A system includes a generator and at least one device. The generator includes a waveform oscillator and a blanking pulse generator. Each device includes a transmit antenna, a receive antenna, an antenna unit, a mixer and a detector. The antenna unit includes a receiver coupled to the receive antenna, an amplifier coupled to the receiver and a transmitter coupled to the transmit antenna and the blanking pulse generator. The mixer has inputs coupled to the amplifier and the waveform oscillator. The detector is coupled to the mixer.