Adjustment of Sentinel-2 Multi-Spectral Instrument (MSI) Red-Edge Band Reflectance to Nadir BRDF Adjusted Reflectance (NBAR) and Quantification of Red-Edge Band BRDF Effects

David Roy,Hankui Zhang,Zhongbin Li

doi:10.3390/rs9121325

Abstract

Optical wavelength satellite data have directional reflectance effects over non-Lambertian surfaces, described by the bidirectional reflectance distribution function (BRDF). The Sentinel-2 multi-spectral instrument (MSI) acquires data over a 20.6° field of view that have been shown to have non-negligible BRDF effects in the visible, near-infrared, and short wave infrared bands. MSI red-edge BRDF effects have not been investigated. In this study, they are quantified by an examination of 6.6 million (January 2016) and 10.7 million (April 2016) pairs of forward and back scatter reflectance observations extracted over approximately 20° × 10° of southern Africa. Non-negligible MSI red-edge BRDF effects up to 0.08 (reflectance units) across the 290 km wide MSI swath are documented. A recently published MODIS BRDF parameter c-factor approach to adjust MSI visible, near-infrared, and short wave infrared reflectance to nadir BRDF-adjusted reflectance (NBAR) is adapted for application to the MSI red-edge bands. The red-edge band BRDF parameters needed to implement the algorithm are provided. The parameters are derived by a linear wavelength interpolation of fixed global MODIS red and NIR BRDF model parameters. The efficacy of the interpolation is investigated using POLDER red, red-edge, and NIR BRDF model parameters, and is shown to be appropriate for the c-factor NBAR generation approach. After adjustment to NBAR, red-edge MSI BRDF effects were reduced for the January data (acquired close to the solar principal where BRDF effects are maximal) and the April data (acquired close to the orthogonal plane) for all the MSI red-edge bands.

Highlights

The Sentinel-2A and Sentinel-2B satellites carry the Multi Spectral Instrument (MSI) that sense medium-resolution multi-spectral data designed for global land surface monitoring [1]
The interpolated red-edge band parameter values are intermediate between the Moderate Resolution Imaging Spectral radiometer (MODIS) red and NIR bidirectional reflectance distribution function (BRDF) parameter values and increase with wavelength because the MODIS BRDF parameter values are greater at the NIR than at the red wavelengths (Table 1)
The Polarization and Directionality of the Earth’s Reflectances (POLDER) red-edge (765 nm) parameters derived by linear interpolation, as (4), of the POLDER red and NIR fixed global annual spectral BRDF model parameters (Table 2) were f_iso = 0.2040, f_geo = 0.0299, and f_vol = 0.1094

Summary

Introduction

The Sentinel-2A and Sentinel-2B satellites carry the Multi Spectral Instrument (MSI) that sense medium-resolution multi-spectral data designed for global land surface monitoring [1]. The Sentinel-2A and -2B MSI remote sensing systems are in sun-synchronous polar orbits and acquire images over a 20.6◦ field of view that results in directional reflectance effects over non-Lambertian surfaces. These effects, often described by the bidirectional reflectance distribution function (BRDF), can be non-trivial. In a recent study of 17 million pairs of forward and back scatter reflectance observations, pronounced visible, near-infrared, and short wave infrared BRDF effects were documented across the 290 km wide MSI swath [12]. MSI red-edge BRDF effects are quantified using the same data that were extracted over 20◦ × 10◦ of southern Africa for two 10 day periods in January 2016 (acquired close to the solar principal plane when BRDF effects are maximal) and in April 2016 (acquired close to the orthogonal plane when BRDF effects are minimal)

Methods

Results

Conclusion