Abstract
Copernicus Sentinel-3 Surface Topography Mission embarks a two-channel microwave radiometer combined with 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 in-flight calibration of a single instrument is the critical point to achieve the expected performances. In the context of a constellation, the inter-calibration is even more important. After a presentation of the instrument design, we present the diagnoses used for the calibration of Sentinel-3A, using vicarious calibration over specific areas and double difference methods. The inter-calibration of Sentinel-3B with Sentinel-3A is performed during the tandem phase, using the residual differences of co-located measurements. Finally performances are assessed at crossover points with two parameters, first the wet troposphere correction by comparison with Jason-3; secondly on the Sea Surface Height by difference of variance. Analysis results have shown that Sentinel-3A is well calibrated, consistent with other instruments, and that Sentinel-3B is calibrated within 0.4 K with Sentinel-3A as a reference. Performances and stability fulfill the requirements for both missions.
Highlights
Microwave Radiometers on-board satellite topography missions are combined with the altimeter in order to correct the altimeter range for the excess path delay resulting from the presence of water vapour in the troposphere
When the antenna temperature is higher than the reference temperature, the Microwave Radiometer (MWR) is operating in a conventional Dicke mode: no noise is injected to the antenna path
Ghz single difference (f) CLWC channel single difference. These results show the in-flight calibration is successful and that S3A brightness temperatures are consistent with other instruments, according to the diagnoses used in this study
Summary
Microwave Radiometers (hereafter MWR) on-board satellite topography missions are combined with 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 environment of the antenna is different on-ground than it will be in-flight and the range of brightness temperatures is not fully representative as the coldest test temperature cannot reach the cold sky temperature All these reasons explain why on-ground calibration is required but not sufficient and a careful assessment should be performed after launch. We followed a strategy similar to the one used for European Remote-Sensing Satellite (ERS), Envisat and AltiKa [4] for Sentinel-3A in-flight calibration It is based on several analyses applied to several missions and covering the full range of brightness temperatures:. In a study on the detection of MWR calibration drifts, Ruf [5] demonstrated that the coldest ocean temperatures can be observed under low wind, cloud free and low humidity conditions These points can be used to detect instrumental drifts. Geophysical performances of MWR corrections with respect to the model are assessed by computing the difference of variance of Sea Surface Height (SSH) at crossover points
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