1. A method for positioning a surgical instrument during stereotactic surgery, comprising: providing a guidance fixture that includes an upper portion, including an instrument guide for moving the surgical instrument along a constrained trajectory relative to the upper portion, and includes an adjustable base supporting the upper portion, including a mounting base and an adjustment mechanism; attaching the guidance fixture to a body, including attaching the mounting base to the body; determining using a remote sensing device the orientation of the upper portion of the guidance fixture in relation to the body; displaying a representation of a relationship of the constrained trajectory and a target point in the body; and aligning the guidance fixture, including aligning the orientation of the upper portion to achieve a desired relation between the constrained trajectory and the target point, including rotating the upper portion about a central axis of the mounting base and pivoting the upper portion to adjust an angle between the upper portion and the central axis.