Initial Polarimetric Radio Occultation Results from Spire’s Nanosatellite Constellation: Satellite Payload, Collection, and Calibration

Abstract Radio occultation of Global Navigation Satellite System (GNSS) signals is a remote sensing technique that provides precise thermodynamic measurements of Earth’s atmosphere, which are routinely assimilated into numerical weather prediction models. Polarimetric radio occultation (PRO) extends the traditional technique by using differences in the linear polarization components of the occulted GNSS signal to extract information about the hydrometeor content along the signal path. In early 2023, Spire launched the first three nanosatellites capable of collecting PRO measurements from low-Earth orbit. This paper highlights the initial collection, processing, and calibration of PRO measurements from Spire satellites for the first time. Three Spire satellites equipped with a PRO payload are capable of producing a total of over 2000 PRO measurements per day, which is approximately 10 times the amount currently available to the community through the Radio Occultation and Heavy Precipitation with PAZ (ROHP- PAZ ) instrument. PRO measurements are collected globally from all four of the major GNSS constellations. An antenna pattern analysis shows that the instrument provides stable measurements that require minimal calibration to demonstrate sensitivity to hydrometeors as determined by collocations with other precipitation products. Furthermore, Spire’s PRO data can be used to retrieve atmospheric bending angle profiles with the same statistical quality as Spire’s traditional RO profiles used in operational numerical weather prediction models. Significance Statement We have demonstrated the novel implementation of polarimetric radio occultations (PROs) on board three small Spire satellites using navigation signals from Global Navigation Satellite System (GNSS) constellations collected by a new antenna. The advantage of PRO compared to standard radio occultations (ROs) is its unique sensitivity to hydrometeors (snow crystals, rain droplets, etc.) along the ray path. We show that the antenna patterns are stable and do not appreciably modify the raw measurements, that the novel use of multiple GNSS constellations is successful, and that PRO datasets can also be used for conventional RO applications, namely, the retrieval of atmospheric bending angle, refractivity, temperature, pressure, and humidity. Future studies will seek to quantify the impact of PRO information when these data are assimilated into numerical weather prediction (NWP) models.