Echo-ranging electronic boundary system

A ranging boundary system for animals, comprising:

• a beacon having a beacon transmitter operable repetitively to emit an ultrasonic wave transmission with at least one of a frequency and pulse characteristic, as an ultrasonic interrogation signal propagating from the beacon, the beacon having a beacon receiver for detecting an ultrasonic reply signal with at least one of a frequency and pulse characteristic, and a processor for determining a time between emission of the ultrasonic interrogation signal and reception of the ultrasonic reply signal;
• a portable transponder mountable on an animal, the portable transponder having an ultrasonic transponder receiver for detecting the ultrasonic interrogation signal of the beacon, and a transponder transmitter operable to emit the ultrasonic reply signal in response to reception of the ultrasonic interrogation signal of the beacon;
• a processor associated with the beacon, the processor timing a period between emission of the ultrasonic interrogation signal from the beacon and reception of the ultrasonic reply signal at the beacon, said period representing a distance between the beacon and the animal;
• a stimulus generator associated with the portable transponder, the stimulus generator applying a selected stimulus to the animal under selected conditions;
• wherein the processor is programmed to determine a time interval between emission of the ultrasonic interrogation signal and receipt of the ultrasonic reply signal, the processor applying a minimum threshold to the time interval and signaling to the stimulus generator when the time interval approaches a predetermined boundary level above said minimum threshold.