# Automatic Detection of Marine Litter: A General Framework to Leverage Synthetic Data
## 2 Materials and Methods
### Case Study on Sentinel-2
#### 2.2.4 Aliasing Artifacts in Sentinel-2 Data  
Beyond illustrating the sampling process, Figure 3 also showcases the reproduction of the aliasing effect occurring in the S2 sampling process. Within the same cell, several jitters can be chosen (100 possible jitters for a band with GSD = 10 m). Depending on the jitter, the final value is not the same. In Figure 3b,d, it is possible to see that the final result will be different if the jitter is 5, 5 or 1, 1. Therefore, depending on the jitter, the value can be drastically different. This is how the loss of information or artifacts occur in the S2 sampling process. By generating different scenes in different conditions, we can investigate the range of sizes sensitive to aliasing. To assess the effect of aliasing over one scene, it is necessary to observe the results of the sampling with all the possible jitters and to see how the reflectance values are affected. For a band with a GSD of 10 m, all of the 100 jitters are used for the sampling; for a band with a GSD of 20 m, all of the 400 jitters are used and for a band with GSD of 60 m, all of the 3600 jitters are used. In order to collect the result of all of the jitters and to compare it, the scene described in Section 2.2.3. (patch size of 30 m \(\times\) 30 m) has been sampled as many times as needed by going through all the jitters and without applying noise as the focus is put on the sampling process. For each sampling, only the value of the pixel with the highest  
Figure 5: Simulated images of a 30 m \(\times\) 30 m plastic patch made out of wet bottles frame for B01 and B02 only; with and without noise. Figure 6: In orange, the spectral signature extracted from a 28 m diameter target deployed for the PLP21. Image filename: S2A_MSIL2A_20210611T085601_N0300_R007_T35 SMD_20210611T121904. In blue, the spectral signature of simulated plastic with the inputs mentioned in Section 2.2.3. The fraction of plastic was set to 0.6 to match the fraction of plastic of the HDPE mesh. reflectance value was collected (recall that the seawater has a minimum reflectance). All these values have been displayed in boxplots, one boxplot for each band, and are presented in Figure 7. The blue line corresponds to the spectral signature of wet bottles frame [25] after the convolution with the PSFs; the closer the reflectance values are from this line, the more accurate the sampling of the satellite is. The dark line corresponds to the seawater spectral signature from [35]. For bands B02, B03, B04, and B08 (GSD = 10 m), the boxplots are centered around the same reflectance value and are close to the spectral signature of plastic (blue line). This means that for the particular size 30 m \(\times\) 30 m, the data collected by S2 are not subject to aliasing artifacts. The boxplots of bands B01, B09, and B10 are more spread meaning that the sampling of the scene depends on the jitter and thus that these bands are sensitive to the aliasing effect. As for the bands with a GSD of 20 m, the boxplots are spread but the values meet with the spectral signature of plastic. All in all, based on the simulations, a plastic patch of size 30 m \(\times\) 30 m can be accurately detected by S2 for the bands with a GSD of 10 m. The results of the other bands are less exploitable. As a comparison, Figure 8 shows the same plots but with a plastic patch of size 10 m \(\times\) 10 m. This time, the blue line does not meet any of the boxplots and the boxes for the bands of 10 m resolution have spread values. The boxplots of bands B01, B09 and B10 (GSD = 60 m) seem to be centered around the same value but this is because no plastic is detected at all. A 10 m \(\times\) 10 m plastic patch made out of plastic bottles such as the one deployed by [10] is strongly affected by aliasing. Though small plastic patches such as a 10 m \(\times\) 10 m patch are observable from S2, the reflectance values are not reliable because of aliasing. By comparing the results from  
Figure 8: Results of all the possible jitters for each band by selecting the maximum reflectance values.