ABSTRACT The Giant Magellan Telescope (GMT) will place seven prim ary mirror segments of 8.4 m diameter on a common mount to form a single co-phased aperture of 25 m. 1 High order adaptive optics (AO) using an adaptive secondary mirror that is segmented in the same way as the primary will correct the telescope’s imaging to the diffraction limit in the near infrared. 2 Critical to the performance of the telescope will be real-time correction of atmospherically-induced optical path differences between the primary mirror segments. Measur ing these errors is challenging because of the large gaps between the segments, where the aberrated wavefront is not explicitly measured by the AO sensors, which are approximately 30 cm even at their narrowest points. In this paper we show that it will be feasible to estimate the path differences between the segments from the commands sent to the adaptive secondary mirror while the AO is runni ng in closed loop. These commands will be an approximate representation of the open-loop atmospheric wavefronts. We have investigated the value of the approach with real-time closed-loop deformable mirror command data from the first-light AO system now running on the Large Binocular Telescope (LBT).