How It Works
The Recco system consists of two parts: a reflector integrated into clothing, boots, helmets, and body protection worn by skiers and riders; and a detector used by organized rescue teams.
The reflector consists of a small, flat capsule, about 1/2" by 2" by 1/16" thick, which contains a pair of foil aerials, joined by a diode. The size of the aerials makes the unit a tuned circuit resonating at one specific frequency. The reflector is passive meaning it has no batteries and it never has to be switched on. Recco recommends users be equipped with two reflectors placed on opposite sides of the body. Many garment manufacturers now place one in one jacket sleeve and one in the opposing side trouser leg. Many snowboard and ski boot manufacturers place one in each boot.
The detector sends out a highly directional signal on that frequency from an aerial projecting from the front of the unit, and if the signal ‘hits’ a reflector it is bounced back. And, due to the diode the returned signal is doubled in frequency - harmonic radar. Thus the detector tells the operator that it is pointing at a reflector, and not just a piece of metal the right 'length'.
The returned signal is translated into an audio tone whose volume is proportional to the returned signal, and by means of a volume control a trained rescue operator can literally go straight to the buried reflector once a signal is detected.
Recco's newest detector - 9th generation released in 2009 - also includes an avalanche rescue beacon receiver. The R9 detector allows a single rescuer to perform both the search for Recco reflectors and 457 kHz avalanche beacons at the same time.
The Recco signal easily passes through air, dry snow and ice. Liquid water absorbs the signal, so in spring time when the snow is wet—liquid water present—the range is decreased. Rescuers need to make slight adjustments to their search tactics when searching in wet snow conditions.
The Recco system transmits on 917 MHz at around 1.5 watt and uses the tuned dipole diode transponder to double this signal to 1834 MHz. It is this 1834 MHz return that is used by the Recco receiver in the search unit. The transponders are not that precise and a signal a few MHz higher or lower will also be doubled, thus a signal at 915 MHz will yield a return at 1830 MHz. As the frequency range of the Recco device falls fully within the Amateur Radio 900 MHz band this opens up an opportunity to experiment. And due to the somewhat broad nature of the Recco transponders it is possible to transmit far enough away from the target frequency that there would not be an issue, indeed the Recco unit is totally unaffected by a signal at 915 MHz.
Read more about this topic: RECCO
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