Research advance on worldwide agricultural UAVs in 2001-2020 based on bibliometrics

Unmanned aerial vehicles (UAVs) have become essential to promote the digital process of agriculture in the artificial intelligent era. This review aims to clarify the development trends of agricultural UAVs covering the cutting-edge technologies at the research frontiers, as well as technical achievement and transformation at home and abroad. Five typical databases were selected for the document searching, including Web of Science Core Proceedings, China National Knowledge Infrastructure, Derwent Innovations Index, the China National Intellectual Property Administration patent database, and World Intellectual Property Organization Patent Scope. A statistical analysis was performed on the journal articles and invention patents related to agricultural UAVs that published in the above-mentioned databases over the past 20 years from 2001 to 2020 using statistical and bibliometric indicators. Two keywords were used in the search strategy, including “drone” and “agriculture”. Mathematical statistics showed that agricultural UAVs entered a high speed of research progress since 2014, and then reached a high fever period in 2016-2018. The literature publication per year is 1 232 with 1 970 patents in 2020. More than 4 000 papers and nearly 7 000 patents have been published in total. Europe has a relatively strong research strength for agricultural UAVs. China and the United States were the main research countries on agricultural UAVs, where paper outputs exceeded 45% (China accounts for 21.43% and the United States accounts for 24.60%) of the total, while Chinese, Japanese, and American companies contributed more patents. Particularly, Chinese patents accounted for around 80% of the total. Spain first laid the theoretical foundation for agricultural UAVs. Chinese and American institutions cooperated with each other relatively closely, followed by Australian institutions. The major publications in the field included Remote Sensing, Precision Agriculture and Computers and Electronics in Agriculture. In the past five years, Chinese studies were more active in this field. The subject areas of agricultural UAV papers were mainly related to remote sensing (30.22%) and environmental science (24.79%) in terms of bibliometrics and scientific mapping. The technical areas of patents were mainly related to aircraft and agriculture, forestry, animal husbandry, and fishery, specifically including B64D - aircraft for releasing substances (30.03%), B64C - aircraft for special purposes (23.66%), and A01M - liquid spraying equipment (21.01%). Both papers and patents on agricultural UAVs were divided into two aspects: UAV platform construction and agricultural applications. Agricultural applications were further divided into two major purposes: agricultural information collection and transportation. The current research frontier of agricultural drones lied in the mining of agricultural big data and the construction of agricultural digital models, whereas, the practical application was inclined to low-altitude remote sensing spraying and energy endurance. The latest research and technology application were “plant height” and “animal management and care”. The application scenarios are ever-increasingly wider and richer with the rise of emerging technologies, such as 5G, Internet of Things, and blockchain. Meanwhile, there are still some difficulties and challenges on the deeper integration of technologies. UAVs can also serve the new promising applications in animal husbandry and fishery in the future.