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AHI's automatic sun avoidance function

Around midnight during the spring and autumn eclipse periods, sunlight can directly enter the AHI sensor when the sun and the earth occasionally lie in an approximate line with the satellite. To avoid this, the AHI has a function by which part of the observation area can be skipped based on prediction of the sun's position and potentially affected areas.

This operation is performed not only in Full Disk observation but also in Japan Area and Target Area observation, resulting in the output image with a partial lack. When the skipped area is expected to cover the whole domain of Target Area observation, the observation will not be performed.

Prediction and results

The pages linked to below provide information on sun avoidance prediction for the forthcoming eclipse period. Actual observation data are also shown for the 2015 spring eclipse period.

Solar interference prediction (click to open/close)

Note: Prediction is based on the assumption of ideal satellite position and attitude. This can cause discrepancies between prediction and actual observation.

Understanding figures on the prediction/results page

Figures with a white background are output from actual observation. Skipped earth areas are shown in white.

Figures with a black background show prediction of the sun's position and potential areas of solar interference. The details are outlined below.

  • Small circles represent the predicted position of the sun at the beginning and the end of the timeline.
  • Large circles represent 3 degrees of angular distance from the center of small circles at the beginning and end of the timeline. (Angular distance is measured as observed from the satellite.)
  • Small/large circles are shown in white when no need for sun avoidance is predicted.
  • Small/large circles are shown in red when a need for sun avoidance is predicted. Overlaps with large circles can be considered solar interference areas.
  • Angular distances between the center of the sun and the geocenter at the beginning and end of the timeline are shown at the lower left and lower right, respectively.
Prediction Observation How to read figures (Click observation figure to enlarge)
  • The predicted position of the sun is south-west of the earth.
  • The need for sun avoidance is not expected (Prediction), and output images will have no missing data (Result).
  • The sun is 16.8 degrees from the geocenter at the beginning of the timeline.
  • The sun is 15.2 degrees from the geocenter at the end of the timeline.
  • The predicted position of the sun is behind the earth.
  • The need for sun avoidance is not expected (Prediction), and output images will have no missing data (Result).
  • The sun is 6.0 degrees from the geocenter at the beginning of the timeline.
  • The sun is 4.6 degrees from the geocenter at the end of the timeline.
  • The predicted position of the sun is north-east of the earth.
  • The need for sun avoidance is expected (Prediction), and output images will have missing data (Result).
  • The sun is 9.6 degrees from the geocenter at the beginning of the timeline.
  • The sun is 11.0 degrees from the geocenter at the end of the timeline.

Stray light

Stray light (a term given to reflected or diffused sunlight entering the sensor) can also be seen in AHI images during eclipse periods. Examples are shown on the Impacts of spacecraft activity and calibration on Himawari-8/9 Images page.