Sentinel-3A Early Payload Assessments

The Sentinel-3 mission is devoted to operational oceanography and global land applications, thanks to its payload composed of a set of optical and microwave instruments to measure variables such as sea-surface topography, sea and land-surface temperature, ocean colour and land colour with high-end accuracy and reliability. Especially for land and costal applications, the geometric performance of the optical instruments OLCI and SLSTR in terms of absolute geolocation and registration plays a crucial role for the development of successful retrieval services. For this reason this paper aims at explaining the approach, the tools and the first results obtained after few months of Sentinel-3 commissioning.

The main driver for the overall geometric performance is the accuracy of the satellite Geometric Model (GM). In simple words, the GM expresses mathematically the position and the orientation of OLCI and SLSTR, including the line of sight of their detectors from which the image is reconstructed. The GM is estimated on ground by the satellite prime contractor but it needs to be characterized and verified again after launch.

The major tools is the S3Geocal, which has been procured by the Sentinel-3 project team specifically to assess and calibrate the GM using OLCI and SLSTR L1b data. Particularly important is also the L1c product, which is the synergy between OLCI and SLSTR. Processing and analyzing this product, the S3 commissioning team is able to assess the registration capability between OLCI and SLSTR.

Together with the Geocal, the S3 toolbox is used because it offers a collection of tools for a quick but effective analysis of pixel measurements and metadata before the more complex processing is initiated in the Geocal.

Finally some preliminary results are showed and discussed.

The Sentinel-3 Mission (S-3) is part of the Global Monitoring for Environment and Security (GMES/Copernicus) European initiative. For oceanic applications, the S-3 mission will deliver continuity to existing ESA ERS, Envisat and CryoSat missions. The S-3 mission will provide ocean/land colour data, sea/land surface temperatures and sea surface and land ice topography at least at the performance level of corresponding Envisat instruments, the Medium Resolution Imaging Spectrometer (MERIS), the Advanced Along-Track Scanning Radiometer (AATSR) and the Envisat Radar Altimeter (RA). The topography payload consists in a Doppler altimeter SRAL, a microwave radiometer MWR, and 3 instruments for precise orbit computation purposes (GNSS, DORIS and laser reflector). The Sentinel-3 satellites (A and B) will fly on a new orbit with a 27-days cycle.

The SRAL instrument is a nadir Ku/C altimeter, with two operating modes: the low resolution mode (LRM), used on all past oceanographic missions and the high resolution mode (SAR) used for the first time on CryoSat. The SAR mode (also called Delay Doppler) provides a better along track resolution (around 320m) with respect to the LRM mode.

The Sentinel-3A satellite will be launched in December 2015, and CNES (with the support of CLS) will provide a strong support to ESTEC project for the topography calibration and validation during the commissioning phase. In this paper, we will present a first data quality assessment performed on CNES and CLS side, focusing on deep ocean and using classical CalVal analysis tools. This will include mono mission Cross Over analysis, Sea level residuals but also comparison to the Jason reference mission and the SARAL/AltiKa mission. A specific focus will be given on the spectral analysis and a comparison to others missions will be presented.

The Sea and Land Surface Temperature Radiometer on Sentinel-3A underwent an extensive pre-flight calibration campaign that was completed a few months before the launch date of late 2015.  The calibration and characterization was to ensure that all the necessary calibration parameters are measured before launch, but more importantly that the end-to-end system performance and calibration model is fully verified against traceable calibration sources before launch. As an infrared sensor, this is essential for understanding all sources of uncertainty that affect the instrument calibration which cannot be directly measured once on-orbit. 

In this paper we present the results of measurement of the spectral responses of all channels, the optical alignment, the vis-swir channel radiometric calibration and the TIR radiometric calibration. 

The Sentinel-3 SRAL/MWR surface topography mission will serve primarily the marine operational users but will also allow the monitoring of sea ice and land ice, as well as inland water surfaces, using novel observation techniques.
A two-channels microwave radiometer (23.8 and 36.5 GHz) is combined to the altimeter in order to correct the altimeter range for the excess path delay resulting from the presence of water vapour in the troposphere. The radiometer will perform measurements of brightness temperatures in both bands interpolated at the location of the altimeter footprint. The wet tropospheric correction is retrieved from both brightness temperatures and altimeter backscattering coefficient to take into account the surface roughness using an empirical method based on simulated parameters (brightness temperatures and altimeter backscattering ratio) and neural networks. Another algorithm is implemented for the retrieval of the wet tropospheric correction with additional inputs (sea surface temperature and temperature lapse rate) to improve the retrieval over specific areas such as upwelling regions. For the retrieval of the other microwave radiometer parameters (water vapor content, cloud liquid water content, the atmospheric attenuation of the altimeter backscatter coefficient in Ku band), a three input algorithm is implemented.
We will present here a first assessment of the MWR performances over ocean as well as coastal areas, sea ice and inland water.
The in-flight calibration during the commissioning phase aims at providing quantitative information on the accuracy and the precision of their measurements. On the long-term, it will be used to assess the stability of the S-3 MWR instrument.
However, the main difficulty for microwave radiometry lies in the lack of references: natural targets are neither well known nor homogeneous enough and each in-flight instrument has its own calibration strategy. We have therefore based the calibration of AltiKa radiometer on a combination of comparisons to other instruments (AMR on Jason-2, AMSU-A on MetopA, AltiKa/MWR on SARAL, AMR on Jason3) directly over Amazonian forest. Over ocean, a statistical study of the coldest ocean points is performed as well as double-difference analysis using simulations as a common reference (using ECMWF analyses and UCL radiative transfer model) to compare Sentinel-3A radiometer to other instruments.

The Ocean and Land Colour Imager (OLCI) is one of the key instruments on-board the Sentinel-3 mission. The mission consists of four satellites each having a design lifetime of 7.5 years starting with Sentinel-3A. The Sentinel-3B satellite will be launched 18 month later and the two Sentinel-3 C and D satellites are in preparation as replacement units for the A and B satellites. The Ocean and Land Colour Imagers (OLCI) on-board each of the satellites are high accuracy visible near-infrared pushbroom imaging spectrometer designed to provide continuity of land and ocean colour data following ENVISAT-MERIS. In the following the OLCI design and the instrument characteristics are described. Then, an overview about the characterization and calibration activities is given. Finally, first results from the commissioning phase of OLCI-A are concluding the paper.