Unmanned Aerial Vehicles in Smart Agriculture: Applications, Requirements, and Challenges[J] IEEE SENSORS JOURNAL 2021(21) Reddy Maddikunta, Praveen Kumar Hakak, Saqib Alazab, Mamoun Bhattacharya, Sweta Gadekallu, Thippa Reddy Khan, Wazir Zada Pham, Quoc-Viet