CTX in-flight calibration and data dissemination

The Context Camera (CTX) on board NASA's Mars Reconnaissance Orbiter (MRO) has been in orbit since 2006 and has so far delivered more than 130,000 images. The images are one of the most popular data sets for planetary geologists because the data cover almost the entire planet and have good radiometric resolution, allowing very detailed interpretation of surface features. Since the beginning of the mission, the images have exhibited a darkening effect from the centre of the images towards the edges, creating visible seam lines when multiple images are stitched together. Due to the symmetric decrease in reflectance plots averaged over all lines, this problem is often referred to as "frowning" (see Figure 1 left). Since the standard calibration routines of the Integrated Software for Imagers and Spectrometers (ISIS) only include flatfield files for the first year of the mission, there are no quick and easy standard methods to correct for these artefacts. In this work, we provide an extended in-flight radiometric calibration and the resulting flatfield files that can be used directly in the ISIS environment (see correction example in Figure 1 right). The files are updated regularly and are permanently available in this repository: https://dx.doi.org/10.17169/refubium-37236 .

Figure 1: left: CTX image N05_064260_1638 with standard ISIS calibration applied (top) and curve plot of all averaged lines (bottom); right: after additional in-flight calibration the image (top) shows less darkening to the borders and the downward trent in the plot has been removed.

In addition, we are in the process of updating our "integrated Mars analysis and research system" (iMars) to include the full set of CTX images, which will be readily processed and made available for download in GIS-compatible formats. As with the previous system, users can select the footprints and visualise the data directly in the map view. Special tools for switching between images with multiple coverage provide an excellent infrastructure for analysing surface changes and seasonal or interannual variations.  We have made a complete overhaul of the graphical interface, which is accessible under https://maps.planet.fu-berlin.de/ctx . 

This work is supported by the German Space Agency (DLR Bonn), grant 50 OO 2204, on behalf of the German Federal Ministry for Economic Affairs and Energy. We thank the HPC Service of Freie Universität Berlin for computing time.