Extraction and Evaluation of Ionospheric Electron Density Using Undifferenced Uncombined GPS Radio Occultation Phase Measurements

Global positioning system (GPS) radio occultation (RO) is a widely used technique for deriving ionospheric electron density profiles (EDPs), especially the first constellation observing system for meteorology, ionosphere, and climate (COSMIC-1) satellites, enriching the amount of measurements over the ocean. In standard data processing, COSMIC-1 EDPs are retrieved based on a dual-frequency (L1 and L2 band) combination of carrier phase measurements (dual-frequency method), which may introduce and amplify tracking errors, particularly from the noisier or unstable L2-band signal. This study proposes an alternative method called the undifferenced uncombined (UDUC) approach for electron density retrieval, which uses only the L1-band carrier phase along with 1-s COSMIC-1 clock offset products. Our analysis shows that the EDPs derived from the UDUC method are highly consistent with those obtained from the dual-frequency method, the international reference ionosphere (IRI) 2020 model, as well as ionosondes. Statistical validation of the peak electron density of the F2 layer (NmF2) and its height (hmF2) shows strong correlations between the UDUC and dual-frequency methods, with Pearson correlation coefficients of 0.92 (0.96) for NmF2 and 0.88 (0.92) for hmF2 during the years of 2009 (2013). Furthermore, the UDUC method is effective for retrieving EDPs even when the L2-band signal is unstable, whereas EDPs derived from the dual-frequency method tend to fluctuate significantly.