A UAV-Based Sensor System for Measuring Land Surface Albedo: Tested over a Boreal Peatland Ecosystem

Abstract

A multiple sensor payload for a multi-rotor based UAV platform was developed and tested for measuring land surface albedo and spectral measurements at user-defined spatial, temporal, and spectral resolutions. The system includes a Matrice 600 UAV with an RGB camera and a set of four downward pointing radiation sensors including a pyranometer, quantum sensor, and VIS and NIR spectrometers, measuring surface reflected radiation. A companion ground unit consisting of a second set of identical sensors simultaneously measure downwelling radiation. The reflected and downwelling radiation measured by the four sensors are used for calculating albedo for the total shortwave broadband, visible band and any narrowband at a 1.5 nm spectral resolution within the range of 350-1100 nm. The UAV-derived albedo was compared with those derived from Landsat 8 and Sentinel-2 satellite observations. Results show the agreement between total shortwave albedo from UAV pyranometer and Landsat 8 (R 2 = 0.73) and Sentinel-2 (R 2 = 0.68). Further, total shortwave albedo was estimated from spectral measurements and compared with the satellite-derived albedo. This UAV-based sensor system promises to provide high-resolution multi-sensors data acquisition. It also provides maximal flexibility for data collection at low cost with minimal atmosphere influence, minimal site disturbance, flexibility in measurement planning, and ease of access to study sites (e.g., wetlands) in contrast with traditional data collection methods.There are several operational surface albedo products from satellite remote sensing with spatial resolution ranging from approximately 0.5-25 km and temporal frequencies of daily to monthly [9]. Most notably these products include Moderate Resolution Imaging Spectroradiometer [10], Advanced Very High Resolution Radiometer [11], Polarization and Directionality of the Earth Reflectance [12] and Meteosat [13]. However, coarse resolution products are not detailed enough for studying ecological processes in small dynamic ecosystems such as wetlands. Retrieving surface albedo from high spatial resolution remotely sensed data (e.g., Landsat 8 and Sentinel-2) is methodologically challenging, in part because of the narrow angular sampling and the incomplete spectral sampling from a limited number of wavebands [2] and demands in-situ calibration and validation datasets. In-situ albedo measurements have long been using sensors mounted on research towers. These measurements, though, demand a high logistical requirement and only a few surfaces can be characterized and measured [2]. In order to account for a wide range of surface cover types, aerial based sensor system can play an important role in measuring surface albedo. Measurements from unmanned aerial platforms effectively bridges the observational scale between point-based tower measurements and satellite imageries [14].Unmanned aircraft systems (UAS) are small planes capable of carrying small-sized sensors [15] and [14,16] use UAS mounted spectrometers or cameras to measure reflect...

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