Landsat Missions
An optical leak is a manufacturing flaw in an instrument that allows light from unwanted sources to illuminate the detectors while they are collecting data. While this can be a major concern on some satellites, the known optical leaks on the Landsat satellites are minor.
L5 TM IC Light Leak
The TM light leak is a secondary image next to the internal calibrator lamp pulse. It is caused by an incomplete masking of the optical path by the IC shutter. It appears only in bands 5 and 7 and only in the IC region, thus it can only be found in Level 0 data. The TM light leak causes difficulties in using the IC pulse for calibration of bands 5 and 7 but has no other effect on the data.
Figure 1. Landsat 5 TM band 7 IC data showing the IC light leak.
Click to view larger image.
L7 Ghost PASC
On Landsat 7, an extra PASC signal appears in dark data about one WRS row behind the first PASC glint. This "ghost PASC" is dimmer than the real PASC glint and is dimmer in some bands due to spectral effects, making it appear blue in RGB images made with band combinations 321 or 543 (browse images). The ghost PASC is caused by unwanted reflections from the sun in the partial aperture solar calibrator optics. It only appears in L7 reflective band night imagery, during or immediately after PASC collections. The L7 ghost PASC has no effect on calibration. Because it only occurs in reflective bands during PASC collections, any imagery it obscures is nighttime data.
Figure 2. Landsat 7 ETM+ browse imagery showing the L7 ghost PASC.
Click to view larger image.
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://landsat.usgs.gov
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Page Contact Information: landsat@usgs.gov
Page Last Modified: May 13, 2008
