PRELIMINARY PHOTOGRAMMETRIC CONTROL OF MESSENGER ORBITAL IMAGES OF MERCURY

Key products to be generated from the Mercury Dual Imaging System (MDIS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft include a global image base map and digital elevation model (DEM). In a preliminary effort toward this end, the Astrogeology Science Center (ASC) of the U. S. Geological Survey processed a subset of images acquired with the monochrome narrow-angle camera (NAC) and the multi-spectral wide-angle camera (WAC, G filter) of MDIS during a portion of the first solar day (5 April – 10 May 2011). Covering more than a third of the surface of Mercury at a mean resolution of 180 m/pixel, 4542 images (2781 NAC, 1761 WAC) were photogrammetrically controlled using the Integrated Software for Imagers and Spectrometers package developed by the ASC. This process is defined by two fundamental tasks: (1) the measurement of tie points between overlapping images by automated image matching techniques and (2) refinement of image attitude and/or position using a least-squares bundle adjustment. The bundle adjustment provides triangulated ground coordinates (latitude, longitude, and radius) for all tie points. In this first triangulation, over 200,000 line/sample image measurements were generated for 52,000 ground points. In the adjustment, spacecraft position was rigidly constrained, attitude angles constrained to ±2°, and the radius coordinate of all ground points constrained to ±10 km in accordance with an estimated elevation range for Mercury. The adjustment converged in four iterations with a line/sample residual rms of 0.25 pixels in image space. Of the final triangulated ground points, nearly 5000 were rejected due to high uncertainty in the radius coordinate (resulting from narrow intersection angles). The rms of adjusted uncertainties for the remaining ground points were 100, 145, and 310 m in latitude, longitude, and radius respectively. Generation of an image mosaic and DEM of the coverage area are in progress. The points range from 80°S to 86°N latitude and from 158°E to 358°E longitude. Hypsometry of the region is characterized by a symmetric, unimodal distribution and a dynamic range of 10.5 km. Minimum, maximum, mean, and median elevations are -5.0, 5.5, -0.2, and -0.18 km relative to the mean radius of Mercury (2440 km) adopted by the mission.