# Exploring the Spectral Variability of Estonian Lakes Using Spaceborne Imaging Spectroscopy
## 2 Materials and Methods
### Satellite Data  
In this study, hyperspectral spaceborne images were obtained from two advanced spaceborne missions: PRISMA and EnMAP. These sensors provide contiguous spectral measurements in the visible to short-wave infrared range, offering detailed information on  
Figure 1: Study areas: Lake Vörtsjarv and Lake Peipsi. Blue dots represent the validation points; blue boxes represent the areas targeted for various applications with corresponding IDs.  
the optical properties of inland and coastal waters that are crucial for assessing water quality parameters such as Chl-a, TSM, PC, and CDOM. PRISMA--developed and operated by ASI--was launched in 2019 and delivers hyperspectral imagery with 231 spectral bands spanning the 400-2500 nm spectral range at a spatial resolution of 30 m [71; 72]. It is particularly valuable for environmental monitoring due to its ability to capture small variations in water color and biogeochemical processes [43]. EnMAP--launched in 2022 and operated by the German Aerospace Center (DLR)--provides 224 spectral bands across the 420-2450 nm range at a spatial resolution of 30 m. Designed for environmental and resource monitoring, EnMAP supports the detection of complex aquatic processes, including eutrophication and sediment transport, by capturing high-resolution spectral signatures [73]. The satellite also features a high signal-to-noise ratio and rigorous radiometric calibration, making it suitable for time-series analysis and comparative studies [74]. The combination of PRISMA and EnMAP data could enhance the ability to monitor inland waters at regional scales, enabling spatially comprehensive and temporally consistent assessments [75].  
As mentioned above, S2 and S3 multispectral data were used as a reference to validate the hyperspectral data, highlighting the advantages that can be gained by integrating the latter to obtain a more accurate characterization of water quality. Rrs data gathered from the S2 Multispectral Instrument were used, considering 8 spectral bands (443-842 nm) spatially resampled at 10 m. Rrs data provided by S3 Ocean and Land Colour Instrument were also used, considering 12 spectral bands (442-865 nm) with spatial resolution of 300 m. S2 and S3 multispectral images were acquired on the same day as the PRISMA and EnMAP overpasses, or within one day thereof. The dataset of the available hyperspectral spaceborne images used in this study is summarized in the following table (Table 1), which includes the application IDs and aims, the acquiring sensors, and the acquisition dates.  
Regarding Sentinel data, two S2 images (22 July 2024 for Application n.6 and 17 August 2022 for Application n.5, see Table 1) and four S3 images (19 September 2020, 8 June 2021 and 21 May 2022 for Application n.1 and 4 April 2020 for Application n.7) were exploited in this study.  
\begin{table}
\begin{tabular}{c c c c} \hline \multicolumn{4}{c}{**Lake Peipsi**} \\ \hline
**Appl. ID** & **Appl. goal** & **Sensor** & **Date(s)** \\ \hline
1 & Rrs validation & PRISMA & 19 September 2020, 8 June 2021, 18 July 2021, 21 May 2022 \\
2 & OWT classification & PRISMA & 19 September 2020 \\
3 & Velikaja River influence & PRISMA & 24 June 2020, 9 May 2021 \\
4 & Emajögi River influence & EnMAP & 16 May 2024 \\
5 & PC conc. map + S2 & PRISMA & 16 August 2022 \\ \hline \multicolumn{4}{c}{**Lake Vörtsjärv**} \\ \hline
**Appl. ID** & **Appl. goal** & **Sensor** & **Date(s)** \\ \hline
6 & Rrs validation & EnMAP & 20 June 2024, 21 July 2024 \\
7 & PC conc. map + S3 & PRISMA & 4 April 2020 \\
8 & Aquatic vegetation & EnMAP & 21 July 2024 \\
9 & Ice coverage & PRISMA & 18 March 2022 \\ \hline \end{tabular}
\end{table}
Table 1: Spaceborne data of Lake Peipsi and Lake Vörtsjärv: application ID, application goal, sensor, acquisition date(s).