Climate Data Records from Meteosat First Generation Part II: Retrieval of the In-Flight Visible Spectral Response

Abstract

How can the in-flight spectral response functions of a series of decades-old broad band radiometers in Space be retrieved post-flight? This question is the key to developing Climate Data Records from the Meteosat Visible and Infrared Imager on board the Meteosat First Generation (MFG) of geostationary satellites, which acquired Earth radiance images in the Visible (VIS) broad band from 1977 to 2017. This article presents a new metrologically sound method for retrieving the VIS spectral response from matchups of pseudo-invariant calibration site (PICS) pixels with datasets of simulated top-of-atmosphere spectral radiance used as reference. Calibration sites include bright desert, open ocean and deep convective cloud targets. The absolute instrument spectral response function is decomposed into generalised Bernstein basis polynomials and a degradation function that is based on plain physical considerations and able to represent typical chromatic ageing characteristics. Retrieval uncertainties are specified in terms of an error covariance matrix, which is projected from model parameter space into the spectral response function domain and range. The retrieval method considers target type-specific biases due to errors in, e.g., the selection of PICS target pixels and the spectral radiance simulation explicitly. It has been tested with artificial and well-comprehended observational data from the Spinning Enhanced Visible and Infrared Imager on-board Meteosat Second Generation and has retrieved meaningful results for all MFG satellites apart from Meteosat-1, which was not available for analysis.

Climate Data Records from Meteosat First Generation Part I: Simulation of Accurate Top-of-Atmosphere Spectral Radiance over Pseudo-Invariant Calibration Sites for the Retrieval of the In-Flight Visible Spectral Response

“…A similar verification performed on Meteosat-7 data confirmed the issue with the characterization of the MVIRI/VIS band SRF. The outcome of this paper is a key result for the reconstruction of the in-flight SRF and the recalibration of MVIRI data described in companion papers [6,16].…”

“…The generation of consistent MFG FCDR with fully traceable uncertainty is one of the main objectives of this project. In this context, a new reverse engineering method was developed for the recovery of the MVIRI/VIS spectral response and its uncertainty accounting for the temporal ageing effects [6]. This method requires essentially two pieces of information: the Digital Count (DC) value of selected pixels over PICSs and the corresponding simulated Top-Of-Atmosphere (TOA) spectral radiance in the MVIRI/VIS band.…”

“…2. SRF retrieval The MVIRI/VIS band SRF and its spectral degradation is estimated from the extracted DC values and simulated spectral radiances [6]. 3.…”

“…This paper describes the first step of this process. Steps 2 and 3 are described in companion papers in this issue [6,16].…”

Radiance Uncertainty Characterisation to Facilitate Climate Data Record Creation

“…The key contributions of the paper are (i) to propose a set of summary information on errors and their correlations that it is feasible and sufficient to provide, and (ii) to summarise the mathematics and computing techniques required to calculate this uncertainty information. References [15][16][17][18][19] illustrate the working out of these concepts in particular cases.…”