  This paper presents Navigation and Rendering Pipeline for Astronautics
(NaRPA) - a novel ray-tracing-based computer graphics engine to model and
simulate light transport for space-borne imaging. NaRPA incorporates lighting
models with attention to atmospheric and shading effects for the synthesis of
space-to-space and ground-to-space virtual observations. In addition to image
rendering, the engine also possesses point cloud, depth, and contour map
generation capabilities to simulate passive and active vision-based sensors and
to facilitate the designing, testing, or verification of visual navigation
algorithms. Physically based rendering capabilities of NaRPA and the efficacy
of the proposed rendering algorithm are demonstrated using applications in
representative space-based environments. A key demonstration includes NaRPA as
a tool for generating stereo imagery and application in 3D coordinate
estimation using triangulation. Another prominent application of NaRPA includes
a novel differentiable rendering approach for image-based attitude estimation
is proposed to highlight the efficacy of the NaRPA engine for simulating
vision-based navigation and guidance operations.
