Assessment of the performance of a visual payload mounted on a small satellite involved in an in-orbit servicing mission during the observation phase.

Illumination analysis tool for preliminary assessment of proximity operations. Interest in graphical simulations to support proximity operations is increasing along with the number of private and public missions involving robotic spacecraft operating close to each other for commercial or scientific (and military) purposes. Using the open-source computer graphics software Blender, a simulation environment was developed to perform a preliminary assessment of lighting conditions for proximity operations in Low Earth Orbit. The main purpose of the simulator is to guide mission requirements in terms of trim, position and payload performance. The instrument created was used to assess the observation and docking conditions of an observe-and-dock mission of a CubeSat near its mothership. An instrument was sought that accurately simulated the transport of light because the factor of view of the Earth from a spacecraft in low orbit is almost 50%. This makes diffused and reflected light a non-negligible component of scene lighting and under these conditions, indirect lighting (if neglected) could lead to incorrectly marking valid observation states as invalid. Central to our tool’s realism is Blender’s Cycles rendering engine, which is a physically based path tracing engine. Starting from a simulation in Blender previously developed; containing an accurate model of the Earth, in which realistic atmospheric dispersion and decay, a layer of clouds, 3D topological characteristics and different reflective characteristics of water and land mass were implemented; the radiative environment of space around the Earth was initially developed as physically accurately as possible. Sunlight is simulated as a uniform vector field with energy flow 1367 [W/m2] and the spectral characteristics of a black body 5777 [°K] that can be oriented depending on the position of the Sun. The initial prototype focuses on the visual spectral light captured by RGB sensors. The numerical limits of Blender that usually arise when placing objects with scale differences in the many orders of magnitude next to each other have been circumvented, hitting the target observed in the origin of the global frame and moving all other objects around it. An ideal optical sensor model is developed for the evaluation of lighting conditions, to which the various signal disturbances due to geometry or physical phenomena of the system are subsequently added. In order to correctly evaluate the performance of a payload, the rendering outputs are post-processed to extract radiance information that is subsequently fed to a monochrome sensor simulator as output to account for aperture and exposure time. To fill the gaps in terms of physical accuracy of Blender, as it was born as a graphic rendering program, without taking care of the scientific analysis part in depth, a second simulator was created using the PANGU program developed by ESA. PANGU allows users to graphically implement the model, going to evaluate in a physically accurate way the quantities of interest for a lighting analysis, paying particular attention to the numerical aspect of the results as well as to an excellent graphic display. At the end of the work, a comparison was made between the two simulators to understand their differences and strengths. The graphics simulator can help high-level decision-making for orbit, trim, and payload selection, but it lacks the performance and real-time effects needed for GN&C software-in-the-loop simulation.