title: Calculate distance to RFID tag?
body: <p>Is there a way to calculate/estimate the physical distance to a long-distance passive RFID tag when reading it with a tag reader? E.g. to determine the order of books in a shelf, or telling if one object is close or far away.</p>
  
  <p>If the answer is 'No - not according to the standard', would it be possible to build a reader with this feature? (I guess the only way to achieve this would be to measure the time between call and response very precisely).</p>