Technology that troops use to locate friendly forces may give parents a strategic advantage. With help from defense contractor Harris Corp. in Melbourne, Fla., a Long Beach (Calif.) startup has developed a beeper system that warns parents when a child strays more than 15 feet. Existing products measure radio-wave signal strength. But they have been plagued with false alarms caused by interference from walls and store security systems.

A+H International's BeeperKid works differently. The child's transmitter sends two signals: One identifies the child. The other, A+H says, is a low-frequency signal that drops off sharply in intensity beyond 15 feet. The parent's unit has to receive both or an alarm goes off. A+H won't say why the low-frequency signal drops off sharply, a phenomenon that seems to defy principles of radio transmission. BeeperKid sells for $150.

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