Two calibration methods are the main suppliers of extraterrestrial standards: the high altitude balloon and the high altitude aircraft. Both methods require minimum data correction due to the small residual air mass at the altitude where the calibration is performed.
2.1.1 High Altitude Balloon
Calibrations are performed on board stratospheric balloons flying at altitudes of around 36 km, where the illumination sun conditions are very close to AMO. Cells to be calibrated are directly exposed to the sun, mounted on supports with sun trackers. Currently, two institutions, JPL-NASA in the USA  and CNES in France [3, 4], are conducting, on a yearly basis, these calibration campaigns. The main differences between the two calibration institutes are the position of the cells, which in the case of JPL-NASA is mounted on the balloon apex and in the case of CNES, is a gondola hanging from the balloon. Both institutes correct calibrated data taking into account the effect of temperature and the variation of illumination due to the Earth-Sun distance variation over the year. CNES also corrects its calibrated data, taking into account the effect of the residual atmosphere.