The CEOS IVOS sub-group members worked with partners around the world to establish a set of CEOS endorsed globally-distributed reference standard test sites for the post-launch calibration of space-based optical imaging sensors. Based on the decision from the CEOS IVOS-19 meeting that was help in Phoenix, AZ, at present there are eight instrumented sites and five pseudo-invariant test sites.

The CEOS instrumented sites are provisionally being called "LANDNET". These instrumented sites are primarily used for field campaigns to obtain radiometric gain and these sites can serve as a focus for international efforts, facilitating traceability and cross-comparison to evaluate biases of in-flight and future sensors in a harmonized manner.

The pseudo-invariant desert sites have high reflectance and are usually made up of sand dunes with low aerosol loading and, practically, no vegetation. Consequently, these pseudo-invariant reference standard test site can be used to evaluate the long-term stability of a sensor and to facilitate cross-comparison of multiple sensors.