Long Yang

The burgeoning development of low earth orbit (LEO) constellations for positioning, navigation, and timing in recent years has brought about new opportunities for ionospheric sounding below the LEO satellite altitudes. This study established the regional bottomside ionospheric map (RBIM) using navigation augmentation signals from two CENTISPACE ^TM LEO satellites during April 1-7, 2023, under moderate solar activity and quiet geomagnetic conditions. Accuracy was subsequently validated through comparison with multiple datasets, including Global and Regional Ionospheric Maps (GIMs and RIMs) constructed from ground-based GNSS observations, as well as the differential slant bottomside electron content (dSBEC) derived from LEO observations. The vertical bottomside electron content (VBEC) fitted by two distinct methods, i.e., grid map and polynomial methods, achieved fitting residuals with root mean square (RMS) values of 1.2 TECU and 0.7 TECU, respectively. The RBIM precision evaluated by LEO dSBEC reaches 1.0 TECU. Comparing the VBEC from our RBIM to the GIM/ RIM indicates an RMS level mostly within 3-8 TECU. More importantly, the established RBIM facilitates the analysis of the proportional variation of the VBEC over the total electron content. For the one-week LEO observations, an average VBEC proportion is derived at 84% during daytime and 56% during nighttime, indicating a reduction of 33.3%. This result is more realistic than that calculated from the empirical International Reference Ionosphere (IRI-2020) model (5.3%) and is in good agreement with the previous studies. Thus, the RBIM can not only benefit LEO navigation augmentation but also provide significant observations on the vertical distribution of electron content.

Low Earth orbit (LEO) satellites benefit future ground-based positioning with their high number, strong signal strength and high speed. The rapid geometry change with the LEO augmentation offers acceleration of the convergence of the precision point positioning (PPP) solution. This contribution discusses the influences of the LEO augmentation on the precise point positioning—real-time kinematic (PPP-RTK) positioning and its integrity monitoring. Using 1 Hz simulated data around Beijing for global positioning system (GPS)/Galileo/Beidou navigation satellite system (BDS)-3 and the tested LEO constellation with 150 satellites on L1/L5, it was found that the convergence of the formal horizontal precision can be significantly shortened in the ambiguity-float case, especially for the single-constellation scenarios with low precision of the interpolated ionospheric delays. The LEO augmentation also improves the efficiency of the user ambiguity resolution and the formal horizontal precision with the ambiguities fixed. Using the integrity monitoring (IM) procedure introduced in the first part of this series of papers, the ambiguity-float horizontal protection levels (HPLs) are sharply reduced in various tested scenarios, with an improvement of more than 60% from 5 to 30 min after the processing start. The ambiguity-fixed HPLs can generally be improved by 10% to 60% with the LEO augmentation, depending on the global navigation satellite system (GNSS) constellations used and the precision of the ionospheric interpolation.

Currently, low Earth orbit (LEO) satellites are attracting great attention in the navigation enhancement field because of their stronger navigation signal and faster elevation variation than medium Earth orbit (MEO) satellites. To meet the need for real-time and precise positioning, navigation and timing (PNT) services, the first and most difficult task is correcting errors in the process of precise LEO orbit and clock offset determination as much as possible. Launched in 29 September 2018, the CentiSpace-1 (CS01) satellite is the first experimental satellite of LEO-based navigation enhancement system constellations developed by Beijing Future Navigation Technology Co. Ltd. To analyze the impact of the attitude model, carrier phase wind-up (PWU) and phase center variation (PCV) on precise LEO orbit and clock offset in an LEO-based navigation system that needs extremely high precision, we not only select the CS01 satellite as a testing spacecraft, but also the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO). First, the dual-frequency global positioning system (GPS) data are collected and the data quality is assessed by analyzing the performance of tracking GPS satellites, multipath errors and signal to noise ratio (SNR) variation. The analysis results show that the data quality of GRACE-FO is slightly better than CS01. With residual analysis and overlapping comparison, a further orbit quality improvement is possible when we further correct the errors of the attitude model, PWU and PCV in this paper. The final three-dimensional (3D) root mean square (RMS) of the overlapping orbit for GRACE-FO and CS01 is 2.08 cm and 1.72 cm, respectively. Meanwhile, errors of the attitude model, PWU and PCV can be absorbed partly in the clock offset and these errors can generate one nonnegligible effect, which can reach 0.02~0.05 ns. The experiment results indicate that processing the errors of the attitude model, PWU and PCV carefully can improve the consistency of precise LEO orbit and clock offset and raise the performance of an LEO-based navigation enhancement system.