CaSiO3 perovskite is the third most abundant mineral in the Earth's lower mantle (below 670 km). It also occurs as a major phase at lower pressures and temperatures in subduction zones, where, for example, it may form ~25 wt% of subducted mid-ocean ridge basalt. The crystal structure, ferroelastic phase transitions, elastic properties and equation of state of CaSiO3 perovskite are, therefore, of great significance for the interpretation of seismological data. The phase diagram of CaSiO3 perovskite is currently in a state of utter confusion; computer simulations and experimental studies disagree widely, both as to the symmetry of the various phases and the P/T conditions under which they occur. We propose to use HRPD to make an accurate structure determination of metastable CaSiO3 perovskite at atmospheric pressure and 4 K, so as to constrain future computer simulations of the material.