Why does Landsat 7 ETM+ change its gains?
For every band, ETM+ instrument can operate in either high (H) or low (L) gain state. Gain state strongly depends on the illumination (sun elevation) at ground level and on cover type. The gain mod system consists of an on-board pre amplifier which acts as a filter; the low gain mod prevents the sensors saturation but reduces the histogram dynamic. The two modes differ by about a factor of 1.5 for the seven reflective bands and 2.0 for the emissive band 6. As an example, Landsat 7 image bands acquired over snow areas in high gain mode can show sensor saturation for bands 1 (blue), 2 (green), 3 (red) and less frequently 4 (near infrared). However, the same bands acquired in low gain mode do not show sensor saturation, but their poor dynamics are sometimes the cause of a striping defect.
