further interference resulted in the loss of signal and no distance being reported by the receivers.
Figure 2. Interference with Barryvox receiver.
Figure 3. Interference with Pieps DSP receiver.
THE IMPACTS OF AVALANCHE TRANSCEIVER INTERFERENCE
When the interference sources were at a distance of 30cm or greater, none of the sources (excluding a cordless drill we used experimentally) had any significant impact on the distance readings when the receiver's antenna was aligned with the transmitter's antenna. Rotating the transmitter at 90° weakened the received signal, causing the distance readings to increase and show greater variability and, in a couple of cases, resulting in a loss of signal.
As the distance of the interferer was decreased, the variability of the distance readings increased and, in some cases, resulted in the loss of signal. Generally, the impact of the interference source was greater when the transmitter was rotated 90° due to the weaker signal.
The greatest interference was recorded by electric motors. The cordless drill we tested rendered transceivers useless when it was 50cm from the receiver. We included a cordless drill as an interference source only because the high-power PWM circuits used to drive the brushless motor can be a fantastic source of broadband EMI. While it is a great tool for illustrating the effects of electromagnetic interference, it is unlikely to be on the scene of an avalanche.
The auto-focus motors on the cameras were also quite disruptive, but because of the short duration to focus, we were unable to accurately determine their effect. Though not part of this test, the magneto on a running snowmobile may also be a major source of interference. Studies by BCA have indicated that users should be at least one meter from a running snowmobile when performing a search.
Display screens appeared to be the second greatest source of receiver interference. We suspect this is caused by the display electronics present in all smartphones and tablets. The larger the screen, the greater the interference.
LED lights were also a significant source of interference. Just like display screens, LED lights have a refresh (flicker) rate that generates interference. The level of interference was greatest when the antennae were perpendicular to each other. Potential sources of interference needed to be more than 30cm from the receiver to pick up a clean signal 20m away.
Another surprise was how significantly foil impacted the receiver in a search. When a small square of aluminum foil was placed over or under the receivers, the effectiveness of both units was seriously degraded. Foil is an avalanche transceiver's kryptonite. The foil did not have a major impact on transmitters unless the unit was literally wrapped in foil.
An interesting result is that the interference produced by an iPhone was similar when airplane mode was enabled and disabled. In both cases, the display was on.
This ruled out the cellular, WiFi, or Bluetooth links as the interference source. The likely source was either the display driver or the switching regulator used to power the display. Both of these types of circuits switch currents on and off rapidly and therefore can produce electromagnetic interference (EMI) across a broad frequency range. Similarly, the LED headlamps use pulse width modulation (PWM), where the current is rapidly switched on and off with a variable duty cycle to control the power delivered to the LEDs; this can produce wideband EMI. Electric gloves and boots also use PWM to control heat levels.
Looking across all the interference sources tested, it is unlikely that any of them produce a strong interfering signal directly at the 457kHz transmit frequency used by the beacons. So why does the LED headlamp or an active display cause interference? The likely culprit is saturation or overloading of the beacon's receiver analog circuitry prior to the digital signal processor (DSP) that is used to isolate the 457kHz tone.
THE UNEXPECTED IMPACT OF FOIL ON AVALANCHE TRANSCEIVERS
One unexpected result was the impact of aluminum foil on both the transmitter and receiver. Non-ferrous material such as aluminum foil should be largely invisible to it, yet when either the transmitter or receiver was placed on top of a square of foil, it caused the reported distance to vary.
When the transmitting beacon was covered or placed on top of the foil, the distance readings reported by the receiver showed little change from the baseline distance readings. When the transmitting beacon was placed between the two layers of foil in a clamshell configuration, the distance readings showed a significant impact. Placing the beacon in its holster reduced the impact of the foil clamshell somewhat, possibly due to the increased distance between the beacon antenna/circuitry and the foil.
It is harder to repeat this experiment on the receiver since the foil obscures the display in many of the configurations. A general observation was that if the foil is placed very close to the receiver, the distance readings increase in value and variability.
Based on these results, concerns such as the impact of a foil-wrapped energy bar in a pocket near a transmitting beacon are likely not significant. Most skiers will not wrap their beacons in foil, so the significant degradation with a clamshell configuration is not of practical significance. It is unclear what impact clothing with heat reflective foil layers, such as gloves, may have. This is mostly of concern when handling the search beacon (receiver) with such gloves.
When looking at transmitter interference, all but one source of interference made almost no difference when placed within 30 cm of the transmitter. When the strongest source of interference, the operating cordless drill, was placed close to the transmitter, a normal search was not possible. Based on this, low to moderate interference does not appear to threaten the effectiveness of the transmitter. Strong interference severely impacts the effectiveness of the transmitter.
Receivers are a different beast. Searchers need to be aware of potential sources of interference and make reasonable efforts to keep them away from the receiver. Electrically heated and foil-lined gloves can easily be missed and can effectively impair a transceiver search. Electronic displays should be kept more than 30cm from the transceiver or turned off. Headlamps may be necessary for the rescue, but keep them at arm’s length from the receiver. More powerful search lights should stay out of the searcher's immediate vicinity until the transceiver search is complete. VHF Radios can be used by the searcher so long as there is some distance maintained between the radio and the receiver.
It is useful to understand how to continue a rescue with potential interference. What do we do in a rescue where interference is suspected? Our first reaction would be to check the rescuer's gloves. If they are electrically heated or foil-lined, swap them for a different pair or send a different searcher. If this is not the problem, modify the search pattern.
We tested searching with a receiver that was experiencing a moderate level of interference and found a workable solution. Moderate interference was randomly defined as a five-watt VHF radio on receive, a GPS unit with an active display screen, and an LED headlamp, all within 10cm of the receiver. Finding a signal on a 40m grid search was unreliable. Tightening the initial grid search to 20m did provide a consistent initial contact signal with the transmitter. Once the signal is detected, physically mark the location (with a wand or ski pole). At this point, you may attempt a standard search.
Don't be surprised if the signal is lost, as the direction arrows may be ineffective due to the interference. Return to the reference point (the marked spot) and estimate the best direction to proceed. Ignore the arrows on the receiver and watch the numbers on the unit. Begin a grid search until the signal becomes significantly stronger. Once you feel you have a strong signal (we found 15m or less), return to a standard search.
We also attempted to locate a transmitter with a high level of interference. We randomly defined a high level as a running cordless drill, large display screen, GPS unit with the display on, and a VHS radio, all within 10cm of the transmitter. Neither a standard search pattern nor a reduced search pattern could reliably detect a signal. By applying a 5m micro-strip search to the area, we were able to locate the transmitter using only the distance numbers and a full grid search. The direction indicated by the arrows was useless. The numbers indicated on the transceiver did not reflect even an approximation of the transmitter's distance from the receiver. At one meter, the unit recorded a 4 m distance. Having said this, the smallest numbers displayed on the receiver did represent the best place to begin probing, and a grid search provided an effective strategy to locate the victim.
A heavily polluted signal can still provide valuable search information.
We hope that this is enough information to begin finding a meaningful understanding of avalanche transceiver interference and possible solutions in an emergency. One day at the Banff airfield is insufficient to solve the problem, but maybe it can start to show us the way forward.
For the victim, low to moderate interference near the transmitter does not appear to impact a transceiver search significantly. High levels of interference may crush the effective range of the unit and render the directional arrows on the receiver useless. This is an unlikely scenario, but is certainly possible in industrial settings and may become a growing concern with electrically heated clothing.
For the rescuers, low to moderate interference can affect the receiver but appears to be manageable. Keep electric motors and generators well away from the searcher. Display screens and LED lights should remain more than 30cm from the receiver and only be used by the searcher if it is necessary to conduct the signal search. Electrically heated and/or foil-lined gloves may impair or ruin a signal search. VHF radios are not a major source of interference.
If you suspect that interference is impacting the avalanche transceiver search:
1. Tighten the initial search grid to 20m until the signal is acquired.
2. Physically mark the location where the signal is first detected.
3. From the marked location, grid search using only the numbers on the transceiver until you have a strong signal (15m or less)
4. Finish by using a normal induction search.
5. If strong interference is suspected, consider having a second searcher begin a 5m microstrip search in likely burial locations.
Hopefully, this information is useful. We look forward to seeing future research develop more effective strategies and a better understanding of avalanche transceiver interference.
Disclaimer: This is not a thoroughly researched peer reviewed study. These tests were conducted by a working, professional electrical engineer and a seasoned ski guide. Our data is too limited to give any hard numbers or definitive statements. Having said this, we do believe we have done enough testing to begin to identify emerging patterns and apply this to potential search and rescue strategies. We hope this work leads to a better understanding of the problem and initiates more research for search and rescue applications.