### Impacts of Viewing Geometry on SEVIRI's Detection Performance  
The sub-pixel fraction occupied by a fire (and thus the probability of detecting a fire) decreases as pixel area increases with increasing scan angle [66]. Therefore the combination of SEVIRI and MODIS viewing geometries will inevitably affect the evaluation of SEVIRI's detection performance--particularly at extreme MODIS scan angles where nominal SEVIRI and MODIS pixel areas are nearly equivalent in the CAR.  
SEVIRI's errors of omission and commission presented in Table 2 were calculated without regard to scan angle. Although SEVIRI view zenith angles in ROI 1 are greater than in ROI 3 (corresponding to larger ground footprints), SEVIRI's omission rate is considerably lower in ROI 1 than in ROI 3. Since larger SEVIRI pixels would imply greater errors of omission, this contradiction suggests that other factors besides SEVIRI's view zenith angle (e.g., active fire characteristics) have a greater influence on SEVIRI's active fire detection performance across the CAR.  
Independent of the ROI in which fire pixels were detected, Figure 10a demonstrates that the evaluation of SEVIRI's omission rate depends on MODIS scan angle. The decrease in SEVIRI's omission rate as a function of MODIS scan angle is attributed to the decreased ability of MODIS to detect low FRP pixels towards the scene edges [57,67]. After accounting for atmospheric transmissivity, the minimum FRP measured by SEVIRI in all ROIs was 9.1 MW (Table 1). For comparison, MODIS measured no less than \(\sim\)10 MW for fire pixels detected in all ROIs above a scan angle of 33 \({}^{\circ}\) (Figure 10b). Although the minimum detection limit of MODIS at the swath edges (\(\sim\)50 MW) is well above SEVIRI's minimum detection limit, SEVIRI's omission rate evaluated at MODIS scan angles above of 50 \({}^{\circ}\) does not completely drop to zero, but instead only decreases to \(\sim\)18%. Upon closer examination, the majority (89%) of SEVIRI's errors of omission located at the MODIS swath edges were confounded by sunlight, uncertainties in the background characterization, or cloud cover.  
The rise in the commission rate of the SEVIRI FTA detection algorithm towards the MODIS scene edges (Figure 10a) is also attributed to an increase in the MODIS active fire omission rate away from nadir. Since SEVIRI is able to detect active fire pixels with an FRP less than the MODIS minimum limit of \(\sim\)50 MW at the swath edges, it is quite possible that SEVIRI errors of commission at extreme MODIS scan angles may not be false alarms, but rather true fires that MODIS failed to detect due to the increased pixel size at the MODIS scene edges.  
To report a more genuine estimate of SEVIRI's true active fire detection performance, sensor-to-sensor comparisons were limited to MODIS scan angles within \(\pm\)18.3 \({}^{\circ}\)of nadir (_i.e._, within \(\pm\)235 km of MODIS nadir). Within this central portion of the MODIS swath, where the MODIS active fire detection performance has been well characterised using co-incident ASTER observations [16,44,45,46], SEVIRI's omission and commission rates in ROIs 1, 2, 3, and 4 were recalculated as 34%, 52%, 70%, and 82%, and 4%, 11%, 14%, and 10%, respectively. If accumulated over all ROIs, then SEVIRI's overall omission and commission rates in the CAR evaluated near MODIS nadir were 59% and 9%,respectively, which agree quite well with previously reported monthly values ranging between 54%-57% and 6%-8%, respectively [33].