Validation of Sentinel-3 OLCI Integrated Water Vapor Products Using Regional GNSS Measurements in Crete, Greece

George Petrakis,Costas Kokolakis,Xenophon Frantzis,Panagiotis Partsinevelos,Stelios Mertikas,Achilleas Tripolitsiotis

doi:10.3390/rs12162606

George Petrakis, Costas Kokolakis + Show 4 more

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https://doi.org/10.3390/rs12162606

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Abstract

Water vapor is one of the essential variables in monitoring the Earth’s climate. The Ocean and Land Color Instrument (OLCI) on-board the Copernicus Sentinel-3 missions measures the Integrated Water Vapor (IWV) column over land and ocean surfaces. Post-launch calibration and validation of satellite measurements constitutes a key process in the operational phase of Earth observation satellites. This work presents the external and independent validation of OLCI-A IWV product using the regional network of continuously operating Global Navigation Satellite System (GNSS) comprised 10 stations distributed over the island of Crete in the eastern Mediterranean. The Sentinel-3A/-3B OLCI imagery that captures in a single scene the entire area of Crete has been examined. For each OLCI image, the IWV value of cloud-free pixels containing the GNSS stations have been derived and compared against simultaneous GNSS-derived measurements. The absolute as well as the relative bias between OLCI-A and OLCI-B IWV measurements have been determined. There is a good agreement between OLCI and GNSS with a bias of −0.57 mm ± 2.90 mm for OLCI(A) and +2.42 ± 3.41 mm for OLCI(B). The results of this regional validation activity are compared against other studies and the regular validation carried out at the Sentinel-3 Mission Performance Center. This work concludes that the accuracy of the OLCI IWV products is within its design requirements. The potential synergy between Sentinel-2 and Sentinel-3 IWV products is also discussed.

Highlights

Water vapor is one of the essential climate variables [1] which seems to have a strong impact on the Earth’s hydrological cycle and correlates to the planet’s energy balance
This work presents the external and independent validation of Ocean and Land Color Instrument (OLCI)-A Integrated Water Vapor (IWV) product using the regional network of continuously operating Global Navigation Satellite System (GNSS) comprised 10 stations distributed over the island of Crete in the eastern Mediterranean
This work attempts to validate the integrated water vapor observed by OLCI in Sentinel-3 against GNSS measurements conducted through a permanent network

Summary

Introduction

Water vapor is one of the essential climate variables [1] which seems to have a strong impact on the Earth’s hydrological cycle (evaporation, condensation and precipitation) and correlates to the planet’s energy balance. The rise of the Earth’s temperature increases evaporation from ocean and inland waters and subsequently builds up water vapor in the atmosphere. Active microwave sensors, such as satellite altimeters, require corrections to be applied to their range observations to compensate the signal propagation delay in the atmosphere and especially their wet troposphere component. Each Copernicus Sentinel-3 mission carries a dual-frequency microwave radiometer (MWR) to determine those wet-tropospheric corrections for the operating synthetic aperture radar altimeter (SRAL); a primary payload of the Sentinel-3A/-3B [11]. These satellite radiometers operate well over the ocean, they do not function properly over land masses. Their proper operation is restricted to sea and water regions farther than about 20–25 km from the coasts [12]

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