On May 31, 2003, the Scan Line Corrector (SLC), which compensates for the forward motion of Landsat 7, failed. Subsequent efforts to recover the SLC were not successful, and the failure appears to be permanent. Without an operating SLC, the Enhanced Thematic Mapper Plus (ETM+) line of sight now traces a zig-zag pattern along the satellite ground track. As a result, imaged area is duplicated, with width that increases toward the scene edge.

The Landsat 7 ETM+ is still capable of acquiring useful image data with the SLC turned off, particularly within the central part of any given scene. The Landsat 7 ETM+ therefore continues to acquire image data in the "SLC-off" mode. All Landsat 7 SLC-off data are of the same high radiometric and geometric quality as data collected prior to the SLC failure. The SLC-off effects are most pronounced along the edge of the scene and gradually diminish toward the centre of the scene (Figure 2). The middle of the scene, approximately 22 kilometres wide on a Level 1 (L1G, L1Gt, L1T) product, contains very little duplication or data loss, and this region of each image is very similar in quality to previous ("SLC-on") Landsat 7 image data. Landsat 7 data collected after May 31, 2003 is know as "SLC-off data."