An improved apparatus and method for position sensing of a medical tracker device are presented. The apparatus and method are designed to constrain, rotate, and track the position of a medical tool. To improve the position sensing of the tracker, a joint that provides two degrees of freedom may use a revolute gear pair together with a rotary sensor for angular motion sensing and a sliding/prismatic assembly together with a linear magnetic sensor for linear position sensing. Thus, the linear motion sensing and the angular motion sensing are decoupled.