Study of the spatial sensitivity of the VenSpec-U spectral imager onboard EnVision using an instrumental model

The VenSpec-U spectrometer onboard EnVision will be dedicated to the study of Venus’ upper atmosphere, with four main science goals: monitoring the sulfured species SO₂ and SO, the UV absorber, and performing observations of the cloud-top with a high spatial resolution thanks to a pushbroom observation strategy. In this article, we study the imaging capabilities of the instrument, as part of the global performance assessments undertaken to estimate the compliance of the current design with the science requirements. This work involves a combination of models to simulate the whole acquisition process, from the observed scene at the cloud-top of Venus, to the consideration of the instrument’s behavior inducing degradation of the images and spectra: outputs of a Large Eddy Simulation model are used to compute maps of atmospheric parameters related to VenSpec-U’s science goals; a radiative transfer model (RTM), initially developed for the SPICAV/Venus Express data analysis and adapted for VenSpec-U, translates these parameters into spectral radiance maps; and a newly developed instrument forward model simulates the optics and detector effects as well as onboard data processing steps. By modifying the maps of RTM parameters, we assess contrast limits below which the small-scale variations of the atmospheric properties can no longer be retrieved accurately. We then evaluate these thresholds under various conditions, including different latitudes, spacecraft altitudes, and spatial samplings. These detection limits appear in accordance with the expected performance.