Abstract: A system provides a mobile transmitter and a plurality of boundary tags for receiving communication from the mobile transmitter. Preferably the mobile transmitter is coupled to a mobile unit, which can be an inanimate object capable of moving or being moved, or a living being. The boundary tags mark the boundaries of an area within which the mobile unit is allowed to move. The mobile transmitter transmits a signal to a boundary tag. The system determines if the mobile transmitter has come into proximity of a boundary tag by receiving a reflected modulated signal from the boundary tag. If the mobile transmitter has come into proximity of a boundary tag a response is generated. The response can be a stimulus or an alert, or both. The response can be generated at a central control station, or at a mobile transceiver that includes the mobile transmitter and a mobile receiver. In one embodiment, the boundary tag modulates a reflection of the signal transmitted by the mobile transmitter.