A wavefront sensor design is presented. The wavefront sensor will be applied typically to adaptive correction of a ground based astronomical telescope. It comprises a pair of crossed biprisms to act as star image splitters in the telescope image plane, splitting the incident wave into four beams. The biprisms are followed by a lens which forms four pupil images. These exhibit intensity map differences related to the phase gradient of the incident wave, providing that the point spread function overlaps the biprism crossing. This allows standard wavefront reconstruction algorithms to be used to reconstruct the phase map over the aperture. The results from a computer diffraction simulation of this system are presented to show the useful range of the sensor. In addition, a design for an experimental prototype system is presented.