The invention relates to a ranging system for measuring the distance between an interrogator and a transponder. The transponder includes: a signal receiver for receiving a challenge signal from an interrogator; a signal processor for processing the challenge signal and generating a response signal in response to the challenge signal; a buffer for storing the response signal generated by the signal processor; and a signal transmitter for sending the response signal stored in the buffer when the signal processor receives a ranging signal from the interrogator, wherein a time interval between the challenge signal and the ranging signal is known to both transponder and the interrogator.