Why buy four anchors if three anchors can calculate a position?

Answer

1) Tags should operate inside the anchor coverage area. Four anchors can cover a rectangular area rather than a triangular one; the source describes an approximately doubled coverage area. 2) Wireless ranging can fail. With four anchors, a position can still be calculated when ranging to at least three succeeds; a three-anchor setup needs all three measurements to succeed. 3) A four-anchor, one-tag setup can be reconfigured as three anchors and two tags to learn multi-tag operation. 4) Four anchors can support 3D positioning when the installation spans length, width and height in a 1:1:1 arrangement.