  The accelerating expansion of the Universe is one of the most surprising and
potentially profound discoveries of modern cosmology. Measuring the
acceleration well enough to meaningfully constrain interesting physical models
requires improvements an order of magnitude beyond on-going and near-term
experiments. The Supernova/Acceleration Probe (SNAP) has been conceived as a
powerful yet simple experiment to use Type Ia supernovae and weak gravitational
lensing to reach this level of accuracy. As fundamentally different causes for
the acceleration map into very small differences in observational parameters
for all relevant cosmological methods, control of systematics is especially
important and so has been built into the SNAP mission design from the very
beginning.
