Description of Time Delayed Imaging (TDI)

The IMAGE satellite is spin stabilized and the WIC and SI imagers are pointing perpendicular away from the rotational axis. For imaging the fixed Earth this requires a compensation for the apparent motion of the imaged object. This is done with the Time Delayed Imaging technique. Every 1/30 of a second the cameras capture a single frame. Between two consecutive frames the satellite has rotated 0.1 degrees and the center of the image points to another point on the surface of the Earth. The images will be added in a virtual memory by shifting consecutive images by the necessary amount of pixels so that the individual pixels which image one and the same area on the surface of the Earth are added together. This adding of frames is necessary because the emission of the desired photons is too weak to produce bright images with only 1/30 s of exposure and because the whole Earth should be imaged.


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Last update: 24 June 1997

by Harald U. Frey, nospam_hfrey@ssl.berkeley.edu