### SEVIRI Errors of Omission and Commission  
The performance of the FTA detection algorithm is evaluated in each ROI by quantifying SEVIRI's errors of omission and commission relative to MODIS. All MODIS active fire pixels (regardless of their detection confidence) were assigned to an individual SEVIRI image coordinate (_i.e._, column and row number) depending on whether the centre latitude and longitude of a MODIS active fire pixel fell within the ground footprint of a SEVIRI thermal pixel. SEVIRI's pixel boundaries were simply drawn at a distance halfway between adjacent SEVIRI pixel centres (Figure 4). For every concurrent observation, a record was kept of whether SEVIRI or MODIS detected a thermal anomaly at each SEVIRI image coordinate. Since the GSD of SEVIRI is coarser than MODIS at nadir, it is possible for a SEVIRI thermal pixel to contain several MODIS active fire pixels for any concurrent observation. All information about MODIS active fire pixels is preserved at each SEVIRI image coordinate. Despite its simplicity, this remapping technique remains unsatisfying due to the shape, orientation, and overlap of the IGFBP and point spread function (PSF) of both sensors. In reality, SEVIRI's detectors are diamond shaped, oriented at 45 \({}^{\circ}\) with respect to the E-W and N-S directions, and the PSF of SEVIRI at the SSP extends \(\sim\)5 km from the pixel centre [60]. Hence SEVIRI is theoretically sensitive to thermal anomalies located up to 5 km from the nominal pixel centre [60], and SEVIRI's active fire detection pattern may resemble a plus sign or a square ring in image space [60]. Meanwhile the focal plane layout and scanning design of MODIS induces (i) a 2 km triangular PSF that overlaps in the along-scan direction; and (ii) substantial scan-to-scan overlap at scan angles greater than 24 \({}^{\circ}\)[42]. Consequently it is possible for MODIS to detect the same fire in adjacent along-scan pixels [44] as well as in consecutive scans [42,57]. Figure 4: SEVIRI pixel boundaries (solid and dashed lines) are drawn at a distance halfway between adjacent pixel centres (filled circles). These artificial boundaries do not coincide with the orientation and overlap of SEVIRI’s point spread function (PSF), which is shown for the centre pixel located at the sub-satellite point. To mitigate these caveats, a 3 \(\times\) 3 window is inspected around the target SEVIRI pixel, and SEVIRI's detection performance is evaluated based on the MODIS active fire pixels found within this 3 \(\times\) 3 window (Figures 4 and 5). In addition, the status flags of the centre (target) SEVIRI pixel and the eight surrounding SEVIRI pixels are retrieved from the FRP-PIXEL quality file (Figure 5). Inspecting this 3 \(\times\) 3 window crudely--but conveniently--accounts for uncertainties in (i) the geometry and overlap of the PSF of SEVIRI and MODIS; and (ii) the geolocational accuracy of both active fire products. Figure 5: Illustration of the technique used to compare simultaneous SEVIRI and MODIS observations of the same fire activity within ROI 2 (Figure 1a) on 18 November 2008 at 09:30 UTC (shown in the native SEVIRI image coordinate system). For each SEVIRI image coordinate that contains a SEVIRI or a MODIS active fire pixel, a 3 \(\times\) 3 window is centred on the target pixel and the adjacent SEVIRI pixels are inspected to evaluate SEVIRI’s detection performance. In addition, the status flags of the SEVIRI pixels within the 3 \(\times\) 3 window are retrieved from the FRP-PIXEL quality file and are used to explain SEVIRI’s active fire errors of omission. Label (**A**) represents a confirmed SEVIRI active fire pixel (_i.e._, no omission or commission error). Label (**B**) highlights a SEVIRI error of omission surrounded only by cloud-free land pixels that are not potential fire pixels. Label (**C**) also indicates a SEVIRI error of omission, but here the error is attributed to the presence of cloud pixels. Label (**D**) is an example of two SEVIRI errors of commission.