After materialization, the Astrosextant Rectifier will attempt to confirm that a TARDIS has arrived at the correct coordinates. Gallifrey (or maybe just the Death Zone) shows up as "no place" on the TARDIS instruments. The Spectrum-Mixer can roughly determine the current spatial (and presumably temporal) location by analyzing the position of near by stars. A damaged Sterometer will reduce the accuracy of the Rectifier to within a few thousand light years.