VACNT versus Black Velvet: a coating analysis for the next-generation Earth Radiation Budget radiometer

Climate on Earth is determined by the Earth Radiation Budget (ERB), which quantifies the incoming and outgoing radiative energy fluxes at the top-of-atmosphere (TOA). The ERB can be monitored from space by non-scanning wide field-of-view radiometers (WFOV), or by scanning narrow field-of-view radiometers. Recently, WFOV radiometers have gained renewed interest as illustrated by the development of the RAVAN and SIMBA 3U CubeSats. RAVAN uses a Vertically Aligned Carbon Nanotubes (VACNT) coating, while the SIMBA CubeSat uses a novel cavity-based geometry with a Black Velvet coating. Both VACNT and Black Velvet are diffuse coatings, but when applied to flat sensors, the VACNT coating has a significantly lower reflectivity in comparison to classic diffuse or specular black coating materials. When used on a cavity radiometer, it is currently unclear if a VACNT coating would improve the measurement accuracy compared to other diffuse coatings, such as Black Velvet. In this paper, we therefore investigate the potential benefits of using the VACNT coating as an alternative for Black Velvet, in our in-house developed radiometer. Our analysis includes the evaluation of the influence of the cavity geometry as well as the coating absorption factor. The comparison of the VACNT with the Black Velvet coating is based on the absorption factor of the cavity that is determined using radiation view factor calculations. Scattering and stray-light analyses are carried out using commercially available ray-tracing software (ASAP®R, Breault Research). We evaluated whether the coating or the geometry is the main contributing factor to the performance of the radiometer. As a conclusion, we observed that for cavity-type radiometers, the difference in the cavity absorption factor between Black Velvet and VACNT becomes negligible, favoring the use of Black Velvet, since Black Velvet has a long space heritage and appears more user friendly from a fabrication point of view, as it can be deposited in an easier and more reproducible manner on the radiometer cavity walls, including non flat surfaces.