Potential of UAV-Based Active Sensing for Monitoring Rice Leaf Nitrogen Status oleh Songyang Li, Xingzhong Ding, Qianliang Kuang, Syed Tahir Ata-UI-Karim, Tao Cheng, Xiaojun Liu, Yongchao Tian, Yan Zhu, Weixing Cao, Qiang Cao