The FX measures the distance with a beam of light, but the velocity of this light varies according to the index of refraction of light in the atmosphere. This index of refraction varies according to the temperature and pressure. Near normal temperature and pressure conditions:
With constant pressure, a temperature change of 1°C: an index change of 1 ppm. With constant temperature, a pressure change of 3.6 hPa: an index change of 1 ppm.
To perform high accuracy measurements, it is necessary to find the atmospheric correction factor from even more accurate temperature and pressure measurements and perform an atmospheric correction