In an era when the number of Earth-observing satellites is rapidly growing and measurements from these sensors are used to answer increasingly urgent global issues, it is imperative that scientists and decision-makers rely on the accuracy of Earth-observing data products. The characterization and calibration of these sensors are vital to achieve an integrated Global Earth Observation System of Systems (GEOSS) for coordinated and sustained observations of Earth. The U.S. Geological Survey (USGS), as a supporting member of Committee on Earth Observation Satellites (CEOS) and GEOSS, worked with partners around the world to establish an online catalogue of prime candidate world-wide test sites for the post-launch characterization and calibration of space-based optical imaging sensors. The online catalogue provides easy public web site access to this vital information for the global community. This paper further describes the catalogue, the test sites, and the methodologies to utilize the test sites. It also provides information regarding access to the online catalogue and plans for further development of the catalogue in cooperation with calibration specialists from agencies and organizations around the world. Through greater access to and understanding of these vital test sites and their use, the validity and utility of information gained from Earth remote sensing will continue to improve.