A device for monitoring distances having an interrogator and a transponder in wireless communication with the interrogator, the transponder having a signaling device that is activated based on the distance between the interrogator and transponders. The interrogator sends an encoded signal to the transponder and determines the distance to the transponder based on the delay of the transponder's response. The interrogator instructs the transponder to activate a signaling device when the distance exceeds a predetermined setting. The present invention is directed towards applications that require one-foot precision and accuracy.