The Recco (http://www.recco.com) system is a passive search device. A small circuit board is sewn into clothing or glued onto the boots. It has the advantage that the the circuit doesn't need batteries and can't be forgotten or turned off. To be effective the rider should always wear two reflectors on both sides of the body.
Now where did I leave those ski boots?
However the system requires special equipment and although this is available in many ski resorts the rescue services must first transport it to the avalanche site. This delay is critical and therefore the system should not be considered by riders going far from the pistes.