Ortovox have recently advised users of the ORTOVOX F1 Plus, manufactured in the early 1990s, that their beacons cannot be serviced and may need to be replaced. A recent study by Thomas S. Lund (Signal Strength vs. Signal Timing - The Avalanche Review - Vol 26/2) questions whether it is time to retire other analogue beacons including the venerable Ortovox F1 given that there may be compatibility problems with more recent digital beacons.
Please read ORTOVOX comments on the F1/F1 Plus
Lund’s article examines the problems of signal overlap in multiple burial situations. Admittedly a rare case, a recent study showed that pure multi-burial situations made up less than 1% of American and Austrian incidents.
Modern digital avalanche beacons such as the Barryvox Pulse, Pieps DSP and Ortovox S1 use signal strength and signal timing analysis to isolate avalanche beacons in multiple burial situations. The microprocessor in the beacon looks for a pattern in the signals to identify each transceiver. Once a transceiver is identified its signal can marked and eliminated from the search. This works quite well but runs into problems when the pulses of beacons overlap. In the worst case both beacons can be “marked” at the same time. Beacons can use other techniques such as frequency differences to mitigate these problems.
The probability of overlap varies widely depending on the number and characteristics of beacons. Lund analyzed a collection of 24 Ortovox F1 and 24 Tracker DTS beacons. These are the most common models in the field today. The Tracker DTS has a narrow pulse (88-93ms) and a precise pulse period 784 (+/- 10) ms. The F1 has a long pulse width (331 to 401ms) and a wide range of pulse periods: 1210 (+/- 103) ms.
The test looked at collections of 2, 3 and 4 beacons. When 4 beacons were used the probability of a signal being masked over a 1 minute period is 16% for the Tracker DTS and 60% for the Ortovox F1. For the F1 there is a 10% probability of overlap over a 5 minute window. The Tracker DTS should have relatively low probability of overlap given the short pulse width but the precise timing of pulses means that it can take a relatively long period for pulses to move out of synch. BCA, along with other manufacturers, is now randomizing the duty cycle of pulses.
In the case of the F1 the long pulse periods greatly increase the chance of overlaps and when three beacons of similar pulse widths were grouped the overlap period could last up to an hour! Lund suggests that marking should not be used where members of the group are wearing analogue beacons with long pulse widths or in the case of external rescue. He also recommends that all members of the group should have different beacons to increase randomization.
A study by the ANENA in 2001 showed that the Ortovox F1 and M2 had issues with frequency drift with age and when exposed to temperature variations as could be encountered in an actual burial. As the frequency drifts from the 457KHz standard the signal strength will appear to diminish to searching beacons giving a much reduced range. Transceivers with wide bandwidths, such as the BCA Tracker are obviously less affected by these issues. We have encountered digital beacons that are unable to give accurate direction information with out of spec beacons.
The problem of signal timing is important to groups where members are using one of the new digital beacons capable of identifying multiple signals. It should be remembered that multiple burial situations are quite rare in practice. Trail-head beacon tests should include a range check and digital beacon owners should be aware of compatibility issues with older beacons such as the F1 and not rely too heavily on the technology in a search scenario. External rescuers should also assume the presence of older beacon types when arriving at a search site.