1. A method for using Radio Frequency ID (RFID) tags to determine the position of an object, comprising: receiving RF signals from an array of RFID tags at a RFID tag reader, wherein a mask of known size and shape blocks RF signals from a subset of the array of RFID tags, so that the RF signals from the subset of RFID tags do not reach the RFID tag reader; determining a pattern of obscured RFID tags in the array of RFID tags; and analyzing the pattern of obscured RFID tags to determine the position of the object, wherein the analysis is based on a known spatial relationship between the object, the mask and the array of RFID tags.