Initial Polarimetric Radio Occultation Results from Spire’s Nanosatellite Constellation: Independent Assessment and Potential Applications

Abstract Global Navigation Satellite System (GNSS) polarimetric radio occultation (PRO) is an extension of the traditional radio occultation (RO) technique that collects two linear and orthogonal polarization components of the occulting GNSS signals in order to retrieve precipitation information along with the standard RO thermodynamic products. This technique has already been demonstrated with the RO and Heavy Precipitation (ROHP) experiment aboard the PAZ satellite, a large platform launched in 2018. In early 2023, Spire launched the first three nanosatellites capable of collecting PRO measurements from low-Earth orbit. This study, performed independently from the data providers, assesses the Spire PRO observations comparing them with ancillary information from global precipitation missions. The results are compared with those obtained from the ROHP- PAZ instrument, showing a good agreement between the three Spire nanosatellites and PAZ . Furthermore, unlike PAZ , Spire nanosatellites are able to collect measurements from the four major GNSS constellations and are placed in orbits that are convenient for obtaining coincident observations from the different nanosatellites. A study of the potential scientific and meteorological applications of such a small PRO constellation is also presented, with emphasis on the resulting clusters of observations around interesting meteorological events, such as tropical cyclones (TCs) or mesoscale convective systems (MCSs). Examples of such clusters and their statistics are provided, highlighting the potential impact of expanding the set of quality observations over these extreme events by means of CubeSat constellations. Significance Statement This study has assessed and explored the potential meteorological applications of three commercial nanosatellites with Global Navigation Satellite System (GNSS) polarimetric radio occultation capability. It is the first time that such measurements are obtained from CubeSats, and the assessment presented here has been performed independently from the data providers. Comparisons with the proof-of-concept mission Radio Occultation and Heavy Precipitation (ROHP)- PAZ and IMERG precipitation products demonstrate that Spire polarimetric radio occultation (PRO) observations are sensitive to the presence and intensity of precipitation and its vertical structure. Furthermore, applications using the three CubeSats as a mini-constellation that delivers spatially closely collocated measurements with a close and controlled temporal spacing are assessed. The results show potential for scientific studies and emphasize the need to work toward using these data for assimilation into NWP models.