<a href="https://en.wikipedia.org/wiki/Landsat_7" class="extiw" title="w:Landsat 7">Landsat 7</a> Scan Line Corrector operation from the Landsat 7 Science Data Users Handbook
<ul><li>The Scan Line Corrector (SLC) is an electro-optical mechanism composed of two parallel nickel-plated beryllium mirrors set at an angle on a shaft. The shaft, driven by a torquer which has primary and redundant tachometers, rotates about an axis normal to the axis of the scan mirror in a sawtooth fashion. The SLC is positioned behind the primary optics and compensates for the along track motion of the spacecraft that occurs during an active SMA cross track scan. A rectilinear scan pattern is produced using the SLC instead of the zig-zag pattern that would be produced without it.</li></ul>
NASA/Goddard Space Flight Center
<a rel="nofollow" class="external free" href="http://landsathandbook.gsfc.nasa.gov/payload/prog_sect3_3.html#section3.3.3">http://landsathandbook.gsfc.nasa.gov/payload/prog_sect3_3.html#section3.3.3</a>