Sunday, 28 December 2014

Transceivers and interference

Major snowfalls have been happening across the Western Alps (50cm+ here in the Vanoise yesterday!) and there should be no need to remind everyone that the avalanche risk will be high for some time to come until the newly fallen snow has time to settle, and even then there will be some persistent weak layers under the new snow due to the thin early-season snowpack...

Conservative slope selection will need to be applied, along with careful group management and it goes without saying that those venturing off piste and into the mountains would be advised to carry avalanche safety kit and know how to use it.

On that note - this autumn I revisited 2 papers presented at the International Snow Science Workshop in 2013, concerning potential interference on transceivers from consumer electronics. This is a subject that is becoming more and more relevant with the increasing carrying of smart phones (particularly with touch screens), along with GPS, POV cameras, video cameras etc - the basic premise is that any device consuming power generates a magnetic field and this can compromise transceiver function.

The papers both took a systematic approach to studying the effects both on transmit and receive mode, and the results take a bit of sifting through - so here is a 'laymans' summary of the results and recommendations - unsurprisingly these match closely the observations I have made in non-scientific tests over a number of years:

Transmit mode:

Possible effect = loss of range.

Recommendations = minimum distance of 20cm between possible interfering objects and Transceiver

Note 1: Objects found to have an effect on the transmit range included: alloy bottle, food wrapper, shovel blade, belt buckle
Note 2: Range reductions of more than 30% were uncommon, so applying some reduction to manufacturers recommended search strip width should account for this

Receive mode:

Possible effects = loss of range, loss of accuracy of direction/distance readings

Recommendations:
- minimum distance for no interference: 1m for phone with active screen, 70cm phone with screen off, 60cm camera in active mode. (With these devices switched off there was no discernible effect)
- watches (GPS or smart watches) had a significant influence within 10-15cm (hold your transceiver in the opposite hand!)
- avoid storing transceiver close to magnetic field as this can magnetise the antenna over time.

Note: significant loss in range was observed in receiving transceivers


Further notes:

- at its furthest range a transceiver is at its most sensitive and so more prone to interference, for example a digital camera 36cm away from a receiving transceiver creates noise of similar level to the received transceiver signal!
- analog function allows increase in range as human hearing is more efficient at detecting/discerning 'real signals' from noise, so can cope with a higher signal-to-noise ratio

In summary: it is strongly recommended to keep phones and other electrical items switched off whenever possible, and certainly this should be standard protocol in the event of an avalanche incident.


References:

Meister and Dammert
"Effect of consumer electronics on avalanche transceivers"
Genswein, Atkins et al.
"Recommendation on how to avoid intereference issues in companion and organised avalanche rescue"





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