The Supernova / Acceleration Probe ( SNAP ) is a proposed space-based experiment designed to study the dark energy and alternative explanations of the acceleration of the Universe ’ s expansion by performing a series of complementary systematics-controlled astrophysical measurements .
We here describe a self-consistent reference mission design that can accomplish this goal with the two leading measurement approaches being the Type Ia supernova Hubble diagram and a wide-area weak gravitational lensing survey .
This design has been optimized to first order and is now under study for further modification and optimization .
A 2-m three-mirror anastigmat wide-field telescope feeds a focal plane consisting of a 0.7 square-degree imager tiled with equal areas of optical CCDs and near infrared sensors , and a high-efficiency low-resolution integral field spectrograph .
The instrumentation suite provides simultaneous discovery and light-curve measurements of supernovae and then can target individual objects for detailed spectral characterization .
The SNAP mission will discover thousands of Type Ia supernovae out to z = 3 and will obtain high-signal-to-noise calibrated light-curves and spectra for a subset of > 2000 
supernovae at redshifts between z = 0.1 and 1.7 
in a northern field and in a southern field .
A wide-field survey covering one thousand square degrees in both northern and southern fields resolves \sim 100 galaxies per square arcminute , or a total of more than 300 million galaxies .
With the PSF stability afforded by a space observatory , SNAP will provide precise and accurate measurements of gravitational lensing .
The high-quality data available in space , combined with the large sample of supernovae , will enable stringent control of systematic uncertainties .
The resulting data set will be used to determine the energy density of dark energy and parameters that describe its dynamical behavior .
The data also provide a direct test of theoretical models for the dark energy , including discrimination of vacuum energy due to the cosmological constant and various classes of dynamical scalar fields .
If we assume we live in a cosmological-constant-dominated Universe , the matter density , dark energy density , and flatness of space can all be measured with SNAP supernova and weak-lensing measurements to a systematics-limited accuracy of 1 % .
For a flat universe , the density-to-pressure ratio of dark energy or equation of state w ( z ) can be similarly measured to 5 % for the present value w _ { 0 } and \sim 0.1 for the time variation w ^ { \prime } \equiv dw / d \ln a| _ { z = 1 } .
For a fiducial SUGRA-inspired universe , w _ { 0 } and w ^ { \prime } can be measured to an even tighter uncertainty of 0.03 and 0.06 respectively .
Note that no external priors are needed .
As more accurate theoretical predictions for the small-scale weak-lensing shear develop , the conservative estimates adopted here for space-based systematics should improve , allowing even tighter constraints .
While the survey strategy is tailored for supernova and weak gravitational lensing observations , the large survey area , depth , spatial resolution , time-sampling , and nine-band optical to NIR photometry will support additional independent and/or complementary dark-energy measurement approaches as well as a broad range of auxiliary science programs .