Abstract
The Sentinel-3 tandem project represents the first time that two ocean colour satellites have been flown in the same orbit with minimal temporal separation (~30 s), thus allowing them to have virtually identical views of the ocean. This offers an opportunity for understanding how differences in individual sensor uncertainty can affect conclusions drawn from the data. Here, we specifically focus on trend estimation. Observational chlorophyll-a uncertainty is assessed from the Sentinel-3A Ocean and Land Colour Imager (OLCI-A) and Sentinel-3B OLCI (OLCI-B) sensors using a bootstrapping approach. Realistic trends are then imposed on a synthetic chlorophyll-a time series to understand how sensor uncertainty could affect potential long-term trends in Sentinel-3 OLCI data. We find that OLCI-A and OLCI-B both show very similar trends, with the OLCI-B trend estimates tending to have a slightly wider distribution, although not statistically different from the OLCI-A distribution. The spatial pattern of trend estimates is also assessed, showing that the probability distributions of trend estimates in OLCI-A and OLCI-B are most similar in open ocean regions, and least similar in coastal regions and at high northern latitudes. This analysis shows that the two sensors should provide consistent trends between the two satellites, provided future ageing is well quantified and mitigated. The Sentinel-3 programme offers a strong baseline for estimating long-term chlorophyll-a trends by offering a series of satellites (starting with Sentinel-3A and Sentinel-3B) that use the same sensor design, reducing potential issues with cross-calibration between sensors. This analysis contributes an important understanding of the reliability of the two current Sentinel-3 OLCI sensors for future studies of climate change driven chlorophyll-a trends.
Highlights
Sentinel-3 is a multi-satellite Copernicus mission featuring a number of instruments, including an optical radiometry instrument, focused on measuring the Earth’s surface over land and oceans with the aim of supporting environmental and climate monitoring [1]
We performed an analysis to understand the effect of sensor uncertainty on estimation of potential chl-a trends in the Sentinel-3A and B Ocean and Land Colour Imager (OLCI) sensors, including two versions of Ocean and Land Colour Imager (OLCI-A), using data from the tandem phase
From this analysis of synthetic chl-a trends, we were able to determine that any differences in uncertainty between the two sensors will have only a limited effect on chl-a trend estimation
Summary
Sentinel-3 is a multi-satellite Copernicus mission featuring a number of instruments, including an optical radiometry instrument, focused on measuring the Earth’s surface over land and oceans with the aim of supporting environmental and climate monitoring [1]. During the period from June to October 2018, Sentinel-3A and Sentinel-3B were flown in a tandem configuration with a separation of just 30 s in their overpass time. This tandem phase was designed to ensure the consistency of data between the two satellites in support of establishing reliable long-term climate data records in accordance with the Global Climate Observing System’s Climate Monitoring Principles [2]. This paper focuses on assessing the ability of the OLCI data to provide a reliable climate data record which can be used to study long-term chlorophyll-a (chl-a) change
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