Calculation and Evaluation of BRF Correction Factors for Railroad Valley Playa

Abstract

Applications ranging from military reconnaissance to climate studies depend on the data collected by spaceborne remote sensors. Vicarious calibration is one method by which these sensors are validated. At the Radiometric Calibration Test Site (RadCaTS) in Railroad Valley Playa, Nevada, several autonomous ground viewing radiometers (GVRs) measure the surface reflectance at a nadir view. These data can then be used to calibrate sensors that view the playa from space. The process works well for sensors that view the playa at nadir, but uncertainty increases when the sensors view the playa at large zenith angles due to the non-Lambertian nature of the playa surface. However, if the bidirectional reflectance factor (BRF) of the surface is determined through models or measurement, it is possible to calculate a correction factor that converts a nadir reflectance value to the expected reflectance at an arbitrary view angle. Using preliminary surface measurements taken by the University of Lethbridge Goniometer System II (ULGS-II), correction factors were calculated which improved the agreement between the bottom of atmosphere (BOA) reflectances determined by RadCaTS and those measured by the Sentinel-2A and -2B Multispectral Instrument (MSI) for a view zenith angle (VZA) of 11°.