A Mercury surface radiometric model for SIMBIO-SYS instrument suite on board of BepiColombo mission

Abstract

The BepiColombo mission represents the cornerstone n.5 of the European Space Agency (ESA) and it is composed of two satellites: the Mercury Planetary Orbiter (MPO) realized by ESA and the Mercury Magnetospheric Orbiter (MMO) provided by the Japan Aerospace Exploration Agency (JAXA). The payload of the MPO is composed by 11 instruments. About half of the entire MPO data volume will be provided by the Spectrometer and Imagers for MPO BepiColombo Integrated Observatory System (SIMBIO-SYS) instrument suite. The SIMBIO-SYS suite includes three imaging systems, two with stereo and high spatial resolution capabilities, which are the Stereoscopic Imaging Channel (STC) and High Resolution Imaging Channel (HRIC), and a hyper-spectral imager in the Vis-NIR range, named Visible and near Infrared Hyper-spectral Imager (VIHI). In order to test and predict the instrument performances, a radiometric model is needed. It consists in a tool that permits to know what fraction of the incoming light is measured by the detector. The obtained signal depends on the detector properties (such as quantum efficiency and dark current) and the instrument transmission characteristics (transmission of lenses and filter strips, mirrors reflectivity). The radiometric model allows to correlate the radiance of the source and the signal measured by each instrument. We used the Hapke model to obtain the Mercury reflectance, and we included it in the radiometric model applied to the STC, HRIC and VIHI channels. The radiometric model here presented is a useful tool to predict the instruments performance: it permits to calculate the expected optical response of the instrument (the position in latitude and longitude of the filter footprints, the on-ground px dimensions, the on-ground speed, the smearing and the illumination angles of the observed points), and the detector behavior (the expected signal and the integration time to reach a specific SNR). In this work we derive the input flux and the integration times for the three channels of SIMBIO-SYS, using the radiometric model to obtain the source radiance for each Mercury surface area observed.