satellite radiance pair regression slopes based on SNO and off-nadir SNO radiances were rather similar. demonstrated that the Advanced Very High Resolution Radiometer A. Relaxing the view angle-matching cone allows multiple off-angle matching op- portunities for each ground track intersect. A strategy to increase the number of ray-matched samples is to broaden the boresight cone diameter. Such few matches cannot possibly resolve the temporal degradation of the GEO visible sensor but serve only to validate postlaunch calibration. These cycles equate to less than 40 matches per year. The SCIAMACHY nadir and limb scan-duty cycle reduces the potential matches by half (see ). Each collocation is usually sampled within 15 min once or twice per month owing to the 35-day ENVISAT ground track repeat cycle. The corresponding SCIAMACHY footprint hyperspectral radiances convolved with the GEO SRF are also computed.
Any time the appropriate SCIAMACHY footprint center is within 40 km (precise ray matching) of one of the four a priori SCIAMACHY/GEO colocations, an average is computed using the GEO radiances contained inside the SCIAMACHY footprint. Because the line that defines the cross track intersects the GEO subsatellite point, a nearly exact solar and azimuthal geometry match between the two instruments is ensured. The GEO VZA range across the footprint is ∼ 2 ◦, whereas for SCIAMACHY, it is ∼ 15 ◦. This approach matches the GEO VZA with the center SCIAMACHY footprint VZA. 4 illus- trates the orientation of the SCIAMACHY footprints along the ENVISAT ground track for the ray-matched footprint on the far west side, which is shown in purple. 4) over the GEO domain where ray matching, or bore sighting, is possible.
four cross-track SCIAMACHY footprints allow only four collocations (blue x’s in Fig.
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