Can we use Commercial RO Spire Data Collected from CubeSats for Climate and Weather Prediction Studies?

The GNSS (Global Navigation Satellite System) Radio Occultation (RO) technique provides unique measurements of the vertical profile of atmospheric density, from which we can derive temperature and, to some extent, humidity in the neutral atmosphere. For this reason, NOAA assimilates approximately 10K RO occultation profiles per day into its numerical weather prediction (NWP) system from COSMIC-2 and other government-sponsored satellites. To provide additional sources of GNSS RO data, NOAA initiated the Commercial Weather Data Pilot (CWDP) program to assess commercially available GNSS RO data. After two rounds of pilot studies, the NOAA awarded the first Indefinite Delivery Indefinite Quantity (IDIQ) contract to GeoOptics and Spire in 2020. Unlike government-sponsored RO missions with more expensive receivers and larger antennae, GeoOptics and Spire used smaller receivers on CubeSats. To use these RO data from commercial CubeSats for climate and weather prediction studies, we must carefully quantify their accuracy, precision, and impact on applications. To answer these and related questions, the NOAA Center for Satellite Applications and Research (STAR) has established a GNSS RO Data Science Center to develop enterprise RO processing algorithms for all RO missions, as has been done for NOAA’s infrared and microwave satellite missions. The STAR RO inversion package can now process data from COSMIC-2, Spire, and many other RO missions.

In this study, we focused on evaluating commercial RO data from Spire. We compared the Spire observations to data from COSMIC-2, high-quality radiosonde data, and model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis version 5 (ERA5). We compared the STAR-processed Spire products with those from UCAR to evaluate the differences in the processing algorithm implementations. The results demonstrate that although Spire has a lower Signal-Noise-Ratio (SNR) in general, the pattern of the lowest penetration height for Spire is similar to that for COSMIC-2. Spire and COSMIC-2 penetration heights are between 0.6 and 0.8 km in tropical ocean regions. Although using different GNSS RO receivers, the precision of Spire profiles is of the same quality as those of COSMIC-2. The retrieval accuracy from Spire is also very compatible with those from COSMIC-2. We estimated the error covariance matrix for Spire, COSMIC-2, and KOMPSAT-5. Results indicate that the COSMIC-2 estimated error covariance values are slightly more significant over the oceans in the mid-latitudes (45^oN-30^oN and 30^oS-45^oS), which may be due to COSMIC-2’s SNR being lower in those latitudinal zones.

The manuscript contents are solely the opinions of the authors and do not constitute a statement of policy, decision, or position on behalf of NOAA or the U.S. government.