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

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 OLCI-A/OLCI-B tandem phase analysis. This analysis provided cross-calibration factors between the two instruments with a very high precision, providing a ‘truth’ from the direct comparison of simultaneous and collocated acquisitions. The long-term monitoring of such cross-calibration is a prerequisite for an operational application of sensors harmonisation along the mission lifetime, no other tandem phase between OLCI-A and OLCI-B being foreseen due to the cost of such operation. This article presents a novel approach for the monitoring of the OLCI radiometry based on statistics of Deep Convective Clouds (DCC) observations, especially dedicated to accurately monitor the full across-track dependency of the cross-calibration of OLCI-A and OLCI-B. Specifically, the inflexion point of DCC reflectance distributions is used as an indicator of the absolute calibration for each subdivision of the OLCI Field-of-View. This inflexion point is shown to provide better precision than the mode of the distributions which is commonly used in the community. Excess of saturation in OLCI-A high radiances is handled through the analysis of interband relationships between impacted channels and reference channels that are not impacted by saturation. Such analysis also provides efficient insights on the variability of the target’s response as well as on the evolution of the interband calibration of each payload. First, cross-calibration factors obtained over the tandem period allows to develop and validate the approach, notably for the handling of the saturated pixels, based on the comparison with the ‘truth’ obtained from the tandem analysis. Factors obtained out of (and far from) the tandem period then provides evidence that the cross-calibration reported over the tandem period (1–2% bias between the instruments) as well as inter-camera calibration residuals persist with very similar proportions, to the exception of the 400 nm channel and with slightly less precision for the 1020 nm channel. For all OLCI channels, relative differences between the cross-calibration factors obtained from the tandem analysis and the factors obtained over the other period are below 1% from a monthly analysis, even below 0.5% from a multi-monthly analysis). This opens the way not only to an accurate long-term monitoring of the OLCI radiometry but also, and precisely targeted for this study, to the monitoring of the cross-calibration of the two sensors over the mission lifetime. It also provides complementary information to the tandem analysis as the calibration indicators are traced individually for each sensor across-track, confirming and quantifying inter-camera radiometric biases, independently for both sensors. Assumptions used in this study are discussed and validated, also providing a framework for the adaptation of the presented methodology to other optical sensors.

OLCI A/B tandem phase: Evaluation of FLEX like radiances and estimation of systematic differences between OLCI-A and OLCI-FLEX

“…The bias of OLCI-FLEX with respect to OLCI-A will be estimated by using our transfer function. The bias should be consistent with the findings of Lamquin et al (2020). Furthermore, this comparison is a test of the calibration of FLORIS.…”

“…We observed a relative difference in measured TOA radiance between OLCI-FLEX and OLCI-A of about comparing OLCI-A and B with their original band settings. We also found a difference of about 5% with different sign at 778.75 nm for the OLCI-FLEX-OLCI-A comparison which is not observed by Lamquin et al (2020). Thus, we conclude that it was not caused by an absolute calibration issue between OLCI-A and OLCI-B but by the processing from L0 to L1 of the OLCI-FLEX data.…”

“…It is applied for vegetated cloud free land pixels, as the main objective of FLEX mission is to retrieve fluorescence emitted by plants. Lamquin et al (2020) showed a systematic bias between OLCI-A and OLCI-B in the tandem constellation data. The bias of OLCI-FLEX with respect to OLCI-A will be estimated by using our transfer function.…”

“…For the georeferencing we used the same method that was suggested by Lamquin et al (2020). They showed that the reprojection of both OLCI-A and OLCI-B on the same regular grid results in a valid georeferencing of OLCI-A and B for the tandem phase data.…”

“…Both satellites flew in the same orbit observing the same geographic target within 30 s. The measurements were taken with the same geometrical and environmental conditions. Thus, a comparison of the radiance data was possible (Lamquin et al, 2020).…”