Wednesday, 17 November 2010

Get Rid of Old Avalanche Beacons - Transceivers

I can unashamedly declare an interest as I promote avalanche and off piste safety and sell Back Country Access products of which the Tracker DTS2 is in my opinion the best Transceiver on the market. Thanks to Steve Achellis author of Mountain Responder for this interesting article. I think it highlights quite well what we discussed on the level 1 Avalanche course last winter when patrols had older transceivers mixed up with digital.  For those wanting a new transceiver public or ski patrol I will give you 10% off the DTS2.

Frequency Considerations - Steve Achellis
Early avalanche beacons transmitted at 2.275 kHz. In 1986 the international standard of 457 kHz was adopted. The Ortovox F2 (which was released in 1980) transmitted and received on both 2.275 and 457 kHz. This allowed it to locate both new and old-style beacons. That was handy in the 1980s, but it is no longer relevant.

A 1997 update to the standard specified that beacons must transmit within ±100 Hz of 457 kHz. In 2001 this allowable variance was reduced to ±80 Hz. Unfortunately, older analog beacons that used ceramic oscillators (e.g., the F1 and F2, but not the M2) can drift outside this ±80 Hz range. Newer beacons use crystal oscillators and transmit very close to 457 kHz.

The drifting usually gets worse as the temperature decreases. For example, a warm beacon inside your jacket may transmit near 457 kHz, but when it cools it may drift outside the ±80 Hz range.

Frequency Testing

I tested 51 avalanche transceivers (15 different models) using a sophisticated spectrum analyzer to see if they transmitted within specifications. I measured the frequency at 70F and 0F to see how temperature affected the frequency. I presented the results of this testing at the International Technical Rescue Symposium (ITRS) in 2007. Suffice it to say that 49 of the 51 beacons were within range. The others were only slightly out of range (-90 Hz  and -100 Hz, and only when cooled to 0F). 

During the testing, I also compared the results of the spectrum analyzer with the built-in frequency tester found in the Pieps DSP.

Frequency Drift and Transceiver Performance

The downsides of a beacon transmitting outside the ±80 Hz range are twofold. Most importantly, a searching transceiver may not receive the wayward beacon's signal at all. Secondly, the range (i.e., distance) at which the wayward beacon can be located will be significantly reduced.

This was first brought to my attention by a friend who successfully located three buried Ortovox F2s during a beacon drill in the morning. That evening he went outside to retrieve the beacons and couldn't receive a signal from any of the beacons! They were eventually located with a different beacon. I suspect the F2s drifted off-frequency as they cooled and that his digital transceiver was being too particular about the required frequency (that model of receiver has since been updated to better handle off-frequency transmitters).

I also tested the distance at which four different beacons could receive a signal from an Ortovox F2 that was transmitting +200 Hz above the normal 457 kHz frequency (i.e., at 457.200 kHz). The results are shown in the following graph. The maroon bars show the distance that the beacon could receive a properly transmitting 457 kHz signal when the transceiver was held perpendicularly (i.e., in worst-case orientation). 

The blue bars show the distance that the beacon could receive the wayward 457.200 kHz signal. Note that I only did these tests on one occasion using these four transceivers. The results should be viewed as indicative of the problem rather than as definitive numerical data.

You can see that the Pieps DSP which could receive a 457 kHz signal at 48 meters did not receive the wayward signal until 20 meters. The Barryvox 3000 received the signal at 11 meters, but only in analog mode (i.e., the direction indicator did not appear). The Tracker DTS had the smallest percent decrease in performance (although the actual distance was less than the DSP), but the distance of 12 meters is also unacceptable. Needless to say, having an transceiver that transmits this far out of range is dangerous.

The Pieps DSP has a feature wherein it does not normally search outside of the ±80 Hz range when first entering the search mode, but if you press the scan button (labeled either "?" or "(((") button it will scan±500 Hz for several seconds. If it finds a beacon, it will lock onto it while flashing "500" in the display. It's a nifty feature, but I doubt many of us would think to toggle this mode if our initial signal search failed. This was added to the DSP in version 2.8 of their software (if you have an older DSP, you can get the software updated). Prior to this upgrade, the DSP had a difficult time locating out-of-frequency beacons (based on word-of-mouth conversations, the old DSP software had shorter range than the Barryvox shown in the above table).

Both the Tracker and Pieps DSP engineers have gone to great lengths to receive signals from out-of-frequency analog beacons. The problem is not with these newer beacons, but with the off-frequency transmissions of the older beacons. It was extremely difficult for me to accept that my faithful Ortovox F1 was transmitting at an unsafe frequency, but I eventually accepted this fact and retired "old blue."
Beginning with version 3.1 of the Pieps software, the DSP can measure the frequency of other beacons. The photograph to the right shows the DSP measuring the frequency of a Tracker DTS. At, we performed more than 100 tests on 51 different beacons at various temperatures and compared the results with a $20,000 spectrum analyzer—the DSP's frequency tester did an excellent job.
To test the frequency using a Pieps DSP:
Switch the DSP into Search mode.
Place the two transceivers close together and motionless.
Press-and-hold the button just below the screen (i.e., the Enter button) until "F" (for Frequency) appears in the display.

The DSP will then display the amount that the transmitting beacon is off. For example, if the screen displays "F020" and the arrow points to the left, it means the transmitting beacon is transmitting 20 Hz below the standard 457,000 Hz, or at 456,980 Hz. In the illustration to the right you can see that this Tracker DTS is transmitting at exactly 457,000 Hz. As long as the number displayed is equal-to-or-less-than 080, the beacon is within specifications.

Additional Reading

Bruce Edgerly & John Hereford of Backcountry Access, Inc. (the company that makes the Trackers) wrote an excellent article on this topic. Read it here. Antennas

Somewhat related to frequency drift, the "continuous carrier" signal can confuse digital transceivers. Read about it here. Antennas

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