A system has a plurality of transceiver units, including a parent unit and one or more portable and wearable child units, for generating an alarm when a child transceiver unit is more than a predetermined distance away from a parent transceiver unit. The distance is determined by the difference in the...http://www.google.fr/patents/US5661460?utm_source=gb-gplus-shareBrevet US5661460 - Distance determination and alarm system
