The read accuracy with respect to a tag location is thus the percentage of all orientations of
the tag at that particular position that can be powered by at least one reader antenna. For a
single reader antenna, the 100a% read accuracy powering region is ?¬?xed with respect to
the reader??™s location. However, with more than one reader antenna, such a region is not
simply the union of the individual 100a% read accuracy powering regions.
Consider Figure 10.5, where a unit ball is used to represent all possible orientations for
an RFID tag with its antenna center located at the center of the ball. Note that the surface of
the ball does not have any signi?¬?cance by itself??”a line joining the center of the ball to any
point on the surface represents one possible orientation for the tag??™s antenna. Now suppose
that a is speci?¬?ed as 0.95, and suppose further that each of the two RFID reader antennas
shown can individually cover only 90% of all of the orientations. The points on the surface
within the conical shadow represent those 10% of the orientations of the tag where it
cannot be powered by the corresponding reader. Based on our de?¬?nition of read accuracy,
the current tag position does not meet the speci?¬?cations for either antenna individually.
Thus this point would not lie within the individual football-shaped powering region
(Figure 10.
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