SURFACE IMAGERY BASED MAPPING AND ROVER LOCALIZATION FOR THE 2003 MARS EXPLORATION ROVER MISSION Kaichang Di, Fengliang Xu, Jue Wang, Xutong Niu, Charles Serafy, Feng Zhou, and Ron Li Mapping and GIS Laboratory, CEEGS, The Ohio State University 470 Hitchcock Hall, 2070 Neil Avenue, Columbus, OH 43210-1275 Tel: (614) 292-6946, Fax: (614) 292-2957 Email: (di.2, xu.101, wang.813, niu.9, Serafy.1, zhou.182, li.282)@osu.edu
Larry Matthies Jet Propulsion Laboratory, California Institute of Technology Mail Stop 125-209, Pasadena, CA 91109 Email:
[email protected]
ABSTRACT This paper presents the technology of mapping and rover localization at the two landing sites, Gusev and Meridiani, for the 2003 Mars Exploration Rover (MER) mission. The rover localization and landing site mapping technology is based on the incremental bundle adjustment of an image network formed by Pancam, and Navcam stereo images. The developed incremental bundle adjustment model supplies improved rover locations and image orientation parameters, which are critical for the generation of high quality landing site topographic mapping products. The rover localization results demonstrate that the bundle adjustment technology is able to correct position errors caused by wheel slippages, azimuthal angle drift and other navigation errors as large as 21%. The mapping products, which are generated at each stop of the rovers, include digital terrain models, orthophotos, and rover traverse maps. These maps and localization information were provided to MER mission scientists and engineers through a Web GIS.
INTRODUCTION In the Mars Exploration Rover (MER) 2003 mission, the twin rovers, Spirit and Opportunity, carry identical Athena Instrument Payloads and engineering cameras to explore the landing sites of Gusev Crater and Meridiani Planum (Squyres et al., 2003; 2004). To support science and engineering operations, it is critical to localize the rovers and map the landing and traversing area with high accuracy (Arvidson et al., 2004). During the mission, The Ohio State University (OSU) team, collaborating with JPL, has been routinely producing topographic maps, rover traverse maps, and updated rover locations to support tactical and strategic operations. These maps and localization data were provided to MER mission scientists and engineers through a Web GIS. Among the various instruments on board the rovers, Pancam (Panoramic Camera) and Navcam (Navigation Camera) stereo cameras are the most important for high-precision landing-site mapping and rover localization. These two stereo-imaging systems are mounted on the same stereo bar of the rover mast. The image sizes of both the Pancam and Navcam are 1,024 × 1,024 pixels. Navcam has a stereo base of 20 cm, a focal length of 14.67 mm, and an effective depth of field of 0.5 m to infinity. Its best focus is at 1 m with a field of view (FOV) of 45 degrees. Pancam has a wider stereo base (30 cm) and a longer focal length (43 mm), making it more effective for mapping medium-to-far objects in the panoramic images. The effective depth of field for the Pancam is 3 m to infinity and the FOV is 16 degrees. The rover localization and landing-site mapping technology is based on the bundle adjustment (BA) of an image network formed by surface imagery, i.e., Pancam and Navcam stereo images. The overall technology is described in (Li et al., 2004a). Before the MER mission, the rover localization and mapping technology had been extensively tested and verified with field test data acquired on earth and actual Mars data from the 1997 Mars Pathfinder mission (Li et al., 2002; Di et al., 2002). After the landing of the two rovers, the OSU team and collaborating scientists and engineers of the mission performed lander localization using rover panoramic images, orbital images, and descent images as well as radio science based localization. The initial results of lander/rover localization, regional mapping using orbital and descent images, and detailed landing-site mapping using surface imagery are reported in Li et al. (2004b). In this paper, a detailed description of the technology of mapping and rover localization using surface imagery is given and updated mapping and localization results at the two landing sites are presented.
CAMERA MODEL AND REFERENCE SYSYTEM As a starting point for photogrammetric processing, the camera models and reference systems must be elucidated. The original camera model of the Pancam and Navcam images is the CAHVOR model, which models the transformation from the object domain to the image domain by using vectors C, A, H, and V and corrects radial lens distortions with a vector O and a triplet R. It has an intrinsic difference with the conventional photogrammetric model but can be converted to the photogrammetric model with sufficient accuracy (Di and Li, 2004a). The model is defined in rover frame, in which the X axis points forward, the Z axis points down, and the Y axis is defined to form a right-handed system. The parameters of the CAHVOR model are stored in the image header. To facilitate rover operations in an extended landing site, individual site frames are defined along the traverse. The X axis of a site frame points to north. The Z axis points down in the normal direction. The Y axis is defined to form a right-handed system. The position and attitude of each rover frame, with respect to its site frame, is defined by three translations and a set of quaternion parameters, which are also included in the image header. The first site frame (Site 0), which is at the lander, is defined as the Landing Site Local (LSL) frame for mapping and rover localization purposes. The relative position of a site frame to its previous site frame is stored in a master file. This primary geometric information is regularly supplied by the onboard Inertial Measurement Unit (IMU) and wheel-odometry–based localization system with infrequent support by sun-finding techniques that improve the azimuth quality. In addition to original rover images with the CAHVOR camera model, linearized images are also provided to facilitate stereo viewing and matching. Since the linearized image is resampled according to epipolar geometry, there is practically no parallax in the vertical direction. The lens distortions have also been corrected in the linearized imagery. Thus, the CAHVOR model is reduced to a CAHV camera model, which does not have lensdistortion components. Linearized Pancam and Navcam images were used for in topographic mapping and rover localization. First, the CAHV camera model was converted to a photogrammetric model that is necessary and is commonly used for topographic mapping and remote sensing (Di and Li, 2004a). The camera model is then transformed from the rover frame to its site frame and subsequently to the LSL by sequential rotations and translations. The resultant imageorientation parameters are used as initial approximations in the BA. Orbital images were available pre- and post-landing. After the landing of the two rovers, the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) aimed at the two estimated lander locations and took a new, highresolution (1 m) cPROTO (compensated Pitch and Roll Targeted Observations) image at the Gusev site on Sol 16 and a ROTO (Roll-Only Targeted Observation) image at the Meridiani site on Sol 13 (Malin, 2004). The landers can be seen from the two high-resolution images. Through the lander position, which is also the origin of the LSL, the LSL can be linked to the Mars body-fixed reference system.
ROVER LOCALIZATION BASED ON BUNDLE ADJUSTMENT OF SURFACE IMAGE NETWORK As indicated above, on-board rover localization is primarily performed by the IMU, wheel-odometry, and sun-finding technology. In cases where the rover experiences slippage caused by traversing loose soil or steep slopes, particularly in a crater, the onboard visual odometry (VO) technique was applied. In this mission, VO has acquired consecutive Navcam stereo pairs (
