OLCI A/B Tandem Phase Analysis, Part 1: Level 1 Homogenisation and Harmonisation

Abstract

Copernicus is a European system for monitoring the Earth in support of European policy. It includes the Sentinel-3 satellite mission which provides reliable and up-to-date measurements of the ocean, atmosphere, cryosphere, and land. To fulfil mission requirements, two Sentinel-3 satellites are required on-orbit at the same time to meet revisit and coverage requirements in support of Copernicus Services. The inter-unit consistency is critical for the mission as more S3 platforms are planned in the future. A few weeks after its launch in April 2018, the Sentinel-3B satellite was manoeuvred into a tandem configuration with its operational twin Sentinel-3A already in orbit. Both satellites were flown only thirty seconds apart on the same orbit ground track to optimise cross-comparisons. This tandem phase lasted from early June to mid October 2018 and was followed by a short drift phase during which the Sentinel-3B satellite was progressively moved to a specific orbit phasing of 140° separation from the sentinel-3A satellite. In this paper, an output of the European Space Agency (ESA) Sentinel-3 Tandem for Climate study (S3TC), we provide a full methodology for the homogenisation and harmonisation of the two Ocean and Land Colour Instruments (OLCI) based on the tandem phase. Homogenisation adjusts for unavoidable slight spatial and spectral differences between the two sensors and provide a basis for the comparison of the radiometry. Persistent radiometric biases of 1–2% across the OLCI spectrum are found with very high confidence. Harmonisation then consists of adjusting one instrument on the other based on these findings. Validation of the approach shows that such harmonisation then procures an excellent radiometric alignment. Performed on L1 calibrated radiances, the benefits of harmonisation are fully appreciated on Level 2 products as reported in a companion paper. Whereas our methodology aligns one sensor to behave radiometrically as the other, discussions consider the choice of the reference to be used within the operational framework. Further exploitation of the measurements indeed provides evidence of the need to perform flat-fielding on both payloads, prior to any harmonisation. Such flat-fielding notably removes inter-camera differences in the harmonisation coefficients. We conclude on the extreme usefulness of performing a tandem phase for the OLCI mission continuity as well as for any optical mission to which the methodology presented in this paper applies (e.g., Sentinel-2). To maintain the climate record, it is highly recommended that the future Sentinel-3C and Sentinel-3D satellites perform tandem flights when injected into the Sentinel-3 time series.

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

“…In the continuity of ESA's MEdium Resolution Imaging Spectrometer (MERIS), it is a good candidate to become a reference instrument for the radiometry in the VNIR, owing to its advanced on-board calibration devices, wide swath, and medium spectral resolution [2]. More description of the instrument and its calibration are provided in [4], we only recall that OLCI measurements are provided across-track (ACT) owing to the combination of five independent cameras totalizing a field-of-view (FOV) of about 1270 km from a tilted geometry of acquisition covering viewing angles from 46.5 • on the west of the FOV to 22.1 • on the east.…”

“…As in [4], the tandem phase datasets used in this work were collected in the frame of the Sentinel-3 Tandem for Climate project. The project data archive contains Sentinel-3A and Sentinel-3B data for all instruments, collected one day per week (Monday) between 25 June 2018 and 18 March 2019.…”

“…This study bases on the results of a homogenisation and harmonisation methodology for the two OLCI-A and OLCI-B payloads developed through the Sentinel-3 for Climate Study (S3TC), an ESA-funded project (s3tandem.eu), and described in [4]. Homogenisation first adjusts the slight spectral and spatial differences of the two sensors.…”

“…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.…”

“…Cross-comparisons of the two sensors have proven very stable over the tandem period [4]. Throughout the Sentinel-3 mission duration, as no other tandem phase is foreseen, the harmonisation of the sensors would however only rely on the individual in-flight calibration of the two sensors and notably through their temporal stability relating the calibration at the time of acquisition to the calibration at the time of tandem cross-comparisons.…”