Abstract
The Sentinel-3 Mission Performance Centre (S3MPC) is tasked by the European Space Agency (ESA) to monitor the health of the Copernicus Sentinel-3 satellites and ensure a high data quality to the users. This paper deals exclusively with the effort devoted to the altimeter and microwave radiometer, both components of the Surface Topography Mission (STM). The altimeters on Sentinel-3A and -3B are the first to operate in delay-Doppler or SAR mode over all Earth surfaces, which enables better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking, whilst the radiometer is a two-channel nadir-viewing system. There are regular routine assessments of the instruments through investigation of telemetered housekeeping data, calibrations over selected sites and comparisons of geophysical retrievals with models, in situ data and other satellite systems. These are performed both to monitor the daily production, assessing the uncertainties and errors on the estimates, and also to characterize the long-term performance for climate science applications. This is critical because an undetected drift in performance could be misconstrued as a climate variation. As the data are used by the Copernicus Services (e.g., CMEMS, Global Land Monitoring Services) and by the research community over open ocean, coastal waters, sea ice, land ice, rivers and lakes, the validation activities encompass all these domains, with regular reports openly available. The S3MPC is also in charge of preparing improvements to the processing, and of the development and tuning of algorithms to improve their accuracy. This paper is thus the first refereed publication to bring together the analysis of SAR altimetry across all these different domains to highlight the benefits and existing challenges.
Highlights
This paper provides an overview of the activities of the Sentinel-3 Mission Performance Centre (S3MPC) in relation to the Surface Topography Mission, which focus on the operation of the Sentinel Radar Altimeter (SRAL) and Microwave Radiometer (MWR) and of the products derived from their data
The validation is based on the Near Real-Time (NRT) operational data from both Sentinel-3A and -3B, using the Surface Topography Mission (STM) product distributed by EUMETSAT in netCDF through their Online Data Access (ODA) system. [For consistency with other meteorological data, validation analysis will be converted to work with the formal BUFR (Binary Universal Form for the Representation of meteorological data) format whenever that becomes available.] The raw data product is collected for 6-hourly time windows centred at synoptic times (00, 06, 12 and 18 UTC)
We only present the regional sea surface height (SSH) bias estimates computed in Senetosa (the analysis of the results in Ajaccio has shown some discrepancies between the high-frequency signals that are observed by the tide gauge and the signals that are provided by the models, which require further investigation)
Summary
The Sentinel-3 satellites in particular provide multi-sensor observations of the Earth’s surface, with sensors using the visible, infra-red and microwave bands [1] (see Figure 1a). In terms of their sensor payload, the Sentinel-3 satellites are successors to ERS-1, ERS-2 and Envisat, but with advances in the capabilities of all the instruments. On 16th Oct. 2018 its orbit was changed, with the ground track drifting until 23rd Nov. 2018 when it was placed into the “interleaved” orbit This meant it measured along a ground track exactly halfway between two neighbouring S3A tracks and 10 days later. This had been determined as the best arrangement for enabling synoptic mapping using two almost identical instruments
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