Sentinel-1 time series data for sunflower (Helianthus annuus) phenology monitoring

The potential of time series of Sentinel-1 (S1) Synthetic Aperture Radar (SAR) data for monitoring crops and their phenological stages has long been recognized. However, sunflower (Helianthus annuus) is one of the crops whose interaction with microwave signal is still not fully understood. Here, we aim to analyze and interpret time series of S1 data for sunflower phenology monitoring. The SAR backscattering coefficients in VH (vertical-horizontal) and VV (vertical-vertical) polarizations as well as their ratio (VH/VV) in both ascending and descending orbits were retrieved for three sites located in three different agro-ecological regions in Ukraine. We observed that sunflower backscattering response differs for the ascending and descending orbits for the VV polarization and VH/VV polarization ratio due to the directional behavior of the flower head. Our analysis did not find major change in VH polarization for the two orbits. We also compared the S1-based sunflower phenology with that of Normalized Difference Vegetation Index (NDVI) derived from optical data acquired by Sentinel-2 (S2). Our analysis revealed a considerable correlation r2 > 0.4 between S1 and NDVI for VH and VV polarizations in both ascending and descending orbits; however, we observed lower correlation (r2 < 0.20) for the VH/VV polarization ratio in descending orbit (VH/VVdes). Based on the characteristics of sunflower phenology, we identified two sunflower phenological stages: beginning of flowering (BoF) and end of flowering (EoF). The difference between S1-based dates using VH/VVdes and reference dates was 4.1 and 4.4 days for the BoF and EoF stages, respectively; the use of VH/VVdes+asc yielded weaker results: 7.4 and 6.1 days. These results suggest that the VH/VVdes is a preferable parameter for identifying sunflower phenological stages such as BoF and EoF.