CropSAT is a free-to-use web application for satellite-based production of variable-rate application (VRA) files of e.g. nitrogen (N) and fungicides currently available in Sweden and Denmark. Even in areas frequently covered by clouds, vegetation index maps from data derived from low-cost or freely available optical satellites can be used in practice as a cost-efficient tool in time-critical applications such as optimized nitrogen use. During the very cloudy year 2015, or more useable images were generated for 2/3 of all arable land in Sweden in the period end of April to the first part of June. By using the relative variation within fields, often images upto a few weeks old can be used as input for the VRA maps, reducing the need for entirely recently acquired images. CropSAT is an interactive decision support system in which the user decides upon the level of input in different index classes. The relationship between satellite data and data from ground-based sensors suggest that simple vegetation index maps can be calibrated by sensors on the ground to produce maps of N uptake. Thousands of users have tested CropSAT which has the potential to be developed further.