Radiometric calibration of the RapidEye Multispectral Imager (MSI) as with all other remote sensing instruments is an essential task in the quantitative assessment of sensor image quality and the production of accurate data products for a wide range of geo-spatial applications. Spatially and temporally pseudo-invariant terrestrial targets have long been used to quantify and provide a consistent record of the radiometric performance of Earth observation systems. The RapidEye cross-calibration approach combines temporal and relative calibration to ensure temporal stability in spectral response between it's 5 identical MSI over time by using a large number of repetitive collects of many pseudo-invariant calibration sites. The approach is characterized by its known reliability which is based on the purely statistical analysis of many ground collects with ground infrastructure or measurement systems not being necessary. The results show that the in-band percent difference in the measured response among all RapidEye sensors is less than two percent. Although the results show some offsets between the different sensors, the response of the RapidEye constellation over a three-year period is very stable.