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We present results of simulations involving a curvature-based wavefront sensor which uses an extended pattern as a reference source. The proposed sensor provides measurements of both symmetric and asymmetric aberration terms by comparing the Fourier transforms of two oppositely defocused images. Symmetric terms such as defocus and astigmatism can be measured without regard to the object distribution. The asymmetric terms, such as tip and tilt, rely on averaging the signal over many atmospheric realizations in order to determine the object phase, or on defining an arbitrary reference phase. Only after removal of the object Fourier transform phase can the asymmetric terms be identified. Although this paper reports on preliminary results, we believe the proposed sensor will be useful for both real-time compensation of atmospheric distortions while imaging the Sun, and post-facto compensation of optical misalignments in Earth-pointing satellites.