Radiometric comparison of multispectral imagers over a pseudo-invariant calibration site using a reference radiometric model

A model is proposed to simulate top-of-atmosphere (TOA) observations in the visible to near-infrared (NIR) spectral range, over a pseudo-invariant calibration site, the so-called Libya-4 site. The model is based on a fully physical radiative transfer model simulating the coupling between a realistic atmosphere and a spectral surface Bidirectional Reflectance Distribution Function (BRDF) model parameterised by 4 free parameters. At first, the model is 'calibrated' on 4 years of MERIS observations by inverting the 4 free parameters of the surface BRDF model that provide the best fit to the MERIS observations. The model mimics the MERIS TOA observations with a precision of approximately 1% RMSE outside water vapour and O-2 absorption features. The inverted BRDF model parameters obtained at MERIS spectral bands are then spectrally interpolated and used as input to the radiative transfer model to simulate observations from ATSR-2, AATSR, A-MODIS, MERIS, POLDER-3 and VEGETATION-2 over the 2002 to 2012 period. Depending on the spectral band considered, AATSR radiometry appears 2% to 3% above the model 'calibrated' on MERIS radiometry, A-MODIS is 0% to 3% below, POLDER-3 is 2% to 4% below and VEGETATION-2 about 4% below. ATSR-2 data during the 2002 to early 2003 period are almost 10% below their simulations. Temporal trends between simulations and observations are also measured for all sensors. The smallest linear trends are observed for the MERIS 3rd reprocessing data (below 1%/decade). The temporal trends obtained from all sensors against the coupled surface-atmosphere model are in line with expected residual errors of instrument degradation model used in temporal extrapolation: larger in blue than in the NIR. The combined temporal trends from all sensors tend to demonstrate that the Libia-4 site is radiometrically stable in the visible to the NIR to better than 1%/decade for the 2002-2012 period, thus quantitatively confirming that it is a terrestrial target particularly adequate for the assessment of the temporal stability of Earth Observation sensors. (C) 2013 Elsevier Inc. All rights reserved.