The STereoscopic imaging Channel (STC) is one of the three channels of SIMBIO-SYS instrument, whose goal is to study the Mercury surface in visible wavelength range. The SIMBIO-SYS instrument is on-board of ESA Bepicolombo spacecraft. STC is a double wide angle camera designed to map in 3D the whole Mercury surface. The detector of STC has been equipped with six filters: two panchromatic and four broad band. The panchromatic filters are centred at 700 nm with 200 nm of bandwidth, while the broad band ones have bandwidth of 20 nm and are centred at 420, 550, 750 and 920 nm, respectively. In order to verify the relative spectral response of each STC sub-channel, a spectral calibration has to be performed during the on-ground calibration campaign. The result consists in the transmissivity curve of each filter of STC as function of wavelength. The camera has been illuminated with a monochromator coupled with a diffuser and a collimator. The images have been acquired by changing the wavelength of the monochromator in the range correspondent to the filter bandwidth. The background images have been obtained by covering the light source and have been used to calculate and subtract the dark signal, fixed pattern noise (FPN) and ambient effects.
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.
BepiColombo is the first ambitious, multi-spacecraft mission of ESA/JAXA to Mercury. It will be launched in October 2018 from Kourou, French Guiana, starting a 7-year journey, which will bring its modules to the innermost planet of the solar system.
The Stereo Camera (STC) is part of the SIMBIO-SYS instrument, the Italian suite for imaging in visible and near infrared which is mounted on the BepiColombo European module, i.e. the Mercury Planetary Orbiter (MPO). STC represents the first push-frame stereo camera on board of an ESA satellite and its main objective is the global three-dimensional reconstruction of the Mercury surface.
The harsh environment around Mercury and the new stereo acquisition concept adopted for STC pushed our team to conceive a new design for the camera and to carry out specific calibration activities to validate its photogrammetric performance. Two divergent optical channels converging the collected light onto a unique optical head, consisting in an off-axis telescope, will provide images of the surface with an on-ground resolution at periherm of 58 m and a vertical precision of 80 m.
The observation strategies and operation procedures have been designed to optimize the data-volume and guarantee the global mapping considering the MPO orbit.
Multiple calibrations have been performed on-ground and they will be repeated during the mission to improve the instrument performance: the dark side of the planet will be exploited for dark calibrations while stellar fields will be acquired to perform geometrical and radiometric calibrations.
The ESA-JAXA mission BepiColombo toward Mercury will be launched in October 2018. On board of the European module, MPO (Mercury Planetary Orbiter), the remote sensing suite SIMBIOSYS will cover the imaging demand of the mission. The suite consists of three channels dedicated to imaging and spectroscopy in the spectral range between 420 nm and 2 μm. STC (STereo Imaging Channel) will provide the global three-dimensional reconstruction of the Mercury surface with a vertical accuracy better than 80 m and, as a secondary scientific objective, it will operate in target oriented mode for the acquisition of multi spectral images with a spatial scale of 65 m along-track at the periherm for the first orbit at Mercury. STC consists in 2 sub-channels looking at the Mercury surface with an angle of ±20° with respect to the nadir direction. Most of the optical elements and the detector are shared by the two STC sub-channels and to satisfy the scientific objectives six filters strips are mounted directly in front of the sensor. An off-axis and unobstructed optical configuration has been chosen to enhance the imaging contrast capabilities of the instrument and to allow to reduce the impact of the ghosts and stray light. The scope of this work is to present the on-ground geometric calibration pipeline adopted for the STC instrument. For instruments dedicated to 3D reconstruction, a careful geometric calibration is important, since distortion removal has a direct impact on the registration and the mosaicking of the images. The definition of the distortion for off-axis optical configuration is not trivial, this fact forced the development of a distortion map model based on the RFM (rational function model). In contrast to other existing models, which are based on linear estimates, the RFM is not specialized to any particular lens geometry, and is sufficiently general to model different distortion types, as it will be demonstrated.
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 . 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° 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.
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 ±20° 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° x 3.2°. 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.
The ESA-JAXA mission BepiColombo that will be launched in 2018 is devoted to the observation of Mercury, the innermost planet of the Solar System. SIMBIOSYS is its remote sensing suite, which consists of three instruments: the High Resolution Imaging Channel (HRIC), the Visible and Infrared Hyperspectral Imager (VIHI), and the Stereo Imaging Channel (STC). The latter will provide the global three dimensional reconstruction of the Mercury surface, and it represents the first push-frame stereo camera on board of a space satellite. Based on a new telescope design, STC combines the advantages of a compact single detector camera to the convenience of a double direction acquisition system; this solution allows to minimize mass and volume performing a push-frame imaging acquisition. The shared camera sensor is divided in six portions: four are covered with suitable filters; the others, one looking forward and one backwards with respect to nadir direction, are covered with a panchromatic filter supplying stereo image pairs of the planet surface. The main STC scientific requirements are to reconstruct in 3D the Mercury surface with a vertical accuracy better than 80 m and performing a global imaging with a grid size of 65 m along-track at the periherm. Scope of this work is to present the on-ground geometric calibration pipeline for this original instrument. The selected STC off-axis configuration forced to develop a new distortion map model. Additional considerations are connected to the detector, a Si-Pin hybrid CMOS, which is characterized by a high fixed pattern noise. This had a great impact in pre-calibration phases compelling to use a not common approach to the definition of the spot centroids in the distortion calibration process. This work presents the results obtained during the calibration of STC concerning the distortion analysis for three different temperatures. These results are then used to define the corresponding distortion model of the camera.
In the context of a stereo-camera, measuring the image quality allows to define the accuracy of the 3D reconstruction. In fact, depending on the precision of the camera position data, on the kind of reconstruction algorithm, and on the adopted camera model, it determines the vertical accuracy of the reconstructed terrain model. Aim of this work is to describe the results and the method implemented to determine the Line Spread Function (LSF) of the Stereoscopic Channel (STC) of the SIMBIOSYS imaging system for the BepiColombo mission. BepiColombo is the cornerstone mission n.5 of the European Space Agency dedicated to the exploration of the innermost planet of the Solar System, Mercury, and it is expected to be launched in 2016. STC is a double push-frame single-detector camera composed by two identical sub-channels looking at ±21° wrt the nadir direction. STC has been designed so to have many optical elements common to both sub-channels. Also the image focal plane is common to the sub-channels and this permits the use of a single detector for the acquisition of the two images, i.e. one for each viewing direction. Considering the novelty of the design, conceived to sustain a harsh environment and to be as compact as possible, the STC unit is very complex. To obtain the most accurate 3D reconstruction of the Mercury surface, a camera model as precise as possible is needed, and an ad-hoc calibration set-up has been designed to calibrate the instrument both from the usual geometrical and radiometrical points of view and more specifically for the instrument stereo capability. In this context LSF estimation was performed with a new method applying a particular oversampling approach for the curve fitting to determine at first the entire calibration system transfer function and at the end the optical properties of the single instrument.
SIMBIOSYS is a highly integrated instrument suite that will be mounted on-board BepiColombo, which is the fifth cornerstone mission of the European Space Agency dedicated to the exploration of the planet Mercury and it is expected to be launched in 2016. The SIMBIOSYS instrument consists of three channels: the STereo imaging Channel (STC), with broad spectral bands in the 400–950 nm range and medium spatial resolution (up to 50 m/px); the High Resolution Imaging Channel (HRIC), with broad spectral bands in the 400–900 nm range and high spatial resolution (up to 5 m/px), and the Visible and near- Infrared Hyperspectral Imaging channel (VIHI), with high spectral resolution (up to 6 nm) in the 400–2000 nm range and spatial resolution up to 100 m/px. The on-ground calibration system has to cover the full spectral range of the instrument, i.e. from 400 to 2000 nm, and the emitted radiance has to vary over a range of four decades to account for both simulations of Mercury surface acquisition and star field observations. The methods and the results of the measurements done to calibrate the integrating sphere needed for the on-ground radiometric testing of the SIMBIOSYS instrument will be given and discussed. Temporal stability, both on short and long periods, spatial and spectral uniformity, and the emitted radiance for different lamp configurations and different shutter apertures have been measured. The results of the data analysis confirm that the performance of the integrating sphere is well suited for the radiometric calibration of all the three different channels of the SIMBIOSYS instrument.
A comparison between the laser angioplasty techniques preliminary expectations and verified limits with ten years of research on the laser technology and six years experience in clinical utilization is reported. Both early enthusiasm and present criticism have been found excessive because the limits of the techniques were intuitable from the beginning and on the other hand a significant clinical usefulness is verifiable in a selected group of patients.