In this chapter, the basic methodology to derive the geophysical parameters from GNSS RO refractivity will be explored in Sect. 6.1. In the following Sect. 6.2, the detailed characteristics of RO observables will be studied. The science application utilizing such RO derived geophysical parameters, spanning from atmospheric dynamic process to weather and climate studies, will be presented from Sects. 6.3, 6.4 and 6.5. Some future missions of GNSS RO and other novel application of the RO techniques will be discussed in Sect. 6.6, such as the airborne/mountain-top GNSS RO and LEO-to-LEO occultation.
