GOES-13 vs. GOES-12 visible channel
The GOES-13 satellite (launched in May 2006) has been brought out of on-orbit storage for a few weeks of testing and evaluation. A comparison of 1-kilometer resolution visible channel imagery (centered on Sioux Falls, South Dakota) from GOES-13 (above) and GOES-12 (below) demonstrates the fact that over time there is some in-orbit degradation of the GOES visible detectors. Note that the network of cities, towns and highways can be seen in the GOES-13 visible image above, especially across northwestern Iowa (in particular, Highway 60 which runs southwest to northeast: Google maps) and southwestern Minnesota — these towns and roads show up due to the contrast between the higher albedo concrete of the towns and road surfaces (and the adjacent ditches/medians) and the lower albedo of the surrounding fields of dense, mature corn crops. These features were less apparent in the GOES-12 visible image below (GOES-12 was launched in July 2001, and has been the operational GOES-East satellite since March 2003).
Part of the difference in the appearance of the scene can be explained by examining a plot of the spectral response function of the visible channels on GOES-12 and GOES-13 (below; thanks, MatG!): the sharper cutoff for wavelengths beyond 0.7µm on the GOES-13 visible channel (red line) makes it less sensitive to the signal from the mature corn crops (green line), allowing greater contrast between the thick vegetation of the agricultural fields and the more sparsely vegetated cities, towns, and highway corridors. Also of significance is the fact that Highway 60 in Iowa was in the midst of a major reconstruction project to expand it from a 2-lane highway to a 4-lane divided highway, making for a wider non-vegetated space that more easily shows up on the 1-km resolution GOES-13 visible imagery.
A 250-meter resolution “true color” RGB composite image from the polar-orbiting Terra MODIS instrument (below) shows even better detail, with the roadways, cities, and towns standing out very well against the surrounding background of dark green corn crops. Even the square grid network of minor county roads (spaced at 1 mile intervals) can be seen in the MODIS image.
