# Improving the RST-OIL Algorithm for Oil Spill Detection under Severe Sun Glint Conditions
## 3 Results
### Results of the Advanced RST-OIL Configuration  
According to the advanced RST-OIL configuration proposed in this study (Section 2.3, Equation (2)), we found that the Aqua image (Figure 4a) was completely acquired under high glint conditions while the Terra one (Figure 4b) showed dual behavior with a zone affected by high glint (top left) and the remaining portion was under glint conditions. The new proposed approach (Equation (3)) was implemented on both previously analyzed images (Figures 7 and 8). The results are shown in Figures 9 and 10. The improvements of the proposed configuration in terms of reliability are evident. In both images, the \(\otimes_{\mathbb{R}^{0}\cdot 0.8}\) values measured in not oil-contaminated areas are always below 2 sigma, which indicates their acquisition in \"normal\" conditions (i.e., homogeneous with those represented by the reference fields). The comparison between an image acquired under glint (or high glint) conditions and the corresponding reference fields enables discrimination between the (absolute) high reflectance values due to sun-satellite geometry and the (relative) increase only ascribable to the oil presence. Results obtained by implementing the new RST-OIL configuration on all the MODIS images available during the considered period (April-May 2010) and acquired under sun glint conditions are shown in Figure 11 together with the corresponding RGB imagery. In all figures, the oil-affected areas are identified and very well isolated with respect to the clean seawater areas. Values of anomalous areas are always statistically significant (ranging from \(\sim\)2 to \(\sim\)8) and consistent when moving from one image to another, which allows for automatic and unsupervised identification as well as effective monitoring of the evolution, in terms of the intensity and extent of the spilled areas. Moreover, false positives do not seem to significantly affect the scenes analyzed in this case, which confirms the good reliability of the improved method, even under these severe sun glint conditions. On the other hand, some limits appear evident: (i) the cloud coverage is likely over-estimated and (ii) the sensitivity of the method is sometimes lower than expected. Regarding the cloud mask, it is worth mentioning that, in this work, only the TIR channel has been used to implement the OCA approach (Section 2.2), where  
Figure 9: (a) ALICE\({}_{\mathbb{R}^{0}\cdot 0.8}\) (\(\otimes_{\mathbb{R}^{0}\cdot 0.8}\)) map relative to the MODIS Aqua image of Figure 4a and (b) the relative trend along the transect. it is generally applied by combining TIR and VIS information to obtain more accurate results (some clouds may, in fact, show signatures only in one spectral region). Unfortunately, in the VIS region, clouds and oil often exhibit similar spectral behavior and, therefore, it was not possible to use the VIS band in this scenario to discriminate among clouds. Therefore, the obtained cloud mask may suffer from a lack of accuracy. Concerning the RST-OIL sensitivity, the underestimation of oil slicks coming from a visual comparison between ALICE\({}_{\texttt{R}^{\texttt{T}}0.8}\) maps and corresponding RGB imagery may be a direct consequence of the previously mentioned cloud mask inaccuracy, since the spilled areas might be simply covered by the (supposed) cloudy radiances. In addition, the oil status, type, and thickness may further affect the spectral behavior and, consequently, the RST-OIL capability in correctly detecting spilled zones. However, it should be stressed in this study that the main aim of the algorithm is not providing a detailed mapping of the spill but promptly detecting the possible oil presence on the scene in order to give a timely alert. In this sense, even a single oil pixel detection represents a good result. In the results shown above, we focused only on oil characterized by a positive contrast compared with surrounding waters. As discussed in Section 3.1, such a condition represents a critical situation for the standard RST configuration.