In short… Retrieving Sentinel-2 data from official repositories (Copernicus Open Access Hub) for time series analysis has recently become particularly laborious, since products older than 30 days cannot be directly downloaded (users have to order them from Long Term Archive, with the chance to retrieve corrupted archives). To bypass these problems, the possibility to search and download Sentinel-2 data from Google Cloud was recently implemented in the R package sen2r.
sen2r is an R package developed by the Institute of Remote Sensing of Environment (IREA) of the Italian National Research Council, with the aim to simplify and speed up several steps commonly needed to process Sentinel-2 data. Recently, an important recognition to the worth and usefulness of this tool came from the ISI journal Computers & Geosciences, publishing the scientific paper “sen2r”: An R toolbox for automatically downloading and preprocessing Sentinel-2 satellite data.
Until August 2019, all Sentinel-2 satellite data could be directly downloaded from the ESA Data Hub, both through the interactive Open Hub or using an API interface.
Recently this policy was changed: typically, only the most recent products are available for direct download, while the oldest ones (level 2A archives older than 18 months and level 1C older than one year) are stored in the so called Long Term Archive, and must be ordered by the user; then, they are made available for download after a while (no messages are sent to the user).
The availability of Sentinel-2 satellite data has been a turning point for a widespread kind of users, which can now take advantage from a dataset sufficiently dense (revisiting time of 5 days) to perform time series analysis, and which is characterised by a decametric resolution that allows to discriminate many natural and agronomic targets.
Data access is free (upon registration to ESA data hub). Although many solutions are available to obtain images (a detailed list is available here), R users should download them manually (or using ESA API Hub) and then perform some preprocessing operations before being able to use them (e.