Advanced Land Imager Solar Calibration Mechanism Anomaly

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Advanced Land ImagerSolar Calibration Mechanism
Anomaly

• Constantine J. Digenis
• 4 November 2002

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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 220 o 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.

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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.

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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.