User Manual - pancroma

9. Landsat 8 Data Sets________________________________________________________________Landsat 8 was successfully launched on February 11, 2013 from Vandenberg AirForce Base in California. It initially achieved stable orbit with its sensors turnedoff. It was gradually maneuvered into its final orbit with its sensors completelypowered up in May, 2013.There are many similarities between Landsat 8 and its predecessor, Landsat 7and significant differences as well. The biggest difference is that Landsat 8works. Landsat 7 has a so-called whiskbroom sensor, meaning that it has amechanical scanning mirror that sweeps from one side of the forward path to theother, successively directing points along the swept path to the sensor. Asowners of computer hard drives know, mechanical elements of electronicsystems are always the weak link and are prone to early failure. That is whathappened to the scanning mirror (actually the scanning mirror corrector calledthe SLC) in 2003. Since that time Landsat images have black stripes of missingdata that must be patched in order to create useful images.Landsat 8 in contrast has what is sometimes called a push broom sensor. Itconsists of an array of sensors called a Sensor Chip Assembly (SCA) that eachcollect data for part of the swath. As a result there are no mechanical parts andthis sensor should be much more reliable. The main Landsat 8 SCA is shownschematically below.The most obvious specification difference to the user between Landsat 8 andLandsat 7 is the dynamic range of the digital number (DNs) output from thesensors. Landsat 7 has 8 bits of dynamic range while Landsat 8 has 12. Thisdoes not mean that Landsat 8 is more accurate than Landsat 7, or that it canreport back a larger number (since any number can be scaled infinitely large). Itmeans that the numbers it returns are known more precisely. Landsat 7 canreport 256 discrete levels of sensed electromagnetic radiation (2 8 =256) whileLandsat 8 can report 2 12 =4096 levels. Essentially, Landsat 7 measures thingswith a ruler that has 256 gradients on it while Landsat 8 measures things with4096 gradients on its ruler. This can theoretically be very useful when trying to16