AUTHOR=de Jong Steven M. TITLE=Grand Challenges in Earth Observation Using Unpiloted Airborne Systems JOURNAL=Frontiers in Remote Sensing VOLUME=1 YEAR=2020 URL=https://www.frontiersin.org/journals/remote-sensing/articles/10.3389/frsen.2020.601737 DOI=10.3389/frsen.2020.601737 ISSN=2673-6187