What we did
An earlier entry in this research area attributed measurement offsets site by site. Part of the evidence behind it compared surface readings with top-of-atmosphere reference values, a mismatch found and fixed in September. This entry rechecks those attributions with surface readings compared against measured surface truth, and adds three new real-data analyses at the same reference sites.
The first rereads the site-by-site evidence on the corrected basis. The second tests whether the offsets follow viewing geometry, using real per-site sun and view angles for each acquisition at six sites. The third runs the reference sites' own measured surface and atmosphere through an independent physics calculation to test whether the atmosphere could explain the offsets.
What we saw
The earlier attributions survive the correction. At LCFR, the step between the two viewing directions is still present on the surface basis, and a fixed detector offset is still ruled out as its only cause. At RVUS, the same question is still open. At BTCN, the mismatch between the small reference target and the much larger area the satellite reading covers is confirmed on the corrected basis.
Viewing geometry is a real effect at more than one site: clearly at RVUS and LCFR, with the same sign but too weakly to be conclusive at BSCN. One shared strength describes it across those sites. In the near-infrared it accounts for the step between viewing directions at RVUS (about seven percent) and largely explains LCFR's offset. It explains only a minority of RVUS's overall offset, and none of GSCN's, where the satellite looks almost straight down.
That leaves an offset of about five percent at RVUS and GSCN that does not depend on geometry. It is not a sensor calibration error: GONA, observed by the same satellites and the same detector, reads close to zero.
The atmosphere does not explain the RVUS offset: its viewing-angle part is too small, and the atmospheric state accounts for only a few percent of the offset there. At GSCN the atmospheric state cannot yet be ruled out.
What it means
Correcting the reference basis did not overturn the earlier site-by-site findings, and the new evidence narrows them further. Viewing geometry is now seen at more than one site, though the result is still preliminary. The sensor and the atmosphere are ruled out as the cause of the largest remaining offset at RVUS. What is left is a smaller, sharper question: at RVUS, is the remaining difference in the reference measurement itself or in the area being compared, and at GSCN, could the atmospheric state also play a part? That is the next step in this research area.