Performance Of The Seawifsvi

SeaWiFS was launched on the SeaStar spacecraft on August 1, 1997. Since mid-September, 1997, it delivers multispectral BRF values collected over all regions of the globe. The wide geographical and long temporal availability of SeaWiFS data enables the verification of 1) the robustness of the

SeaWiFS-VI with respect to large variations in the observation zenith angles and/or rapid changes in atmospheric conditions and 2) the capacity of the index to relate to well-identified ecological patterns.

The polar orbit of SeaWiFS instrument, combined with its wide swath width, permits the observation of sites close enough to the poles more than once per day. For these locations, it is thus possible to compare the original measurements and the derived products from two consecutive orbits, i.e., at about 100 minutes interval. In this period, it is reasonable to expect that the surface has remained essentially the same. Some changes may result from slightly different atmospheric conditions, but the bulk of observed changes must result from variations in the conditions of observation, as the same region is observed from eastward and westward directions (see Figure 3).

Figure 3. Illustration of the trace of two successive orbits of SeaWiFS over Western Europe, for August 7, 1998, at 11:06 and 12:42 UT

For the purpose of this evaluation, we selected data from two consecutive relatively cloud-free SeaWiFS orbits over Northern Europe, acquired on August 7, 1998, at 11:06 and 12:42 UT, respectively. The SeaWiFS-VI values obtained through the procedure described above are displayed in Figure 4 for these two consecutive orbits. The superimposed ellipse on both images delineates the geographical region located approximately between latitude 46° N and 52° N, and longitude 11° E and 15° E, for which further tests are conducted.

Figure 5 (right panel) shows the variations of the SeaWiFS-VI along a particular transect across the mapped data sets for the two consecutive orbits, where the full (dashed) line corresponds to eastward (westward) observation conditions, respectively. Some changes in atmospheric conditions may have occurred in the time period between the two observations, but no significant modifications of the surface properties are expected.

Figure 4. Maps of the SeaWiFS-VI estimated for two successive orbits at 11:06 and 12:42 UT, respectively, of SeaWiFS over Western Europe on August 7, 1998. The left (right) image corresponds to an eastward (westward) observation of the region surrounded by an ellipse

Figure 5. Transects of index values extracted from the region identified by the ellipse in Figure 4. The full (dashed) line refers to observations taken at 11:06 (12:42) UT, respectively. The left (right) panel exhibits the SeaWiFS-VI (NDVI) values estimated from data acquired by SeaWiFS for these two successive orbits on August 7, 1998 on the same transect

Figure 5. Transects of index values extracted from the region identified by the ellipse in Figure 4. The full (dashed) line refers to observations taken at 11:06 (12:42) UT, respectively. The left (right) panel exhibits the SeaWiFS-VI (NDVI) values estimated from data acquired by SeaWiFS for these two successive orbits on August 7, 1998 on the same transect

It can readily be seen that the two SeaWiFS-VI profiles better overlay each other than the corresponding NDVI profiles. Hence, the rectification procedure described earlier has effectively reduced the sensitivity of the optimal index with respect to changes in observation geometry.

The mathematical explanation for these substantial differences in index behavior can be seen in Figure 6, which illustrates the displacements in the spectral space of the data points responsible for an NDVI change of 0.15 between measurements taken from the two successive orbits. The left (right) panel locates these points in the rectified RED-NIR (classical RED-NIR) spectral space and shows the vectors describing the spectral BRF changes during this period. As can be seen, the displacement vectors in spectral space between consecutive orbits occur at significant angles with respect to the NDVI isolines in the original (RED, NIR) space, and are much more parallel to the SeaWiFS-VI isolines in the rectified (RED, NIR) space. This example graphically explains the consistency of the results provided by the SeaWiFS-VI when changing the observation geometry and possibly the atmospheric conditions, and demonstrates the superior performance of SeaWiFS-VI compared to classical indices such as NDVI.

Figure 6. Displacements vectors for the data points corresponding to a change in NDVI equal to 0.15 between the two transects shown in Figure 5, acquired at 11:06 and 12:42 UT, respectively. The (left) right panel displays the displacement of the points into the (rectified) red/near-infrared spectral space. Points leading to an increase (decrease) in NDVI values from 11:06 and 12:42 UT are identified in light gray (dark) arrow

Figure 6. Displacements vectors for the data points corresponding to a change in NDVI equal to 0.15 between the two transects shown in Figure 5, acquired at 11:06 and 12:42 UT, respectively. The (left) right panel displays the displacement of the points into the (rectified) red/near-infrared spectral space. Points leading to an increase (decrease) in NDVI values from 11:06 and 12:42 UT are identified in light gray (dark) arrow

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