Development of a SWIR UAV-based radiometer for validating Landsat 8/9 snow products

This paper summarizes the design and calibration of a radiometer system designed for validating Landsat Band 7 and Sentinel-2 Band 12 (2110 to 2290nm) measurements from an unoccupied aerial vehicle (UAV). This system and these satellite bands are often used to measure snow reflectance, which is critical to the Earth’s energy and water balance. The system uses two radiometers, one for measuring upwelling radiance [W/(m² sr)] and one for measuring downwelling irradiance [W/m²]. The upwelling radiometer was constructed with a single-pixel photodiode and a lens assembly to maximize the field of view for measuring snow-reflected radiance. The downwelling radiometer uses the same detector with a ground-glass diffuser to measure all-sky downwelling irradiance. The upwelling and downwelling radiometers use custom interference filters to achieve the closest match to the satellite spectral bands. The upwelling system has a half-angle field of view of 3.43 degrees. When flown at an above grove level of 120m, this corresponds to a ground swath of diameter 14.39m, a spatial resolution over twice the 30m spatial resolution of a Landsat Band 7 pixel. The instrument is intended to be deployed in colder climates; therefore, a temperature calibration was conducted to reduce uncertainties in the radiometric calibration due to large deviations in the operating temperature.