RECCO

The RECCO rescue technology is an electronic method that facilitates organized rescue teams locating people buried by an avalanche or lost in the outdoors. 
History.
The RECCO rescue technology was developed in response to an avalanche in Åre, Sweden, in 1973, in which Magnus Granhed was involved in the unsuccessful rescue of two buried skiers. Starting in the winter of 1978–1979, Granhed collaborated with Bengt Enander's team at the Department of Electromagnetic Theory, Royal Institute of Technology in Stockholm
to develop a snow-penetrating harmonic radar system that could be used to detect a passive reflector carried by the skier. Granhed formed RECCO AB in 1983 and created the first functional prototype. In 1987, a woman was localised with the RECCO rescue technology from a helicopter in Lenzerheide, Switzerland, in the first live rescue found using the technology.
In 2015, RECCO introduced the SAR-1 helicopter detector that expands the technology to finding a missing person in other outdoor environments. 
Mechanism.
The RECCO rescue technology consists of two parts: a reflector carried by the person and a portable handheld detector used by professional rescue teams, which can be used on foot or from a helicopter.
Reflector.
The reflector is a passive transponder, which does not require batteries or activation, consisting of a diode and an antenna. Its dimensions are 13 mm × 51 mm × 1.5 mm and it weighs 4 g. The company suggests that two reflectors in different locations are ideal.
More than 150 brands integrate RECCO reflectors into their outdoor gear, including ski boots, helmets, hiking shoes, jackets, pants and backpacks.
Handheld detector.
The detector sends out a highly directional signal and if the signal hits a reflector it bounces back, the returned signal is translated into an audio tone if the detector is pointed in the direction of a reflector. Judging from the audio tone a trained professional can determine the location of a buried victim. Due to the diode, the returned signal is doubled in frequency - harmonic radar.
The R9 detector, released in 2009, weighs 900 grams. Its signal penetrates snow and ice, with an estimated range of to 20 meters through snow, which might be reduced in wet snow.
The signal from the detector is also weakly reflected from other electronic devices, such as cameras and cell phones, as well as metallic objects. This effect has allowed the rescue of several buried people who were not carrying a reflector, but also leads to false signals.
Rescue applications.
Avalanche rescue.
In 2015, more than 600 rescue groups, predominantly in ski resorts, used the technology. The professional rescue teams use RECCO detectors as an electronic search method in addition to the avalanche transceiver to find victims in an avalanche. RECCO equipment is included in textbooks on rescue, and a RECCO reflector is included among measures recommended by the International Commission for Alpine Rescue.
According to the company, people totally buried by an avalanche were located using the technology in fifteen incidents during 2005–15. A case report of a live rescue of a buried off-piste skier using RECCO equipment that occurred in Spain in 2015 has been published in the "Journal of Wilderness and Environmental Medicine". The authors commented that the technology was poorly studied compared with some other avalanche safety methods.
Outdoor rescue.
The RECCO SAR helicopter detector is designed for a large-scale search by professional rescue teams for missing persons in open terrain. The missing person must be equipped with a RECCO rescue reflector to be located by the detector signal.
Other applications.
The RECCO technology has also been applied to tracking some amphibian species, especially tropical frogs. The system allows researchers to track small frogs in their natural habitat.