Sentinel-3 OLCI Technical Guide - Validation

Radiometric Validation

The pseudo-invariant calibration sites (PICS) as deserts have been exploited for decades for multi-temporal monitoring and cross-calibration in the solar spectral range. The PICS algorithm aims to simulate the TOA reflectance in the visible to near-infrared (NIR) spectral range of the sensor under test (e.g. OLCI) over a predefined desert site (e.g. CEOS PICS sites). This method makes it possible to perform multi-temporal analysis, as well as a comparison of multiple sensors on the same site over the same or different observation periods.

The PICS method supported by DIMITRI has been successfully performed. The figure below displays time-series of the elementary ratio (estimated gain coefficients) -defined as observed reflectance to the simulated one- over all the PICS calval sites. First, a clear consistency over all the 6 PICS could be seen. The averaged ratios over the time-series –depicted in the top of each plot- show bias values within <4% and uncertainty about ±2.5% for almost all the spectral range of OLCI from Oa01-Oa19 even some bands display about 2% bais (e.g. Oa02, Oa03, Oa05, Oa08 and and Oa17; Figure below). Note that the bias is strongly impacted by the high ratios observed during the first 6 months of the mission.

Sentinel-3A OLCI L1 Val PICS
Sentinel-3B OLCI L1 Val PICS

Time-series of the elementary ratios (observed/simulated) signal from Sentinel-3 OLCI for (top) band Oa8 and (bottom) band Oa17 over six PICS Cal/Val sites. Dashed-green and orange lines indicate the 2% and 5% bias respectively. Error bars indicate the elementary uncertainty of PICS-methodology.

Geometric Validation

Regular monitoring of the geolocation performances is routinely performed with the GeoCal tool, on both NRT and NTC products.

Completion of the time series (started using the partial reprocessing dedicated to validation: 4 days every month between 26/04/2016 and 12/03/2017) and continued afterwards is displayed in the following figure and shows a slight degradation.

OLCI geolocation errors

Time series of geolocation errors for the along-track (blue) and across-track (red) directions since May 2016

Per camera analysis has allowed to detect a significant drift of camera 3 in the along-track direction, yielding to non-compliance of that camera, as shown in the figure below. An updated set of Geometric Calibration Models trained on data from the second half of 2017 has been generated. After validation by the MPC, this has been deployed in the processing centres. The drift of camera 3 should no longer be observed.

OLCI geolocation errors

Residual geolocation errros time series from 04/09/2017 to 02/03/2018: across-track mean (upper) and along-track mean (lower). Camera 3 (violet) is clearly out of the general trend with a significant along-track bias.

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