Satellite scanners operating in Backup bumper mode. Landsat-type sensors such as the Thematic Mapper (TM) and the Enhanced Thematic Mapper Plus (ETM+) are designed to have a fixed line length on every scan controlled by the Scan Angle Monitor (SAM). When the wear on the bumpers is sufficiently large that it is no longer possible to maintain control in the SAM-mode, then the instrument can be put in a backup bumper Mode where the mirror is allowed to scan AT a fixed Frequency of 14 Hz as a free pendulum rather than being controlled on each scan. The calibration shutter is still synchronized with the scan mirror in this mode SO that calibration pulse can be obtained at the end of each scan. However, the variability of the scan length increases in bumper mode. The Landsat 5 TM was placed in the bumper mode in 2002. (Source: Dr. John Barker).