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Description of Driving Event:

The Orbiting Carbon Observatory (OCO), an Earth orbiting satellite mission managed by the NASA/Caltech Jet Propulsion Laboratory, was designed to make precise, time-dependent, global measurements of atmospheric carbon dioxide (CO2). Prelaunch testing of the primary instrument used to detect CO2 from orbit revealed residual image problems with two of the three focal plane array (FPA) detectors (Reference (1)). The residual image in the detector used for the 760 nm Oxygen A-band channel was found to be an intrinsic property of the detector design. The residual image in the 1610 nm Weak CO2 channel was traced to a focal plane fabrication anomaly. The residual image problems in both FPAs were remedied by developing a correction algorithm implemented in the ground data processing system used to processes the science data.

An innovation that proved to be one of the most critical assets of the OCO pre-launch instrument test program was a heliostat (Figure 1) on the roof of the JPL Environmental Test Facility. This facility was installed to allow direct observations of the sun, to validate the spectroscopic calibration against a high-precision reference spectrometer. A heliostat incorporates a mirror that turns to track the sun's apparent motion across the sky and maintain a beam of reflected sunlight pointing toward a target. The OCO heliostat used a pair of mirrors to capture the sunlight and direct it though a window into the thermal-vacuum chamber and into the flight instrument (Reference (2)). While the residual image in the Weak CO2 channel was discovered and characterized using white light, heliostat observations were essential for characterizing and quantifying the detector residual image problem in the A-band Channel. Heliostat observations therefore provided the data needed to correct for its effects in post acquisition data processing. In addition, observations of atmospheric absorption lines obtained with the heliostat played a critical role in verifying the dispersion and instrument line shape function of the OCO spectrometers. It also provided an efficient means to test the throughput of the on-orbit calibration system. Observations of the moon through the heliostat provided a sanity check on the geometric calibration of the instrument, and provided a means to validate instrument slit misalignment.