The Giant Magellan Telescope (GMT) is a 25.4 m diameter ground-based segmented Gregorian telescope, composed of
7 8.4 meter diameter primary mirror segments, and 7 1 meter diameter adaptive secondary mirror segments. Co-phasing
of the integrated optical system will be partially achieved by making real-time measurements of the wavefront of an offaxis
guide star. However, slowly varying aberrations due to thermal and gravitational effects, as well as wind buffeting,
will make it difficult to maintain alignment using real-time optical measurements alone. Consequently, we are proposing
internal metrology systems to maintain the relative alignment of the optical elements. In this paper we describe a
differential capacitive edge sensing system to maintain the relative alignment of the adaptive secondary mirror reference
bodies. We also propose an interferometric system for sensing of the relative displacements of primary mirror segments.