Calibration activities on the BepiColombo High-Resolution Channel (HRIC) of SIMBIO-SYS instrument

HRIC (High Resolution Imaging Channel) is the high resolution channel of the SIMBIO-SYS instrument on- board the ESA BepiColombo Mission. Calibration activities were performed at SelexES premises in spring- summer 2014 in order to check for Channel performances (radiometric performances, quality image and geometrical performances) and to obtain data necessary to setup a calibration pipeline necessary to process the raw images acquired by the channel when in operative scenario

The measurement of the noise-equivalent spectral radiance of SIMBIO-SYS/VIHI spectrometer

We report about the measurement of the Noise- Equivalent Spectral Radiance (NESR) of the VIHI imaging spectromter aboard ESA's Bepi Colombo mission to Mercury. The knowledge of the NESR allows to determine the capability of an optical detector to measure faint signals. A description of the setup used to determine the NESR during the prelaunch calibration campaign is given. The processing of the data col- lected at various operative temperatures and integration times is described. The sensitivity study of the NESR has been performed at the expected detector's temperatures and integration times with the goal to determine the minimum spectral radiance at which VIHI is sensitive during the different observation phases of the mission. A simulation of the expected Signal-to-Noise Ratio (SNR) of VIHI during the different orbital phases is provided

Radiometric calibration of the SIMBIO-SYS STereo imaging Channel

The STereo imaging Channel (STC) is a double wide-angle camera developed to be one of the channels of the SIMBIOSYS instrument onboard of the ESA BepiColombo mission to Mercury. STC main goal is to map in 3D the whole Mercury surface. The geometric and radiometric responses of the STC Proto Flight model have been characterized on-ground during the calibration campaign. The derived responses will be used to calibrate the STC images that will be acquired in flight. The aim is to determine the functions linking the detected signal in digital number to the radiance of the target surface in physical units. The result of the radiometric calibration consists in the determination of well-defined quantities: (1) the dark current as a function of the integration time and of the detector temperature, settled and controlled to be stable at 268 K; (2) the read out noise, which is associated with the noise signal of the read-out electronic; and (3) the fixed pattern noise, which is generated by the different response of each pixel. Once these quantities are known, the photon response and the photoresponse non-uniformity, which represents the variation of the photon responsivity of a pixel in an array, can be derived. The final result of the radiometric calibration is the relation between the radiance of an accurately known and uniform source, and the digital numbers measured by the detector

SIMBIO-SYS : Scientific Cameras and Spectrometer for the BepiColombo Mission

The SIMBIO-SYS (Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem) is a complex instrument suite part of the scientific payload of the Mercury Planetary Orbiter for the BepiColombo mission, the last of the cornerstone missions of the European Space Agency (ESA) Horizon + science program. The SIMBIO-SYS instrument will provide all the science imaging capability of the BepiColombo MPO spacecraft. It consists of three channels: the STereo imaging Channel (STC), with a broad spectral band in the 400-950 nm range and medium spatial resolution (at best 58 m/px), that will provide Digital Terrain Model of the entire surface of the planet with an accuracy better than 80 m; the High Resolution Imaging Channel (HRIC), with broad spectral bands in the 400-900 nm range and high spatial resolution (at best 6 m/px), that will provide high-resolution images of about 20% of the surface, and the Visible and near-Infrared Hyperspectral Imaging channel (VIHI), with high spectral resolution (6 nm at finest) in the 400-2000 nm range and spatial resolution reaching 120 m/px, it will provide global coverage at 480 m/px with the spectral information, assuming the first orbit around Mercury with periherm at 480 km from the surface. SIMBIO-SYS will provide high-resolution images, the Digital Terrain Model of the entire surface, and the surface composition using a wide spectral range, as for instance detecting sulphides or material derived by sulphur and carbon oxidation, at resolutions and coverage higher than the MESSENGER mission with a full co-alignment of the three channels. All the data that will be acquired will allow to cover a wide range of scientific objectives, from the surface processes and cartography up to the internal structure, contributing to the libration experiment, and the surface-exosphere interaction. The global 3D and spectral mapping will allow to study the morphology and the composition of any surface feature. In this work, we describe the on-ground calibrations and the results obtained, providing an important overview of the instrument performances. The calibrations have been performed at channel and at system levels, utilizing specific setup in most of the cases realized for SIMBIO-SYS. In the case of the stereo camera (STC), it has been necessary to have a validation of the new stereo concept adopted, based on the push-frame. This work describes also the results of the Near-Earth Commissioning Phase performed few weeks after the Launch (20 October 2018). According to the calibration results and the first commissioning the three channels are working very well.Peer reviewe

Indoor calibration for stereoscopic camera STC: A new method

In the framework of the ESA-JAXA BepiColombo mission to Mercury, the global mapping of the planet will be performed by the on-board Stereo Camera (STC), part of the SIMBIO-SYS suite [1]. In this paper we propose a new technique for the validation of the 3D reconstruction of planetary surface from images acquired with a stereo camera. STC will provide a three-dimensional reconstruction of Mercury surface. The generation of a DTM of the observed features is based on the processing of the acquired images and on the knowledge of the intrinsic and extrinsic parameters of the optical system. The new stereo concept developed for STC needs a pre-flight verification of the actual capabilities to obtain elevation information from stereo couples: for this, a stereo validation setup to get an indoor reproduction of the flight observing condition of the instrument would give a much greater confidence to the developed instrument design. STC is the first stereo satellite camera with two optical channels converging in a unique sensor. Its optical model is based on a brand new concept to minimize mass and volume and to allow push-frame imaging. This model imposed to define a new calibration pipeline to test the reconstruction method in a controlled ambient. An ad-hoc indoor set-up has been realized for validating the instrument designed to operate in deep space, i.e. in-flight STC will have to deal with source/target essentially placed at infinity. This auxiliary indoor setup permits on one side to rescale the stereo reconstruction problem from the operative distance in-flight of 400 km to almost 1 meter in lab; on the other side it allows to replicate different viewing angles for the considered targets. Neglecting for sake of simplicity the Mercury curvature, the STC observing geometry of the same portion of the planet surface at periherm corresponds to a rotation of the spacecraft (SC) around the observed target by twice the 20\ub0 separation of each channel with respect to nadir. The indoor simulation of the SC trajectory can therefore be provided by two rotation stages to generate a dual system of the real one with same stereo parameters but different scale. The set of acquired images will be used to get a 3D reconstruction of the target: depth information retrieved from stereo reconstruction and the known features of the target will allow to get an evaluation of the stereo system performance both in terms of horizontal resolution and vertical accuracy. To verify the 3D reconstruction capabilities of STC by means of this stereo validation set-up, the lab target surface should provide a reference, i.e. should be known with an accuracy better than that required on the 3D reconstruction itself. For this reason, the rock samples accurately selected to be used as lab targets have been measured with a suitable accurate 3D laser scanner. The paper will show this method in detail analyzing all the choices adopted to lead back a so complex system to the indoor solution for calibration

Preliminary results of the optical calibration for the stereo camera STC onboard the Bepicolombo mission

BepiColombo is one of the cornerstone missions of the European Space Agency dedicated to the exploration of the planet Mercury and it is expected to be launched in July 2016. One of the BepiColombo instruments is the STereoscopic imaging Channel (STC), which is a channel of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIOSYS) suite: an integrated system for imaging and spectroscopic investigation of the Mercury surface. STC main aim is the 3D global mapping of the entire surface of the planet Mercury during the BepiColombo one year nominal mission. The STC instrument consists in a novel concept of stereocamera: two identical cameras (sub-channels) looking at \ub120\ub0 from nadir which share most of the optical components and the detector. Being the detector a 2D matrix, STC is able to adopt the push-frame acquisition technique instead of the much common push-broom one. The camera has the capability of imaging in five different spectral bands: one panchromatic and four intermediate bands, in the range between 410 and 930 nm. To avoid mechanisms, the technical solution chosen for the filters is the single substrate stripe-butted filter in which different glass pieces, with different transmission properties, are glued together and positioned just in front of the detector. The useful field of view (FoV) of each sub-channel, though divided in 3 strips, is about 5.3\ub0 x 3.2\ub0. The optical design, a modified Schmidt layout, is able to guarantee that over all the FoV the diffraction Ensquared Energy inside one pixel of the detector is of the order of 70-80%. To effectively test and calibrate the overall STC channel, an ad hoc Optical Ground Support Equipment has been developed. Each of the sub-channels has to be separately calibrated, but also the data of one sub-channel have to be easily correlated with the other one. In this paper, the experimental results obtained by the analysis of the data acquired during the preliminary onground optical calibration campaign on the STC Flight Model will be presented. This analysis shows a good agreement between the theoretical expected performance and the experimental results

SIMBIO-SYS STC ready for the first light: the radiometric calibration

The Stereo Channel (STC) is a double wide-angle camera developed to be one of the channels of the SIMBIO-SYS instrument onboard of the ESA BepiColombo mission to Mercury. STC main goal is to map in 3D the whole Mercury surface. The geometric and radiometric responses of the STC Proto Flight model have been characterized on-ground during the calibration campaign. The derived responses will be used to calibrate the STC images that will be acquired in flight. The aim is to derive the functions that link the detected signal in digital number to the radiance of the target surface in physical units. The result of the radiometric calibration consists in the determination of well-defined quantities: i) the dark current as a function of the integration time and of the detector temperature, nominally fixed at 268 K; ii) the Read Out Noise, which is associated with the noise signal of the read-out electronic; iii) the Fixed Pattern Noise, which is generated by the different response of each pixel; iv) once these quantities are known, the photon response and the Photo Response Non-uniformity, which represent the variation of the photon-responsivity of a pixel in an array, can be derived. The final result of the radiometric calibration is the relation between the radiance of an accurately known and uniform source, and the digital numbers measured by the detector