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.