# Global GNSS-RO Electron Density in the Lower Ionosphere
## 2 Data and Methods
### Low-Rate (1 Hz) POD and High-Rate (50-100 Hz) RO Data  
Both precise orbit determination (POD) and RO antennas can track GNSS signals at negative elevation angles or, in other words, at \(h_{t}\) below the LEO receiver altitude. Generally speaking, the POD antennas have a wider field of view (FOV), lower SNR, and a coarser sampling resolution, compared to the RO antennas. Depending on the POD receiver design, receiver satellite altitude, and data acquisition schedule, the POD 1 Hz occultations usually have a smaller number of profiles (10-30% in COSMIC-1 podTEC) reaching the \(D\)/\(E\)-region. There are slightly more POD occultation profiles reaching the \(D\)/\(E\)-region when the satellite is in a lower orbital altitude. Thus, the higher RO cutoff tangent height and noisier \(hTEC\) measurements make the POD data less attractive for the \(D\)/\(E\)-region study. In this study, we focus mainly on utilizing the high-rate (50-100 Hz) RO data, of which the sampling and spatiotemporal coverage are much higher than the POD data. The dense spatiotemporal sampling is critically needed to characterize the lower ionosphere variability. In addition, the high-rate data have advantages to detect and correct \(\phi_{ex}\) measurement errors such as clock time and cycle slips in the presence of ionospheric scintillations [42]. Smaller \(\phi_{ex}\) measurement errors help to retrieve a low \(N_{e}\) concentration in the lower ionosphere. In recognition of the high-rate RO measurements for ionospheric observations, the MetOp-A operation conducted a short experiment with the high-rate RO acquisition up to \(h_{t}\) \(\sim\) 290 km during 2020d161-2020d254 (9 June-10 September 2020). The success of the experiment allows the extended acquisition mode to continue from 2021d201 to the decommission in December 2021. As shown later in Section 3, the high-rate provides a significant benefit to the \(N_{e}\) retrievals in the \(E\)- to \(F\)-region transition and the retrieval of \(TEC\). In addition to the extended high-rate profile from MetOp-A, the Spire GNSS-RO constellation, which NOAA uses to fill the polar latitudes outside the COSMIC-2 coverage,has been routinely acquiring the high-rate RO profile up to \(h_{t}\sim 175\) km. Among the earlier missions, the C/NOFS (Communications/Navigation Outage Forecasting System) had a frequent high-rate RO data reaching up to \(h_{t}\sim 165\) km from a low-inclination (\(13^{\circ}\)) elliptical orbit. The high-rate data in the _D/E/F_-region also provide a valuable diagnostic for better understanding and quantifying the RIES of \(\phi_{ex}\) measurements in the ionosphere. The high-rate data help to reduce some of the scintillation-induced \(\phi_{ex}\) measurement noise, but the high-order frequency dependence and multi-path effects remain as a source of the \(\Delta hTEC\) errors in Equation (9) [31]. As shown in Figure 2, \(E_{s}\)-induced phase scintillation can become a large random error source of \(\Delta hTEC\) at \(h_{t}\) below the \(E_{s}\) layer. An RIE from the \(F\)-region can sometimes induce a bias. If the bias is confined in a narrow height region, its impact on the \(N_{e}\) retrieval is generally small. But if the bias is present at all heights, the induced bias can be significant in the retrieved \(N_{e}\). RIES can also have a non-negligible impact on the neutral atmospheric refractivity retrieval. Bending angle (\(\alpha\)), which is used to derive the atmospheric refractivity, is essentially a vertical derivative of the \(\phi_{iono\_free}\) profile, i.e., \(\alpha\cong-d\phi_{iono\_free}/dh_{t}\). Therefore, a bias from RIES would not be a problem for the \(\alpha\) retrieval, but a slope or vertical gradient of RIES would. The RIE-induced \(\alpha\) error has been a major issue in the climate-quality data analysis of the upper stratosphere [43; 44].