PhenoTandem harmonizes new phenology products derived from high resolution optical satellite (Sentinel-2) with the traditional phenological in-situ observations done by volunteers. Since current in-situ observation cannot always be perceived from space, the innovation consists in co-designing with citizen scientists a new protocol that will make in-situ observations interoperate with remote sensing products. This will burst the spatial distribution of remote sensing phenology products sensible to the effects of the climate change in nature while increasing data quality through in-situ validations. This project is part of the RitmeNatura.cat Citizen Science observatory.