Cristina Re

Performance evaluation of DTM area-based matching reconstruction of Moon and Mars

High resolution DTMs, suitable for geomorphological studies of planets and asteroids, are today among the main scientific goals of space missions. In the framework of the BepiColombo mission, we are experimenting the use of different matching algorithms as well as the use of different geometric transformation models between stereo pairs, assessing their performances in terms of accuracy and computational efforts. Results obtained with our matching software are compared with those of established software. The comparison of the performance of image matching being the main objective of this work, all other steps of the DTM generation procedure have been made independent of the matching software by using a common framework. Tests with different transformation models have been performed using computer generated images as well as real HiRISE and LROC NAC images. The matching accuracy for real images has been checked in terms of reconstruction error against DTMs of Mars and the Moon published online and produced by the University of Arizona.

Innovative optical setup for testing a stereo camera for space applications

The Stereo Camera (STC) of the SIMBIO-SYS imaging suite of the BepiColombo ESA mission to Mercury is based on an innovative and compact design in which the light independently collected by two optical channels at ±20° separation with respect to nadir falls on a common bidimensional detector. STC adopts a novel stereo acquisition mode, based on the push-frame concept, never used before on a space mission. To characterize this camera for obtaining the most accurate data of the Mercury surface, standard calibration measurements have been performed. In addition, we also wanted to demonstrate and characterize the capability of the instrument to reconstruct a 3D surface with the desired accuracy by means of the stereo push-frame concept. To this end, a lab setup has been realized with an evaluation model of STC, in which the problem of working at an essentially infinite object distance over hundred km baselines has been overcome by means of a simple collimator and two precision rotators. The intrinsic and extrinsic parameters of the camera have been obtained with standard stereo procedures, adapted to the specific case. The stereo validation has been performed by comparing the shape of the target object accurately measured by laser scanning, with the shape reconstructed by applying the adopted stereo algorithm to the acquired image pairs. The obtained results show the goodness of this innovative validation technique, that will be applied also for validating the stereo capabilities of STC flight model.

DTM generation from STC-SIMBIO-SYS images

The research group with the responsibility of the STereo Camera (STC) for the ESA BepiColombo mission to Mercury, has realized an innovative and compact camera design in which the light collected independently by two optical channels at ±20° with respect to the nadir direction converges on unique bidimensional detector. STC will provide the 3Dmapping of Mercury surface, acquiring images from two different perspectives. A stereo validation setup has been developed in order to give a much greater confidence to the novel instrument design and to get an on ground verification of the actual accuracies in obtaining elevation information from stereo pairs. A series of stereo-pairs of an anorthosite stone sample (good analogue of the hermean surface) and of a modelled piece of concrete, acquired in calibration clean room by means of an auxiliary optical system, have been processed in the photogrammetric pipeline using image correlation for the 3D model generation. The stereo reconstruction validation has been performed by comparing the STC DTMs (Digital Terrain Models) to an high resolution laser scanning 3D model of the stone samples as reference data. The latter has a much higher precision (ca. 20 μm) of the expected in-lab STC DTM (190 μm). Processing parameters have been varied in order to test their influence on the DTM generation accuracy. The main aim is to define the best illumination conditions and the process settings in order to obtain the best DTMs in terms of accuracy and completeness, seeking the best match between the mission constraints and the specific matching aspects that could affect the mapping process.

The pre-launch distortion definition of SIMBIO-SYS/STC stereo camera by rational function models

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.

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° 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.