Short-wave infrared radiometer for airborne validation of satellite snow products

Surface reflectance determines how much solar radiation is retained, making it a critical component of the Earth’s energy and water balance. Several satellites, including Landsat and Sentinel-2, measure surface reflectance; however, the limited temporal resolution and spatial resolution of these satellites introduce uncertainties, especially in areas of high temporal and spatial variability such as mountainous regions dominated by seasonal snowpack, trees, and fluctuating slopes. Unoccupied aerial vehicles (UAVs) provide a platform for frequent measurement of surface reflectance with user-selected coverage during critical times of snowpack change. UAVs can enable higher spatial resolution than satellites while covering a larger spatial area than in situ, tower-based sensors. We present the design and calibration of a custom UAV-based radiometer system for validating the shortwave infrared Landsat Operational Land Imager Band 7 and Sentinel-2 MultiSpectral Imager Band 12 surface reflectance measurements. Two sensors enabled surface reflectance correction: an upwelling radiation sensor was designed to measure scene-reflected radiance [W/(m2sr)], and a downwelling radiation sensor was designed to measure incident irradiance [W/(m2)]. Both radiometers were constructed with single-pixel photodiodes and custom bandpass filters to match the spectral range of Landsat Band 7 (2110 to 2290 nm), and the half-angle field of view of the upwelling system was designed and measured to be 3.43 deg. When flown 120 m above the ground, this corresponds to a ground swath diameter of 14.39 m, a spatial resolution over four times finer than the 30 m Landsat Band 7 pixel. Radiometric and temperature calibrations reduced uncertainties for both sensors. The custom radiometer was flown on a UAV in alignment with a Landsat 9 satellite overpass in Southwestern Montana, United States (45.23 deg, −111.48 deg). The resulting airborne measurements were used to assess the implications of surface topography on radiometer-measured reflectance, where the preliminary airborne measurements indicated a significant correlation between surface reflectance variability and slope angle.