The subject of correction is tied to the procedures called pre-processing or image restoration. The treatment of these modifications is extensive and will not be covered in this Appendix, other than to mention the principal actions normally made in adjusting the DN values. Corrections can be made that affect undue Radiometric variations, i.e., changes in the measured radiances owing to a variety of factors, conveniently divided into natural and instrumental. One natural condition relates to day to day and place to place differences in atmospheric conditions. For example, presence of water vapor influences the radiance, so that its values would not be the same for the same target type if it were present in the tropics as compared with the dry arctic.
Instrument corrections involve variations in detector response and electronic perturbations.
Procedures are available to apply computer-generated corrections for any of these, improving overal image quality. With the advent of micro-computers, the cost of image processing has greatly dropped while quality of the output may be superior to older, large machine methods. Data can now be supplied on 5.25 or 3.5 inch floppy discs, CDs, and other storage devices. Armed with this overview, we are now ready to get you directly involved in the procedures of space image processing.