# Applications of GNSS-RO to Numerical Weather Prediction and Tropical Cyclone Forecast
### RO Missions  
The successful GPS/Meteorology (GPS/MET) experiments conducted in 1995 ushered in the era of RO application for atmospheric measurements, given that these experiments helped derive 0-60 km atmospheric parameter profiles [5]. Subsequently, scientists performed several studies on GPS/MET RO meteorological data and compared the results with those obtained  
Figure 1: Schematic of radio occultation [4]. In the legend, the GNOS-BD (BeiDou) /GPS means global navigation satellite system (GNSS) occultation sounder (GNOS) received signals from Beidou/GPS satellite. The radio occultation (RO) phenomenon occurs when a GPS signal from the GNSS penetrates the Earth’s atmosphere and is then received by low-earth orbit (LEO) satellites behind the earth. using radiosonde [6] and Lidar [7]. These studies have confirmed the accuracy of RO data, although GPS/MET provided only a small amount of RO data. Therefore, a series of RO missions have been scheduled for the following years. These missions include Challenging Mini-satellite Payload (CHAMP) [8; 9]; Gravity Recovery and Climate Experiment (GRACE) and Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) [10]; Scientific Application Satellite-C/D (SAC-C/D) [11]; TerraSAR-X; TanDEM-X; Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/COSMIC-2, also known as FORMOSAT-3/ FO RMOSAT-7); the Meteorological Operational satellite Programme-A/B/C (MetOp-A/B/C); FengYun-3C/D (FY-3C/D) (Sun, 2018); Communications/Navigation Outage Forecasting System(C/NOFS); Korea Multi-Purpose Satellite-5 (KOMPSAT-5); the Indian Space Research Organization spacecraft Ocean Satellite-2 (OceanSat-2); and Spanish PAZ (peace in Spanish). A few missions have been retired, such as COSMIC-1, GRACE, CHAMP, and SAC-C/D, and some missions will be completed by the end of 2020, such as FY-3C, TanDEM-X/TerraSAR-X, KOMPSAT-5, OceanSat-2, and C/NOFS. More missions have been planned for the future such as MetOp Second Generation (MetOp-SG), FengYun-3E/F/G/H (FY-3E/F/G/H), TerraSAR-X Next Generation (TSX-NG), Jason Continuity of Service-A/B (JASON-CS-A/B, also known as Sentinel 6A/6B), and Meteor-MP N1/N2. By 2025, the planned operational missions will provide approximately 14,700 RO soundings per day [12]. Herein, we introduce three primary missions accounting for most of the RO data: COSMIC, MetOp, and FY-3 series. 1. COSMIC. On 15 April 2006, the first satellite constellation COSMIC designed for RO measurements was launched. It marked the beginning of the wide use of RO data in atmospheric and ionospheric sounding research from an operational standpoint [13]. Since then, COSMIC has provided 1500-2000 soundings per day globally, significantly contributing to numerical weather prediction (NWP) and meteorological studies [14]. Later, its follow-up mission was COSMIC-2, launched in June 2019; COSMIC-2 has provided approximately 5000 soundings per day over 35\({}^{\circ}\) N-35\({}^{\circ}\) S, mainly for tropical cyclone (TC) forecasting [15]. Such data are useful for short- and medium-range forecasts in the tropics, given the more accurate measurements of the temperature, humidity, and wind field [16]. The European Centre for Medium-Range Weather Forecasts (ECMWF) started to assimilate COSMIC-2 data at a rate of approximately 5000 soundings per day since March 2020. 2. MetOp. MetOp-A/B/C are three polar-orbiting meteorological satellites that form the space segment component of the overall EUMETSAT Polar System (EPS). MetOp-A was launched in 2006, and its RO data were found to contribute to weather forecasts up to 10 days ahead. This mission was followed by MetOp-B and C. Since March 2019, the ECMWF started to assimilate the bending angle from MetOp-C, with a quality similar to both MetOp-A and B measurements. Currently, the MetOp constellation can provide approximately 2000 soundings per day. As a next step, the EUMETSAT group has planned to launch six MetOp-second-generation (MetOp-SG) satellites. MetOp SG-A is scheduled for launch at the end of 2022. 3. FY-3C/D.