Calibration and Validation of Images from the Mars Reconnaissance Orbiter Mars Color Imager (MARCI) and Context Camera (CTX) Instruments

Computer Science – Performance

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Scientific paper

The MRO CTX instrument is a monochrome (611±189; nm), linear array CCD pushbroom camera with a nominal surface resolution of 6 m/pixel. The MARCI instrument is a 2-D CCD framing camera with 5 visible (420, 550, 600, 650, and 720 nm) and 2 UV (260 and 320 nm) filters, a 180° field of view, and a nominal resolution of about 1 km/pixel at nadir. Following Mars Orbital Insertion (MOI) in March 2006, CTX and MARCI images were acquired for initial instrument checkouts and validation of the pre-flight and in-flight calibration pipeline.
CTX in-flight bias and dark current levels are derived from masked pixels at the edges of the array. A dark current model derived during pre-flight calibration is applied if the masked pixels exhibit a gradient across the field or noise above an acceptable threshold. The CTX flatfield removes residual pixel non-uniformities and a subtle ''jail bar'' effect caused by the CCD's alternating register readout. Radiances are derived from bias, dark, and flat-corrected images using pre-flight scaling factors. Dividing the average radiances by the solar spectral radiance convolved over the CTX filter transmission and applying a Minnaert phase angle correction yields an average I/F level in the CTX post-MOI Mars images near an expected value of 0.2.
Bias and dark current subtraction of the MARCI images uses either a pre-flight model or dark sky data from the far left or far right parts of the field (nominally off the Mars limb). The preflight flatfield data were modified based on in-flight performance to remove residual non-pixel uniformities. Some residual pixel-dependent bias nonuniformities were also corrected using in-flight data. Bias, dark, and flat-corrected images were converted to radiance using pre-flight scaling factors. Phase-corrected 7-filter I/F values for the region of Mars imaged during the post-MOI campaign are consistent with previous data.

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