Optimized identification of worldwide radiometric pseudo-invariant calibration sites

Long-term radiometric stability monitoring of visible and near-infrared Earth observing sensors has been enhanced over the past decade through the use of pseudo-invariant calibration sites on the Earth's surface. Significant work has been done to characterize sites primarily in the Sahara and Middle East desert regions, with some additional work at other locations throughout the world. The work described in this paper attempts to locate those sites that can be considered optimal from a temporal stability measure. To accomplish this, an invariant site identification algorithm was developed that can locate a statistically optimal region in an automated fashion. Results generated from studying virtually all previously identified pseudo-invariant sites indicate that there are six sites in the Sahara and Middle East that can achieve variabilities as low as 2% in the visible and near infrared (VNIR) and 2%-3% in the shortwave infrared (SWIR). The Sonoran Desert site was identified in North America and produces 2%-3% variabilities in the VNIR and 4%-5% in the SWIR. In addition, it has a large amount of historical data available for calibration of historic sensors. Additional sites in Dunhuang, China, and Barreal Blanco, Argentina, show excellent potential for long-term monitoring using high-altitude desert sites as well as potential for improved monitoring in the SWIR region. Lastly, regular acquisition of these sites by current and future Earth observing sensors is strongly recommended for additional reductions in uncertainties associated with these sites, for cost-effective monitoring of sensor stability, and for sensor cross-calibration.