## 2 Materials and Methods
### Satellite Data  
Apart from the collected in situ data, this study employs satellite imagery sourced from the Sentinel-2 constellation, comprising two Earth observation satellites, namely Sentinel-2A and Sentinel-2B, launched in 2015 and 2017, respectively [30]. Both of these satellites orbit the Earth in a sun-synchronous manner and collectively offer a five-day revisit rate over the equator, ensuring data is acquired under consistent viewing circumstances. Additionally, the data is captured at a high spatial resolution and a broad field of view suitable for multispectral observation. Equipped with a Multispectral Instrument (MSI), each satellite can capture reflected radiance across 13 spectral bands ranging from visible to shortwave infrared wavelengths (Table 1). This extensive spectral range combined with high spatial resolution enables comprehensive analysis of land cover, vegetation health, water quality, and other factors. Moreover, the MSI features a 12-bit radiometric resolution, enhancing the capacity of the satellite to distinguish between the intensity or reflectance of the diverse structures being mapped. The Sentinel-2 images were acquired from the ESA's Copernicus Sentinel Scientific Data Hub, which offers comprehensive, unrestricted and openly accessible Sentinel user product data. Figure 4: Satellite and in situ data used for the training and testing stages of SSS and SST for 2024. For the first mission, satellite images captured on 23 August 2021 were used, and images from 23 November 2021 were selected for the second mission. For the float mission, a satellite image captured on 25 June 2024 was utilised. The first two dates were chosen because they correspond to the midpoint of their respective missions, whereas the latter date was selected as it was the closest available image to the in situ measurements. These images were also chosen as they exhibited minimal cloud coverage and sun glint. The 13 spectral bands were extracted from each image. For the first two missions, two additional images were sourced from the Copernicus Sentinel Scientific Data Hub, serving as test sets for predicting SSS and SST for 2022 and 2023 within the designated region and timeframe. These encompassed an image captured on 24 October 2022 and another on 9 October 2023. Furthermore, the satellite image taken on 25 June 2024 was also used for the 2024 test set. Once more, the 13 bands were extracted from these images, and the predictions were based on the in situ measurements obtained in 2021 for the first two years and 2024 for the last year (Table 2). In the analysis of SST, only the spectral bands were employed. Conversely, for the SSS analysis, SST satellite data was incorporated in the training phase, given the strong dependence of SSS readings on SST measurements. The SST satellite data corresponding to the aforementioned dates was retrieved from the Copernicus Marine Data Store (Copernicus Marine Data Store website: [[https://data.marine.copernicus.eu/products](https://data.marine.copernicus.eu/products)]([https://data.marine.copernicus.eu/products](https://data.marine.copernicus.eu/products)), accessed on 13 August 2024), specifically from the Mediterranean Sea High Resolution and Ultra High \begin{table}
\begin{tabular}{c l c c} \hline \hline
**Band** & **Description** & **Central Wavelength (nm)** & **Resolution (m)** \\ \hline B1 & Ultra Blue & 443 & 60 \\ B2 & Blue & 490 & 10 \\ B3 & Green & 560 & 10 \\ B4 & Red & 665 & 10 \\ B5 & Visible and Near-Infrared & 705 & 20 \\ B6 & Visible and Near-Infrared & 740 & 20 \\ B7 & Visible and Near-Infrared & 783 & 20 \\ B8 & Visible and Near-Infrared & 842 & 10 \\ B8a & Visible and Near-Infrared & 865 & 20 \\ B9 & Short Wave Infrared & 940 & 60 \\ B10 & Short Wave Infrared & 1375 & 60 \\ B11 & Short Wave Infrared & 1610 & 20 \\ B12 & Short Wave Infrared & 2190 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Spatial and spectral characteristics of the Sentinel-2 satellites [31].