Consideration of Correlations in Radiometric Measurements of the Environment

Vicarious calibration is a technique that makes use of radiometrically stable targets such as high-altitude dry lakebeds and north African desert sites for the post-launch calibration of a satellite sensor. Top-of-the-atmosphere radiances or reflectances are provided for the calibration of the sensor. The reflectance of a remote sensing vicarious calibration site is measured by ratioing the signal from a ground target to the signal from a reference artifact whose reflectance is known. There can be elapsed times between measurements of a reference artifact on the order of 10 min. For environmental measurements, the solar illumination can vary on time scales relevant to the delay between measurements of a target and a reference artifact, impacting the variance in the measured reflectance. In this work, we explore the impact of a temporal delay between two measurements taken outdoors on the Type A uncertainties in their ratios. A factor of 3 reduction in the Coefficient of Variation of the ratio taken simultaneously versus sequentially with delays on the order of 10 min was realized. Implications for protocols employed to measure the surface reflectance at sites used for the vicarious calibration of aircraft and satellite sensors are discussed.