At-launch Gain Setting Strategy Rationale
The following paragraph was extracted from a paper that describes the rationale behind the at-launch gain setting strategy (Goward, et. al., 1999)
"The 8-year monthly averages of AVHRR visible and near infrared planetary reflectance measurements from the AVHRR data set, at the original 15 km spatial resolution, were used to evaluate the gain settings. Visible (bands 1-3), near infrared (band 4), and shortwave infrared (5 and 7) gains were considered separately. The at-satellite planetary reflectance was converted to at-satellite radiance, based on the solar zenith angle at the time of satellite overpass. For each Landsat WRS scene, the observed spectral radiance was subjected to the two gain states. For each gain state, an entropy statistic was calculated to determine the potential scene contrast in each setting. Where low gain was found to provide substantially greater scene contrast (e.g glaciers in summer), this setting was selected. For all other cases the high gain was selected. This decision process will no doubt yield less than optimum results for some applications, but it was the best compromise to meet the requirement to minimize gain changes while providing generally high quality measurements".
The initial ETM+ gain settings are depicted graphically in Table 6.4. These lasted from launch until July 13, 2000.
