Atmospheric Remote Sensing Using Global Navigation Satellite Systems: From FORMOSAT-3/COSMIC to FORMOSAT-7/COSMIC-2

The world's first Global Positioning System Radio Occultation (GPS-RO) constellation, the joint Taiwan-U.S. Formosa Satellite No. 3 - Constellation Observing System for Meteorology, Ionosphere & Climate (FORMOSAT-3/COSMIC) has demonstrated the value of GPS-RO data in terrestrial weather, space weather, and climate study. The RO measurement technique was dubbed as "the most accurate and stable thermometer in space" because of its high accuracy, high vertical resolution, and high stability, which does not require calibration. Since its launch in 2006, FORMOSAT-3/COSMIC has accumulated over 6.9 million atmospheric and 4.6 million ionospheric RO sounding profiles. Encouraged by the success of FORMOSAT-3/COSMIC, Taiwan's National Space Organization (NSPO) and the U.S. National Oceanic and Atmospheric Administration (NOAA) started the planning for the FORMOSAT-7/COSMIC-2 mission in 2008 as a follow-on program to FORMOSAT-3/COSMIC. The FORMOSAT-7/COSMIC-2 satellite constellation, consisting of six small satellites, was successfully launched in June, 2019 as the world's first operational RO satellite constellation focusing over the tropics. After fourteen years in service, providing valuable data to operational and research communities, FORMOSAT-3/COSMIC was officially decommissioned ten months after FORMOSAT-7/COSMIC-2 has been sent to space, and replaced by the FORMOSAT-7/COSMIC-2. As expected, the first results from FORMOSAT-7/COSMIC-2 showed positive impact on operational weather prediction and has observed interesting phenomenon in the ionosphere. In this paper, we will summarize the progress from FORMOSAT-7/COSMIC-2 up to date.