GNSS-RO Sounding in the Troposphere and Stratosphere

The use of Global Navigation Satellite System Radio Occultations (GNSS-RO) for vertical sounding of temperature and moisture in the atmospheric column has become a standard practice of many numerical weather prediction (NWP) centers. The introduction of this observation has seen broad positive impact on analyses and forecasts. On longer timescales the impact of the introduction of this data type in re-analyses can be clearly seen. Further, the observations can be used without bias correction and the consistency between sensors is very good allowing these observation to serve as anchoring observations. This is particularly helpful to constrain the bias-correction applied to satellite radiances. In the following chapter we explore the fundamentals of the measurement, the derivation of the typical observation which is used in NWP, the assimilation methods and error assumptions which are used, and finally some conjecture on the direction to improve the use of the observations and what future measurement systems may look like.