Program: GN-2013A-Q-35

Progress on measuring dark energy properties with supernovae (SN) was rapid when the samples were small. However, today, the statistical errors from sample size are no longer dominant. Rather, progress is currently limited by the systematic errors introduced by photometry, light curve fitting, and accounting for dust extinction. Observations of SN Ia in the NIR offer the most promising way forward to a more accurate measurement of cosmic expansion history. We have a large {\it HST} program in Cycle 20 to observe SN Ia in the rest-frame NIR at cosmologically interesting distances. We will find the SN with Pan-STARRS, but we need Gemini to determine the type and age, and measure the SN redshift. The final sample will provide constraints on the nature of dark energy competitive with the best current measurements, but with significantly smaller systematic errors.