Figure 1. Example of Internal Calibrator (IC) Intrusion in Landsat 7 Enhanced Thematic Mapper Plus (ETM+) Level-0 imagery.
Click for animation of intrusion from 1999 to 2006. - .gif (799 KB)
Internal Calibrator (IC) Intrusion occurs when the IC shutter on the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) intrudes into the imagery. IC Intrusion is a result of bumper wear, which causes the main mirror scan time to increase and forces the IC shutter to swing slower to remain in synch. Early in the life of the ETM+, intrusions were small, but they have grown with time. In 2005, intrusions grew so large in some bands that the IC pulse became visible in the imagery.
IC Intrusions are largest in Bands 1 and 8 and smallest in Bands 5 and 6, corresponding to the layout of the ETM+ detector array. They only occur on the left side of the image in forward scans. Intrusions are visible in both Level-0 and Level-1 imagery. In Red Green Blue (RGB) imagery, IC Intrusions have a multicolored effect because some bands have dark shutter data in the same pixels that other bands have imagery or the bright IC pulse.
Figure 2. Example of IC Intrusion in Landsat 7 ETM+ Level-0 RGB imagery.
Click to view larger image. - .gif (59 KB)
IC Intrusion is a known artifact that should cause no concern. It is uncorrectable, and the data obscured by the IC shutter intrusions are lost. Monitoring of IC Intrusions may be used as another means of measuring the increasing scan line length, which will eventually end in loss of mirror/shutter synch and force the ETM+ into bumper mode. IC Intrusions do not occur in bumper mode and are not seen in ETM+ imagery after April 2007.