Sentinel-1 time series data for monitoring the phenology of winter wheat
The potential of time series metrics based on optical sensors for monitoring of crops and their phenological stages has long been recognized. SAR (synthetic aperture radar) sensors such as Sentinel-1 can provide dense time series, but their potential for retrieving time series metrics is to date underexplored. The backscatter coefficient in VH (vertical-horizontal) and VV (vertical-vertical) polarization as well as the interferometric coherence of Sentinel-1 was retrieved in a German agricultural area in order to obtain a time series with a temporal resolution of 6 days. The backscatter coefficient ratio of VH and VV polarization was also calculated. Local extrema and breakpoints were detected in the locally smoothed time series data in wheat fields for the years 2017, 2018 and 2019. Breakpoints in the VH/VV ratio time series have high potential for detecting the date of shooting and harvesting of the studied wheat fields. The root mean squared differences between detected date and in situ observed date were 4.6 days in 2017, 5.3 days in 2018 and 9.5 days in 2019 for the shooting and 5.1 days in 2017, 8.2 days in 2018 and 10.4 days in 2019 for the harvest. Ripeness stages could not be detected with a high degree of accuracy. The results suggest that the VH/VV ratio has a high potential for monitoring th phenological stages of wheat fields, which can be used for large-scale crop productivity monitoring and risk assessment in agricultural cropping systems.