LEO Navigation Augmentation Observations for Bottomside Ionospheric Electron Content Estimation: Results From Two CENTISPACE Satellites

The burgeoning development of low Earth orbit (LEO) constellations for positioning, navigation, and timing (PNT) 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 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 Global Navigation Satellite System (GNSS) observations, as well as the differential slant bottomside electron content (dSBEC) derived from LEO observations. The vertical bottomside electron content (VBEC) fit by two distinct methods, i.e., grid map and polynomial methods, achieved fitting residuals with root-mean-square (rms) values of 1.2 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 (TEC). 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.