User Manual - pancroma

68. Tutorial – Gap Filling using Transfer Method________________________________________________________________IMPORTANT NOTE The semi-manual method for registering images in thistutorial has been superceded by the Six File method described in Section 32 ofthis manual. The semi-manual steps described in this tutorial work well but theautomatic method is considerably easier.The newest Landsat satellite (Landsat 7) was actually launched in 1999. ManyLandsat users know that that this valuable earth science asset has beenseriously broken since 2003 when its Scan Line Corrector (SLC), whichcompensates for its forward motion as it scans the earth's surface failed. Thishas resulted in systematic gaps in every Landsat image collected since that time.To understand the problem, it is necessary to know a little about how the Landsatsensor works. As Landsat traverses an (approximate) north-to-south orbit, amirror scans east-to-west collecting data across the orbital path. If uncorrected, azigzag path would be described with wedge-shaped gaps in between the data. Inorder to make the transverse data rows parallel and remove these gaps, amechanical corrector is attached to the mirror. This is the part that failed in 2003.All of NASA's attempts to fix the problem have been unsuccessful.As a result, instead of nice parallel rows (like a well-mowed lawn) theuncorrected gapped pattern is transmitted to the ground station. This cannot becompensated by post-processing at the Landsat ground station because gapdata is physically absent from the scan. Approximately 22% of the data is lost,with more lost information occurring at the edges of the image than at the center,where there is a vertical strip of complete data.Although there is no way to recover the missing gaps, in many cases it ispossible to make these images more useful by filling in the gaps with overlappingdata or data collected before 2003. This is possible because various Landsatsatellites have orbited the earth for so many years that there are multiple many320