  The Nancy Grace Roman Space Telescope (Roman) is an observatory for both
wide-field observations and coronagraphy that is scheduled for launch in the
mid 2020's. Part of the planned survey is a deep, cadenced field or fields that
enable cosmological measurements with type Ia supernovae (SNe Ia). With a pixel
scale of 0".11, the Wide Field Instrument will be undersampled, presenting a
difficulty for precisely subtracting the galaxy light underneath the SNe. We
use simulated data to validate the ability of a forward-model code (such codes
are frequently also called "scene-modeling" codes) to perform precision
supernova photometry for the Nancy Grace Roman Space Telescope SN survey. Our
simulation includes over 760,000 image cutouts around SNe Ia or host galaxies
(~ 10% of a full-scale survey). To have a realistic 2D distribution of
underlying galaxy light, we use the VELA simulated high-resolution images of
galaxies. We run each set of cutouts through our forward-modeling code which
automatically measures time-dependent SN fluxes. Given our assumed inputs of a
perfect model of the instrument PSFs and calibration, we find biases at the
millimagnitude level from this method in four red filters (Y106, J129, H158,
and F184), easily meeting the 0.5% Roman inter-filter calibration requirement
for a cutting-edge measurement of cosmological parameters using SNe Ia.
Simulated data in the bluer Z087 filter shows larger ~ 2--3 millimagnitude
biases, also meeting this requirement, but with more room for improvement. Our
forward-model code has been released on Zenodo.
