## 4 Conclusions  
In this study, three different types of remote sensors were evaluated for their optical quality and profitability for variable rate nitrogen application. A first comparison analyzed the price trend of UAV-derived NDVI image providers in Italy between 2017 and 2020 for arable and specialized crops. In both years arable crops showed lower prices than specialized crops, suggesting differences in survey costs and higher value added to UAV images for specialized crops. A decrease in the average price was observed between 2017 and 2020 for the analyzed service providers. The number of UAV service providers in Italy increased starting from 2015 due to the high interest in this technology by farmers. Due to this latter point, the selection of market operators occurred in the following years. In addition, other sources of multispectral images, such as planes and satellites, compete with UAV image providers. The comparison between UAV, plane and satellite prices per hectare (without taking into account minimum order area, take-off and processing costs) showed that UAV prices were the highest, followed by those of planes and satellites. The optical quality comparison was carried out considering the CV of NDVI simulation and taking advantage of the SNR provided by the manufacturers. Images acquired by high-cost satellite sensors showed optical quality similar to the reference sensors chosen for UAV (Tetracam Micro-MCA) and plane images (Leica DMC III). Conversely, satellite images with lower prices per hectare showed lower optical quality. These findings highlighted that the prices of satellite images were influenced not only by data volume per hectare but in some cases also by optical quality. Free of charge satellites showed different trends. Although the images were free-of-charge, their optical quality was comparable to the referenced sensors for UAVs and planes, demonstrating the consistency of their data. However, the generalized model for satellite image prices was based only on data volume per hectare, as the optical quality influenced the price of only a limited number of satellites. The comparison between the cost of remote sensors and the economic benefits provided by variable rate nitrogen application could be useful for farm managers who need to implement the methodology and tools to detect field variability. Medium-resolution satellites proved to be profitable for variable rate nitrogen applications for blocks bigger than 2.52 hectares. Medium-resolution satellites with spatial resolution between 10 and 15 m are accurate enough for managing variable rate fertilization with commonly used sprayers. In case higher resolution is needed, high-resolution satellites were profitable starting from 13.2 hectares, while very high-resolution ones were profitable starting from 76.8 hectares. The choice between high- and very-high-resolution should be made according to the optical quality of the sensors, which was higher in very high-resolution satellites. Plane-acquired NDVI images proved to be profitable in blocks bigger than 66.4 hectares, thus being suitable for large farms or consortiums which require high spatial resolution images. Despite the widespread growth in UAV NDVI image providers in the last few years, UAV-acquired images did not prove to be profitable for variable rate nitrogen fertilization in grains, as the average price per hectare was higher than the average economic benefits resulting from their use. However, UAVs provide very high-resolution images with suitable optical quality, and, compared to satellites, they are not influenced by cloud cover. For this reason,UAV images could be profitably used for high added value applications that could justify their cost (e.g., variability detection for precision herbicide spraying or foliar fertilizer) or for applications in crops other than grains. In conclusion, the advantages and disadvantages of satellite, plane and UAV remote sensing for variable rate nitrogen application can be highlighted.