Improving the Usability of Galileo and Voyager Images of Jupiter's Moon Europa

Abstract

For a planetary imaging data set to be of maximum use to the science community, the absolute and relative location of the images on the surface of a planet must be known with high accuracy and precision. For some planetary missions in which images are acquired from orbit, image locations derived from reconstructed Spacecraft, Planet, Instrument, C-matrix (pointing), and Events (SPICE) kernels (Acton et al., 2017) are sufficient for many scientific applications (e.g., morphologic analysis). However, the spatial accuracy of such reconstructed data is insufficient for investigating change detection, planning surface mission operation (e.g., rover traverse), or deriving higher-order data products (e.g., digital terrain models, DTMs). For missions in which data are acquired by multiple fast flybys of distant objects (e.g., many missions to the outer Solar System), uncertainties in the spacecraft and body locations and orientations are large enough to result Abstract NASA's Voyager 1, Voyager 2, and Galileo spacecraft acquired hundreds of images of Jupiter's moon Europa. These images provide the only moderate-to high-resolution views of the moon's surface and are therefore a critical resource for scientific analysis and future mission planning. Unfortunately, uncertain knowledge of the spacecraft's position and pointing during image acquisition resulted in significant errors in the location of the images on the surface. The result is that adjacent images are poorly aligned, with some images displaced by more than 100 km from their correct location. These errors severely degrade the usability of the Voyager and Galileo imaging data sets. To improve the usability of these data sets, we used the U.S. Geological Survey Integrated Software for Imagers and Spectrometers to build a nearly global image tie-point network with more than 50,000 tie points and 135,000 image measurements on 481 Galileo and 221 Voyager images. A global least-squares bundle adjustment of our final Europa tie-point network calculated latitude, longitude, and radius values for each point by minimizing residuals globally, and resulted in root mean square (RMS) uncertainties of 246.6 m, 307.0 m, and 70.5 m in latitude, longitude, and radius, respectively. The total RMS uncertainty was 0.32 pixels. This work enables direct use of nearly the entire Galileo and Voyager image data sets for Europa. We are providing the community with updated NASA Navigation and Ancillary Information Facility Spacecraft, Planet, Instrument, C-matrix (pointing), and Events kernels, mosaics of Galileo images acquired during each observation sequence, and individual processed and projected level 2 images.

How Well Do We Know Europa’s Topography? An Evaluation of the Variability in Digital Terrain Models of Europa

“…Unfortunately, absolute positional accuracy and relative elevations (tilts) across or among DTMs are outside the scope of our investigation. The lack of independent elevation data, such as radar or lidar altimetry, for Europa leaves us without the reference needed to assess these factors-and also without the ground data needed to improve the absolute accuracy of DTMs by bundle adjustment (though such adjustment is useful to ensure good relative alignment of groups of images and DTMs; see Bland et al [20]). Instead, we focus on local properties of DTMs, such as their vertical precision and horizontal resolution, which determine what features are likely to be represented reliably.…”

“…In some instances where "a priori" SPICE data is inaccurate, the images were initially photogrammetrically controlled in ISIS. This step is now unnecessary due to the improved image locations provided by Bland et al [20] for the entire Galileo dataset. The images, along with the trajectory and pointing metadata, are then transferred to SOCET SET ® , where they are controlled and bundle adjusted using the SOCET SET ® program Multi-Sensor Triangulation (MST).…”

“…We then photogrammetrically control the image pair in ISIS (using ISIS's jigsaw application to perform the bundle adjustment) and project the data to a common map projection. This control step is necessary because, although ASP includes its own bundle adjustment application, the reconstructed SPICE information for Galileo images is often sufficiently inaccurate that the ASP bundle fails (the "smithed" SPICE kernels of Bland et al [20] were unavailable at the time of this work). We do not perform a second bundle in ASP, as we found this does not improve the solutions.…”

“…The reconstructed SPICE data is often inaccurate due to uncertainty in the spacecraft position and pointing when images were acquired. In some cases, this results in image locations that are inaccurate by 10s to 100s of kilometers [see Bland et al [20] for examples]. These inaccuracies must be corrected either before or during DTM extraction.…”

“…These inaccuracies must be corrected either before or during DTM extraction. Updated SPICE kernels for nearly the entire Europa imaging data set were generated by Bland et al [20], but these were not available when the work described here was completed. Additionally, in many instances, data downlink interruptions resulted in data for a single image being transmitted as two or more separate files that must be reconstructed into a single image before use.…”