Usable observations over Europe: Evaluation of compositing windows for landsat and sentinel-2 time series

Landsat and Sentinel-2 data archives provide ever-increasing amounts of
satellite data. However, the availability of usable observations greatly
varies spatially and temporally. Pixel-based compositing that generates
temporally equidistant cloud-free synthetic images can mitigate temporal
variability, by constructing uninterrupted time series using different
compositing windows. Here, we evaluated the feasibility of using
compositing windows ranging from five days to one year for 1984-2021
Landsat and 2015-2021 Sentinel 2 time series to derive uninterrupted time
series across Europe. We considered separate and joint use of both data
archives and analyzed the spatio-temporal availability of composites
during each calendar year and pixel-specific growing season across a
variety of time windows and hypothesizing data interpolation. Our results
demonstrated opportunities and limitations in the available data records
to support medium- and long-term analyses requiring uninterrupted time
series of composites with sub-annual temporal resolution. Spatial
disparities across different compositing windows provide guidance on the
feasibility of workflows relying on different data densities and on the
challenges in wall-to-wall analyses. The feasibility of consistent time
series based on composites with sub-monthly aggregation periods was mostly
limited to the combined Landsat and Sentinel-2 archives after 2015, yet in
some geographies requires interpolation of up to 50% of data.