Scan Mirror Pulse
Figure 1. Example of Scan Mirror Pulse Anomaly in Landsat MSS scene.
Operational Landsat 5 Multispectral Scanner (MSS) imaging continued until 1999, though downlinking over the United States halted in 1992. In January 1999, the MSS sensor was turned off due to an anomaly in the sensor scanning mechanism. In addition, in 1995 MSS Band 4 was switched off due to low voltage levels.
The MSS Scan Mirror Monitor transmits pulses to the on-board computer at the start of scan, at the mid-scan point, and at the end of scan. When functioning normally, the mid-scan pulse does not preempt imagery at the mid-scan point, and the start-of-scan indicator and end-of-scan marker (which do appear in the raw imagery) are removed during ground processing.
The scanning anomaly first noticed in mid-January of 1999 remained after the Landsat 5 MSS sensor was turned back on in 2012. An investigation showed that the mid-scan marker could not be turned off without also turning off all other pulse markers. Without the start-of-scan and end-of-scan markers, MSS data cannot be successfully processed to higher level products at this time. Thus, the decision was made to acquire L5 MSS data with all three scan mirror pulses.
This results in a normal looking scene except for the pulse at the mid-scan point (4 pixels of black and 4 pixels of white per line per band in the raw imagery). This is in all bands, and will appear as an eight pixel stripe down the center of the scene.
Figure 2. Location of scan marker in the image.
Figure 3. Close-up of scan marker.