Characterization approaches to place invariant sites on si-traceable scales
Understanding Earth's climate requires remote sensing imagery that is consistent across time and multiple countries. A method is described here for a model-based, SI-traceable prediction of at-sensor radiance over selected sites with defensible error budgets. The predicted radiance would be valid for arbitrary view and illumination angles and for any date of interest that is dominated by clear-sky conditions. The effort effectively works to characterize the sites as sources with known top-of-atmosphere radiance allowing accurate intercomparison of sensor data without the need for coincident views. A set of recommendations to ensure future SI-traceable cross calibration using future missions such as CLARREO and TRUTHS are presented. Highly accurate measurements of at-sensor radiance of sufficient quality allow an understanding of spectral and BRDF characteristics of the site. Sufficient historical data are necessary to develop an understanding of temporal effects from changing surface and atmospheric conditions.