Abstract
Imaging surveys will find many tens to hundreds of thousands of Type Ia supernovae in the next decade and measure their light curves. In addition to a need for characterizing their types and subtypes, a redshift is required to place them on a Hubble diagram to map the cosmological expansion. We investigate the requirements on redshift systematics control in order not to bias cosmological results, in particular dark energy parameter estimation. We find that additive and multiplicative systematics must be constrained at the $\text{few}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ level, effectively requiring spectroscopic follow-up for robust use of photometric supernovae. Catastrophic outliers need control at the subpercent level. We also investigate sculpting the spectroscopic sample.