OLCI A/B Tandem Phase Analysis, Part 2: Benefits of Sensors Harmonisation for Level 2 Products

Abstract

This study is a follow-up of a full methodology for the homogenisation and harmonisation of the two Ocean and Land Colour Instrument (OLCI) payloads based on the tandem phase analysis. Sentinel-3B was manoeuvred into a tandem configuration with its operational twin Sentinel-3A already in orbit few weeks after its launch, which was followed by a short drift phase during which Sentinel-3B was progressively moved to a specific orbit phasing of 140° separation from the sentinel-3A. Harmonisation is performed at Level 1 for the radiometric alignment of the OLCI-A TOA radiances to the ones of OLCI-B, considering the slight spectral differences between the two instruments. The benefits of this harmonisation for the main Level 2 products are assessed in the present manuscript for both land and water products. The results validate such benefits showing accuracy between the two sensors after harmonisation better than the products requirements specifications. For the water processing branch, this accuracy opens a path toward an ensemble Sentinel-3 system vicarious calibration with ground-truth measurements. For land products, the tandem phase analysis is also an opportunity to demonstrate that the terrestrial chlorophyll index product requires improvements of the preliminary spectral adjustment of the red-edge channel at 709 nm. As comparisons from the measurements acquired over the tandem phase provides confidence in the alignment of the OLCI-A and OLCI-B series of products, preliminary analysis of the measurements acquired over the drift phase provides the first insights into the sensitivity of the processing algorithms to the geometry of acquisition as well as to calibration residuals of the OLCI field-of-view. As the harmonisation currently performs a radiometric alignment of OLCI-A to OLCI-B, the question of the reference sensor for operational implementation of the harmonisation raises concerns on the individual quality of the calibration of each sensor, notably their across-track consistency. Following the investigations performed at Level 1, where relatively strong calibration residuals are shown between the OLCI cameras and very similarly for both instruments; we discuss the impact of these residuals at L2 using an empirical correction and further conclude with the need to address these problematics with more attention in the future. We conclude with the extreme usefulness of the tandem phase analysis, presently for Level 2 products, and the need to further monitor the temporal stability of the cross-calibration of the OLCI payloads with a view to implementing their harmonisation at operational level.

OLCI A/B Tandem Phase Analysis, Part 3: Post-Tandem Monitoring of Cross-Calibration from Statistics of Deep Convective Clouds Observations

“…In the left-hand side, OLCI-A and OLCI-B reflectance indicators clearly exhibit three main characteristics: first, OLCI-A is clearly brighter than OLCI-B; second, the variability ACT exhibits an empirical BRDF highlighting the need to consider OLCI detector indices (i.e., viewing angles) as independently as possible; three, inter-camera differences show up notably with about 2% differences between camera 4 (detectors 2220 to 2959) and camera 5 (detectors 2960 to 3700) consistently with the observations made in [4,5], further providing evidence of inter-camera calibration differences for both OLCI sensors.…”

“…The left figure shows the absolute values while the right figure shows the difference to the cross-calibration factors of the tandem analysis. Finally, and complementary to the tandem analysis, this methodology allows to trace the ACT variations of an absolute calibration indicator, providing further evidence of calibration residuals between the OLCI cameras, both for OLCI-A and for OLCI-B, as suggested from the analyses at L1 [4] and at L2 [5].…”

“…Validation of this harmonisation has been conducted at Level 1 [4] and its full benefits assessed at Levels 2 and 3 for both Land and Water products [5,6]. Among the output of these analyses, the five OLCI cameras have shown residual calibration differences which can be as high as 2%, remarkably consistently between the two payloads.…”

Evaluation of the CDOM Absorption Coefficient in the Arctic Seas Based on Sentinel-3 OLCI Data

“…We mentioned in the Introduction several works dealing with the uncertainties in OLCI data [24][25][26][27][28][29]. The study [24] is closest to ours regarding approach, research content, and geographically, which combined broad field measurements in the eastern part of the Fram Strait and a transect between northern Norway and southern Spitsbergen in July 2013-2015 with MODIS-Aqua satellite observations, modeling OLCI spectral values using the obtained in situ data.…”

“…ground-truth measurements could solve the problem. The benefit of retrieving the Level 2 data products from the Sentinel-3 Tandem Phase was also considered [26,27].…”