Direct Reflectance Measurements from Drones: Sensor Absolute Radiometric Calibration and System Tests for Forest Reflectance Characterization. - Teemu Hakala; Lauri Markelin; Eija Honkavaara; Barry Scott; Theo Theocharous; Olli Nevalainen; Roope Näsi; Juha Suomalainen; Niko Viljanen; Claire Greenwell; Nigel Fox (Sensors, 2018)