The Lockheed Martin phased-array telescope developed at the Palo Alto Research Laboratory is a 0.75-m imaging system consisting of 9 separate 90-mm telescopes. One of the technology drivers behind this design is the ability to maintain the phasing of the individual telescopes to sub- wavelength tolerances. We demonstrate here the use of the focal-plane method of phase diversity for maintaining the phased-array alignment. The telescope is designed to operate with white light, so the phase diversity concept is extended to accommodate a broad optical bandwidth. A simulation of white-light phase-diverse wavefront sensing is presented as a demonstration of the robustness of the method with respect to sparse pupil and wavelength sampling. The simulation is validated with laboratory experiments using a point source. Finally, a closed-loop experiment is conducted that demonstrates the ability of phase diversity to sense piston error and maintain the alignment of the phased-array system.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.