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On June 23, 2020, the last BDS-3 satellite was launched, which means that the China BDS finished its global system construction. The BDS-3 adopts a new generation global broadcast ionospheric delay correction model (BDGIM) for the single frequency ionospheric delay correction. This paper describes the performance of BDGIM during the period before and after the establishment of the BDS-3 system, in terms of the accuracy of ionospheric delay correction, BDS single-frequency pseudorange positioning and the broadcast model coefficients. To access the performance of BDGIM, the high-precision global ionospheric map (GIM) and the measured ionospheric electron content (TEC) data are selected as references. The accuracy of GPS Klobuchar model and the BDS-2 Klobuchar model are also analyzed. The results show that the accuracy of ionospheric delay correction of the BDGIM did not change significantly before and after the completion of the BDS-3 constellation. Taking the final GIM product released by the International GNSS Service (IGS) as a reference, the ionospheric correction percentages of the BDGIM model in China, the Asia-Pacific and global regions reached 84.5%、74.6% and 64.4%, respectively. Taking the ionospheric TEC measured by BDS and GPS data of 83 global GNSS stations as a reference, the ionospheric correction percentages of BDGIM in China,Asia-Pacific and global regions are 74.3%、70.5% and 68.6%, respectively. When the BDGIM model is applied to BDS single-frequency pseudorange positioning, the three-dimensional positioning accuracy of 2.22、2.66 and 2.96 m has been achieved in China, Asia-Pacific and the global regions, respectively. Different evaluation results show that the average correction accuracy of the BDGIM model is superior to the BDS Klobuchar model and the GPS Klobuchar model.

Galileo adopts NeQuick model for single-frequency ionospheric delay corrections. For the standard operation of Galileo, NeQuick model is driven by the effective ionization level parameter Az instead of the solar activity level index, and the three broadcast ionospheric coefficients are determined by a second-polynomial through fitting the Az values estimated from globally distributed Galileo Sensor Stations (GSS). In this study, the processing strategies for the estimation of NeQuick ionospheric coefficients are discussed and the characteristics of the NeQuick coefficients are also analyzed. The accuracy of Global Position System (GPS) broadcast Klobuchar, original NeQuick2 and fitted NeQuickC as well as Galileo broadcast NeQuickG models is evaluated over the continental and oceanic regions, respectively, in comparison with the ionospheric total electron content (TEC) provided by global ionospheric maps (GIM), GPS test stations and JASON-2 altimeter. The results show that NeQuickG can mitigate ionospheric delay by 54.2%~65.8% on a global scale, and NeQuickC can correct for 71.1%~74.2% of the ionospheric delay. NeQuick2 performs at the same level with NeQuickG, which is a bit better than that of GPS broadcast Klobuchar model.

With the introducing of GPS (Global Positioning System) Block ⅡR-M satellites in 2005, a new civil signal (L2C) was transmitted on L2 frequency, and two new signals (L5I and L5Q) on a new frequency (L5) were also introduced as standard signals with Block ⅡF satellites beginning in 2010. In addition to the timing group delay (TGD) parameter contained in the legacy navigation (LNAV) message, four additional inter-signal correction (ISC) parameters are introduced in the new civil navigation (CNAV) message to provide corrections for L1C/A, L2C and L5 signals with respect to L1P(Y) signal. In this study, the ISC correction models are first developed for GPS single-and dual-frequency navigation users. Thereafter ISCs are validated with the differential code bias (DCB) products of different organizations, and several standard point positioning (SPP) schemes are also carried out to analyze the impact of ISCs on GPS standard positioning. The results indicate that the precision of broadcast ISCC/A is about 0.2 ns, and those of TGD, ISCL2C and ISCL5Q are about 0.5 ns. The positioning accuracy of the SPP solution based on GPS L2C signals applying ISC corrections improves 30.6% and that based on L1C/A and L2C ionosphere-free combination improves 12.2% at 12 multi-GNSS experiment (MGEX) stations during day of year (DOY) 300-310 in 2014, compared to the solutions without the consideration of ISCs. For GPS dual-frequency users, the broadcast ISCs enable a level of accuracy that is competitive with ionosphere-free combination of the present L1P(Y) and L2P(Y) signals.