The time of a forward or reverse scan on an imager such as TM or ETM+. In order to make the geometric Calibration easier, there is an active scan time (AST) and a turn-around time (TAT) for each scan. The Scan-Line-Corrector rotates two mirrors on the optical axis to keep the scan nearly perpendicular to the path of the Satellite on the ground. The active scan time (AST) is controlled as long as the Scanner is in the Scan-Angle-Monitor (SAM) mode, rather than the bumper mode. At a Landsat satellite altitude of 705 Km, an active scan of about 6320 pixels covers approximately 189 km on the Ground and is cropped SO that all the bands cover a common swath width of about 180 Km. Each scan on a TM is a little less than 0.5 Km wide. It is the knowledge of the AST and first and second half scan errors on each scan that allows the TM sensors to be systematically corrected when resampled such that only correction in rotation and translation need to be made for the image as a whole from ground control points. While in the SAM Mode the wear on the bumpers results in increases in TAT but no change in AST. After twenty years in orbit, Landsat 5 TM lost its ability to stay in the SAM mode and had to be operated in the bumper mode. Also see bumper mode and geometric image calibration. (Source: Dr. John Barker)