I can't use active tags, as they would be too bulky for my project. Would I be able to accomplish this using passive tags?
The only way to determine the precise location of a passive tag is to use near-field passive tags with a very short read range and create a grid of antennas, so that you can know which antenna is reading a specific tag at any given time. Think of a chessboard with a tiny RFID reader antenna under each square. The antennas read the tags of the objects above. If you associate each antenna with a particular square, then you can know that one tagged object is on square C4 and another is on square D8, and thus determine their positions relative to one another.
—Mark Roberti, Founder and Editor, RFID Journal