Inland and coastal waters represent complex and variable ecosystems, which offer valuable habitats for a wide variety of aquatic species and are critical to preserve global biodiversity. Their quality and ecological status is a key worldwide issue because of the continuous anthropogenic pressures and environmental changes. The ability to observe waterbodies from space has led Earth Observation (EO) to establish as an important source of information on water quality and ecosystem condition, both for inventory and monitoring. In this context, the aim of this study was to explore the data gathered from PRISMA, the hyperspectral satellite sensor of the Italian Space Agency in orbit since March 2019. A total of nine imagery data, that cover a wide range of inland and coastal water sites distributed across the globe, have been examined during both the commissioning and the operational phase of the mission. This offered the opportunity to explore the effects of the vicarious calibration run, as well as the accuracy of satellite data at different level of processing (Level 1, Level 2). The analysis was achieved by using in-situ reflectance collected from both ad-hoc campaigns and autonomous radiometric stations (AERONET-OC, WISPStation); simulation and atmospheric correction were instead based on radiative transfer codes (6SV, ATCOR). The study was completed by comparing PRISMA data with synchronous measurements from Sentinel-2 and Sentinel-3 OLCI, considering their extensive use in water quality mapping. The ability of PRISMA to assess water quality and bio-geochemistry parameters was finally demonstrated in three case studies (lakes and lagoon) by running both a bio-optical model (BOMBER) parametrized with typical aquatic optics data of the investigated areas and a semi-empirical model. Although the level of noise in the first bands of PRISMA needs further analysis to be reduced, the study has shown promising results. The simulation of water reflectance at the Top of Atmosphere matched well the radiance spectra measured by PRISMA with an overall RMSE of 5-10% with lower performances in the blue region of the spectra. These values agreed also the measures achieved by Sentinel-2, while for Sentinel-3 a lower match was found. This was probably due to the different spatial resolutions between the two sensors. The comparison of Level 2 water reflectance products (distributed by ASI or computed with ATCOR) showed good agreement with in-situ measures for turbid waters and less good agreement for clear waters. Finally, the concentrations of Chlorophyll-a and of Total Suspended Matter obtained from PRISMA images were comparable with in-situ data (with values of MAE of the order of 6% for turbid water and 25% for blue waters). This study also revealed that the calibration activities during the commission improved the radiometry of PRISMA within the investigated VISNIR range for water targets. Although further studies are recommended to improve the qualification of PRISMA, this work has demonstrated that PRISMA data might offer complementary valuable observations to the existing Copernicus for water quality mapping.
Le acque interne e costiere rappresentano ecosistemi complessi e variabili, che offrono habitat preziosi per un'ampia varietà di specie acquatiche e sono fondamentali per preservare la biodiversità globale. La loro qualità e il loro stato ecologico sono una questione chiave a livello mondiale a causa delle continue pressioni antropiche e dei cambiamenti ambientali. La capacità di osservare i corpi idrici dallo spazio ha portato l'Earth Observation (EO) ad affermarsi come un'importante fonte di informazioni sulla qualità dell'acqua e sulle condizioni degli ecosistemi, sia per inventario che per il monitoraggio. In questo contesto, lo scopo di questo studio è stato quello di esplorare i dati raccolti da PRISMA, il sensore satellitare iperspettrale dell'Agenzia Spaziale Italiana in orbita da marzo 2019. Un totale di nove immagini, che coprono un'ampia gamma di siti di acque interne e costiere distribuite in tutto il mondo, sono state esaminate sia durante la fase di commissioning che durante la fase operativa della missione. Ciò ha offerto l'opportunità di esplorare gli effetti della calibrazione vicaria, così come l'accuratezza dei dati satellitari a diversi livelli di elaborazione (Livello 1, Livello 2). L'analisi è stata realizzata utilizzando la riflettanza in situ raccolta sia da campagne ad hoc che da stazioni radiometriche autonome (AERONET-OC, WISPStation); la simulazione e la correzione atmosferica si sono invece basate su codici di trasferimento radiativo (6SV, ATCOR). Lo studio è stato completato confrontando i dati PRISMA con le misurazioni sincrone di Sentinel-2 e Sentinel-3 OLCI, considerando il loro ampio utilizzo nella mappatura della qualità dell'acqua. La capacità di PRISMA di valutare la qualità dell'acqua e i parametri biogeochimici è stata infine dimostrata in tre casi studio (laghi e laguna) eseguendo sia un modello bio-ottico (BOMBER) parametrizzato con i dati tipici dell'ottica acquatica delle aree indagate, sia un modello semi-empirico. Sebbene il livello di rumore nelle prime bande di PRISMA necessiti di ulteriori analisi per essere ridotto, lo studio ha mostrato risultati promettenti. La simulazione della riflettanza dell'acqua nella parte superiore dell'atmosfera ha ben accoppiato gli spettri di radianza misurati da PRISMA con un RMSE complessivo del 5-10% con prestazioni inferiori nella regione blu degli spettri. Questi valori concordano anche con le misure di Sentinel-2, mentre per il Sentinel-3 è stata trovata una corrispondenza inferiore. Ciò è probabilmente dovuto alle diverse risoluzioni spaziali tra i due sensori. Il confronto dei prodotti di riflettanza dell'acqua di livello 2 (distribuiti dall'ASI o calcolati con ATCOR) ha mostrato un buon accordo con le misure in situ per le acque torbide e un accordo meno buono per le acque blu. Infine, le concentrazioni di Clorofilla-a e di Solidi Sospesi Totali ottenute dalle immagini di PRISMA erano comparabili con i dati in-situ (con valori di MAE dell'ordine del 6% per le acque torbide e del 25% per le acque blu). Questo studio ha anche rivelato che le attività di calibrazione durante la fase di commissioning hanno migliorato la radiometria di PRISMA all'interno della gamma VISNIR studiata per i target di acqua. Sebbene siano raccomandati ulteriori studi per migliorare la qualificazione del sensore PRISMA, questo lavoro ha dimostrato che i dati PRISMA potrebbero offrire osservazioni complementari di valore alla missione Copernicus esistente per la mappatura della qualità dell'acqua.
Analysis of hyperspectral Prisma data for the generation of water quality products
FABBRETTO, ALICE
2019/2020
Abstract
Inland and coastal waters represent complex and variable ecosystems, which offer valuable habitats for a wide variety of aquatic species and are critical to preserve global biodiversity. Their quality and ecological status is a key worldwide issue because of the continuous anthropogenic pressures and environmental changes. The ability to observe waterbodies from space has led Earth Observation (EO) to establish as an important source of information on water quality and ecosystem condition, both for inventory and monitoring. In this context, the aim of this study was to explore the data gathered from PRISMA, the hyperspectral satellite sensor of the Italian Space Agency in orbit since March 2019. A total of nine imagery data, that cover a wide range of inland and coastal water sites distributed across the globe, have been examined during both the commissioning and the operational phase of the mission. This offered the opportunity to explore the effects of the vicarious calibration run, as well as the accuracy of satellite data at different level of processing (Level 1, Level 2). The analysis was achieved by using in-situ reflectance collected from both ad-hoc campaigns and autonomous radiometric stations (AERONET-OC, WISPStation); simulation and atmospheric correction were instead based on radiative transfer codes (6SV, ATCOR). The study was completed by comparing PRISMA data with synchronous measurements from Sentinel-2 and Sentinel-3 OLCI, considering their extensive use in water quality mapping. The ability of PRISMA to assess water quality and bio-geochemistry parameters was finally demonstrated in three case studies (lakes and lagoon) by running both a bio-optical model (BOMBER) parametrized with typical aquatic optics data of the investigated areas and a semi-empirical model. Although the level of noise in the first bands of PRISMA needs further analysis to be reduced, the study has shown promising results. The simulation of water reflectance at the Top of Atmosphere matched well the radiance spectra measured by PRISMA with an overall RMSE of 5-10% with lower performances in the blue region of the spectra. These values agreed also the measures achieved by Sentinel-2, while for Sentinel-3 a lower match was found. This was probably due to the different spatial resolutions between the two sensors. The comparison of Level 2 water reflectance products (distributed by ASI or computed with ATCOR) showed good agreement with in-situ measures for turbid waters and less good agreement for clear waters. Finally, the concentrations of Chlorophyll-a and of Total Suspended Matter obtained from PRISMA images were comparable with in-situ data (with values of MAE of the order of 6% for turbid water and 25% for blue waters). This study also revealed that the calibration activities during the commission improved the radiometry of PRISMA within the investigated VISNIR range for water targets. Although further studies are recommended to improve the qualification of PRISMA, this work has demonstrated that PRISMA data might offer complementary valuable observations to the existing Copernicus for water quality mapping.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/164401