Determining the refractivity at the bottom of the atmosphere using radio occultation.

High accuracy of impact height is important to get reliable Radio Occultation (RO) measurements of the atmosphere refractivity. We have made an investigation on how accurately we can measure the impact height at ground level using wave optics simulations, realistic refractivity profiles, a realistic simulator for an advanced RO instrument including noise, and using phase matching for the inversion. The idea of the investigation is to increase the measurement accuracy of impact height at low altitudes and to give reliable measurements even in cases of super-refractive layers. We present statistics on the accuracy and precision of the determination of the impact height at ground, as well as the resulting accuracy and precision in the measured refractivity.