Why do pixel exception values and corresponding confidence flags appear to be set in some visible channel data at night?
At night, noise in the visible channels cannot be distinguished from pixel exception values, and can therefore be masked into the confidence flags. This is not strictly an error in the AATSR processing scheme, but rather an unexpected result of adopting the ATSR-2 processing scheme for AATSR (ATSR-2 visible channel data were not available at night). The AATSR pixel exception values are small negative numbers (ranging from -1 to -8). At night, the noise in the visible channels can mimic these exception values, so that the interpretation of negative visible channel reflectances is ambiguous. Also, because the confidence flags are derived from the union of the exception values across all channels, this can cause the confidence flags to be wrongly set. Note that the latter effect affects any user who is using the confidence flags to filter the data, regardless of whether he is interested in the visible channels or not. Users are advised to take note of this effect and to take into account
