Image construction by Fourier inversion may be hindered by various deficiencies in the data. Examples of defects include data presented on a nonrectangular array of points and missing data values. Among special cases that are commonly encountered are measurements made in polar coordinates and the existence of sectors where data may be entirely missing. An exact method of interpolation analogous to sincfunction interpolation in the rectangular situation is described. However, inverting such data involves not merely a problem of mathematical interpolation, but also account must be taken of the character of the object, the kind of information desired, and the penalties to be associated with errors, considerations that go beyond what is involved under conditions where the one-to-one correspondence between function and transform may be invoked and the Fourier inversion formula is applicable.
Two techniques for automated sea-ice tracking, image pyramid area correlation (hierarchical correlation) and feature tracking, are described. Each technique is applied to a pair of Seasat SAR sea-ice images. The results compare well with each other and with manually tracked estimates of the ice velocity. The advantages and disadvantages of these automated methods are pointed out. Using these ice velocity field estimates it is possible to construct one sea-ice image from the other member of the pair. Comparing the reconstructed image with the observed image, errors in the estimated velocity field can be recognized and a useful probable error display created automatically to accompany ice velocity estimates. It is suggested that this error display may be useful in segmenting the sea ice observed into regions that move as rigid plates of significant ice velocity shear and distortion.< >
The phase of very long radio waves reflected from the ionosphere is very-sensitive to the occurrence of solar flares. The course of the ionospheric disturbance can be followed in detail and a measure of the maximum disturbance given. A method is outlined which may lead to the identification of the radiation which disturbs the ionosphere. Continuous automatic records provide abundant data of great value to the study of solar-terrestrial relations.
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