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Advanced Land Imager Solar Calibration Mechanism Anomaly

Learn about the Advanced Land Imager (ALI) instrument and its solar calibration mechanism anomaly. Discover the innovative technologies integrated into ALI and how the anomaly was assessed and addressed. Gain insights into the impact on data collection and analysis.

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Advanced Land Imager Solar Calibration Mechanism Anomaly

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  1. Advanced Land ImagerSolar Calibration Mechanism Anomaly Constantine J. Digenis 4 November 2002

  2. Advanced Land Imager (ALI) • ALI is an instrument incorporating several new technologies that promise better performance, at a lower cost, of future Landsat missions. • Push broom data collection mode eliminates scan mirror. • ALI includes three NMP Category 1 technologies: • Wide field-of-view (1.25 o x 15 o) telescope that covers the full 185 km Landsat swath width, • SiC mirrors that offer low mass, high stiffness and thermal stability, • Highly integrated focal plane with ten VNIR/SWIR bands and HgCdTe SWIR detectors optimized for operation at 220o K. • ALI also includes an innovative solar calibration that works over the entire dynamic range of the instrument. • All of the above technologies were validated on orbit during the one-year duration primary mission that was completed on 20 November 2001.

  3. Solar Calibration Mechanism • In the first 19 1/2 months on-orbit, ALI had performed 40 solar calibrations flawlessly. • On July 5, 2002, during the 41st solar calibration, the sliding screen failed to complete its travel. • Due to the latency in data reception and processing, three more solar calibrations were performed with the screen appearing in a slightly different , stationary position each time. • Then the solar calibration script was changed, eliminating the activation command to the sliding screen motor. • The last six solar calibrations show the system response to be stable indicating that the sliding screen is stationary and the instrument response is stable.

  4. Anomaly Assessment • We have concluded that the ball nut was stuck on the ball screw causing the sliding screen to get detached from the ball nut. • The screen is now free to slide along the ball screw, however, the stuck ball nut presents a stop in one direction. • The combination of forces acting on the sliding screen, caused it to move towards the stationary ball nut thus keeping the sliding screen stationary. • Solar calibrations can and will continue to be performed albeit at a constant illumination level rather than over the entire dynamic range, as before. • The data need to be analyzed carefully, to detect and interpret any deviations from stability.

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