Abstract: A first mobile unit identification system having a interrogator and responder. The interrogator transmits a given magnitude of a spread spectrum coded signal in standby mode. The responder receives the spread spectrum coded signal and when the magnitude of received signal exceeds a given value, it reflects the transmitted spread spectrum coded signal with ID code when the magnitude of received signal exceeds the given value. The interrogator receives this and then, transmits data to the responder with amplitude modulating the spread spectrum coded signal with the data. The responder detects by amplitude demodulation and stores the detected data. The interrogator may transmit a mode change code. The responder may transmit an end code after transmission of a data train. A second interrogator informs the responder of the standby mode thereof by amplitude-modulated signal having a given cycle.

Abstract: A mobile unit identifying system comprises: an interrogator having: a spread spectrum signal generation circuit for generating a spread spectrum coding signal using a predetermined pseudo noise signal; and a transmitting antenna for transmitting the spread spectrum coding signal; and a responder having: a memory for storing data; and a modulation unit for modulating radio signals including the transmitted spread spectrum coding signal with the data, the interrogator further comprising: a receiving antenna for receiving the modulated radio signals; and a mixer for mixing the received radio signals with the spread spectrum signal to detect the data. The responder has one or two antennas for receiving or reflecting the spread spectrum coding signal with modulation using a predetermined data. Phase adjusting circuits may be provided in both interrogator and responder to distinguished from reflected waves from reflective objects other than the responder.