The Real-Time Working Group (RTWG) of the International GNSS Service (IGS) is
dedicated to providing high-quality data and high-accuracy products for Global
Navigation Satellite System (GNSS) positioning, navigation, timing and Earth
observations. As one part of real-time products, the IGS combined Real-Time
Global Ionosphere Map (RT-GIM) has been generated by the real-time weighting
of the RT-GIMs from IGS real-time ionosphere centers including the Chinese
Academy of Sciences (CAS), Centre National d'Etudes Spatiales (CNES),
Universitat Politècnica de Catalunya (UPC) and Wuhan University
(WHU). The performance of global vertical total electron content (VTEC)
representation in all of the RT-GIMs has been assessed by VTEC from
Jason-3 altimeter for 3 months over oceans and dSTEC-GPS technique with
2

The global ionosphere maps (GIMs), containing vertical total electron content
(VTEC) information at given grid points (typically with a spatial resolution
of 2.5

In order to satisfy the growing demand for real-time GNSS positioning and
applications, the Real-Time Working Group (RTWG) of IGS was established in 2001
and officially started to provide real-time service (RTS) in 2013

The accuracy of RT-GIMs is typically worse than post-processed GIMs due to the
short span of ionospheric observations, sparse distribution of stations,
higher noises in carrier-to-code leveling, or difficulty in carrier ambiguity
estimation in real-time processing mode. While RT-GIMs perform slightly worse
than post-processed GIMs, it is found that RT-GIMs are helpful to reduce the
convergence time of dual-frequency precise point positioning (PPP), and they
strengthen the solution

Recently, one of the IGS RT-GIMs (UPC-IonSAT) has completely changed the
real-time interpolation strategy, with a significant improvement. In addition,
the number of contributing centers has been increased from three to four, thanks to
the participation of Wuhan University. A new version of IGS combined RT-GIM
(IRTG) has been developed to improve the performance and also adapt to the
newly updated IGS-SSR format. In addition, the developed software has been
further parallelized to decrease the latency of IRTG computation to a few
minutes

In order to generate RT-GIMs, the real-time GNSS observations from worldwide stations are received and transformed into slant TEC (STEC). It should be noted that extraction of STEC in an unbiased way can be obtained by fitting an ionospheric model to the observations. With the global distributed STEC, different strategies are chosen for the computation of RT-GIMs.

Currently, two methods are commonly used for the calculation of real-time
STEC. The first method is the so-called carrier-to-code leveling (CCL) as
shown in Eq. (

The brief summary of different IGS RT-GIMs.

The second method is the GF combination of phase-only observations, and the

The strategies for generating RT-GIMs differ between IGS real-time ionospheric analysis centers (ACs). In this subsection, a brief introduction on the generation of RT-GIMs from individual ACs and the strategy comparison between different ACs are given.

The post-processed GIM of CAS has been computed and uploaded to IGS since 2015

The real-time STEC is computed by subtracting estimated DCB in
Eq. (

In the framework of the RTS of the IGS, CNES has computed global VTEC in real time
thanks to the CNES PPP-WIZARD project since 2014. The real-time VTEC is
extracted by pseudorange and carrier phase GF combination as
Eq. (

CNES also uses a spherical harmonic model for global VTEC representation, and the
equation is the same as Eq. (

UPC has been providing daily GIMs in IONEX format to IGS since 1998

From 2011 to 2019, the kriging technique is selected by UPC for real-time VTEC
interpolation. And the spherical harmonic model has been adopted by UPC since
8 September 2019. Recently, a new interpolation technique, denoted atomic
decomposition interpolator of GIMs (ADIGIM), was developed. Since the
global ionospheric electron content mainly depends on the diurnal, seasonal
and solar variation, ADIGIM is computed by the weighted combination of
good-quality historical GIMs (e.g., UQRG) with similar ionosphere
conditions. The database of historical GIMs covers the last two solar cycles
since 1998. The method for obtaining the weights of the linear combination of
past maps is based on Eq. (

The daily rapid and final GIM products have been generated with new WHU
software named GNSS Ionosphere Monitoring and Analysis Software (GIMAS) since
21 June 2018

WHU uses the spherical harmonic expansion model, and the formula is identical
to Eq. (

In the year 2021, WHU is going to focus on how to further improve the accuracy of RT-GIM and update the computation method. The precise WHU RT-GIMs with multi-GNSS data and the application of WHU RT-GIM in the GNSS positioning as well as space physics domain are expected as next steps.

Thanks to the contribution of the initial IGS real-time ionosphere centers
(CAS, CNES and UPC) and globally distributed real-time GNSS stations, the
first experimental IRTG was generated by means of the real-time dSTEC (RT-dSTEC)
weighting technique (normalized inverse of the squared rms of RT-dSTEC error)
in October 2018

In addition, the RT-dSTEC assessment is based on root mean square (rms) of the
dSTEC error calculated by Eq. (

The current status of broadcasting IGS RT-GIMs.

The 25 common real-time stations for RT-dSTEC assessment (in green) and 50 external GNSS stations for dSTEC-GPS assessment (in red).

Data flow for the IGS real-time combined GIM.

Due to the limited number of real-time stations, 25 common real-time stations
that have been used by all the IGS real-time ionosphere centers are selected
for allowing a fair RT-dSTEC assessment. The distribution can be seen as
Fig.

In this section, the performance of IGS RT-GIMs was analyzed and compared
with rapid IGS GIMs as well as IGS combined final GIM. It should be noted that
the RT-GIMs were gathered with BKG Ntrip Client (BNC) software

The ID of compared IGS RT-GIMs.

Before detailing the Jason-3 VTEC and GPS-dSTEC assessment, it should be taken
into account that the GIM error versus Jason VTEC measurements have a high
correlation with the GIM error versus dSTEC-GPS measurements, although
the Jason VTEC measurements are vertical and the dSTEC-GPS measurements are
slanted. As demonstrated in

The VTEC from the Jason-3 altimeter was gathered as an external reference over
the oceans. After applying a sliding window of 16

The recent 3-month data (1 December 2020 to 1 March 2021), containing the two significant events (new contributing RT-GIM (WHU) from 3 January 2021 and the introduction of the new atomic decomposition UPC-GIM (UADG) on 4 January 2021), have been selected to study the consistency and performance of the IGS RT-GIMs.

As can be seen in Fig.

Daily standard deviation of GIM VTEC versus measured Jason-3 VTEC (in TECU), from 1 December 2020 to 1 March 2021.

Standard deviation of GIM-VTEC minus Jason-3 VTEC in Jason-3 VTEC assessment (last two columns) and dSTEC-GPS assessment results of RT-GIMs on 3 January (second and third columns) and 5 January (fourth and fifth columns) in 2021.

The value in bold font means the corresponding RT-GIM has the best performance among the remaining RT-GIMs in each column, and values of irtg are italic for comparison.

In order to investigate the influence of temporal resolution on RT-GIMs over
oceans, different RT-GIMs with full temporal resolution were involved. The
summary of Jason-3 VTEC assessment can be seen in
Table

In addition, dSTEC-GPS assessment in post-processing mode was involved as a
complementary tool with high accuracy (better than 0.1 TECU) over continental
regions on a global scale. In the dSTEC-GPS assessment, the maximum elevation
angle within a continuous arc was regarded as the reference angle in
Eq. (

The distribution of dSTEC-GPS results on 5 January 2021 (after the improvement of the UPC interpolation technique).

As shown in Table

RT-dSTEC assessment of RT-GIMs was automatically running in real-time mode
and used for real-time weighting in the combination of IGS RT-GIMs. In order
to compare with the dSTEC-GPS assessment, the RT-dSTEC assessment with
real-time stations in Fig.

RMSE of RT-GIMs in RT-dSTEC assessment on 3 and 5 January 2021.

The value in bold font means the corresponding RT-GIM has the best performance among the remaining RT-GIMs in each column.

The evolution of real-time weights and daily winning epochs of RT-GIMs.

The GEC, ap and Dst evolution of RT-GIMs from 24 to 29 January 2021 during the low-solar-activity period.

The real-time weights of RT-GIMs can be defined as the normalized inverse of
the squared rms of RT-dSTEC errors and represent the accuracy of RT-GIMs in
the RT-dSTEC assessment. For each RT-GIM, the number of daily winning epochs
is computed by counting the number of epochs within the day when the one
RT-GIM is better than the other RT-GIMs. The evolution of daily winning epochs
of RT-GIMs shown in the bottom figure of Fig.

The global electron content (GEC) is defined as the total number of free
electrons in the ionosphere. Hence the GEC can be estimated from the summation
of the product of the VTEC value and the area of the corresponding GIM
cell. In addition, GEC has been used as an ionospheric index

The IGS real-time combined GIMs during the testing period are available from Zenodo at

In this paper, we have summarized the computation methods of RT-GIMs from four
individual IGS ionosphere centers and introduced the new version of IGS
combined RT-GIM. According to the results of Jason-3 VTEC and dSTEC-GPS
assessment, it could be concluded as follows.

The real-time weighting technique for the generation of IGS combined RT-GIM performs well when it is compared with Jason-3 VTEC and dSTEC-GPS assessment.

The transition of UPC RT-GIM from USRG to UADG is obvious in all involved assessments and also demonstrates the sensibility of the real-time weighting technique to RT-GIMs when the accuracy of RT-GIMs is increased.

The quality of most IGS RT-GIMs is close to post-processed GIMs.

The difference among RT-GIMs with 20

Broadcast real-time rms maps that can be useful for the positioning users.

Increase the accuracy of high-temporal-resolution RT-GIMs. In addition, higher maximum spherical harmonic degrees might be adopted to increase the accuracy and spatial resolution of RT-GIMs.

Coinciding with a much larger number of RT-GNSS receivers in the future, the dSTEC weighting might be improved by replacing the “internal” with the “external” receivers, i.e., not used by any real-time analysis centers. In this way the weighting would be sensitive as well to the interpolation–extrapolation error of the different real-time ionospheric GIMs to be combined. And the resulting combination might behave better.

Increase the number of worldwide GNSS receivers used for the RT-dSTEC up to more than 100. In this way we will be able to study the potential upgrade of the present global weighting to a regional weighting among other potential improvements in the combination strategy.

QL wrote the manuscript. QL developed the updated combination software with contributions from DRD, HY and MHP. QL and MHP designed the research, with contributions from HY, EMM, DRD and AGR. QL, HY, EMM, MHP, ZL, NW, DL, AB, Q. Zhao and Q. Zhang provided the real-time GIMs of the corresponding IGS centers. AH, MS, GW and AS contributed in creating the framework of the real-time IGS service, the ionospheric message format and BNC open software updates. LA suggested the initial idea of this work. AK, StS, JF, AK, RGF and AGR contributed in the generation of rapid and final IGS GIMs used as additional references in the manuscript.

The contact author has declared that neither they nor their co-authors have any competing interests.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

The authors are thankful to the collaborative and friendly framework of the International GNSS Service, an organization providing first-class open data and open products

This research has been supported by the China Scholarship Council (CSC). The contribution from UPC-IonSAT authors was partially supported by the European Union-funded project PITHIA-NRF (grant no. 101007599) and by the ESSP/ICAO-funded project TEC4SpaW. The work of Andrzej Krankowski is supported by the National Centre for Research and Development, Poland, through grant ARTEMIS (grant nos. DWM/PL-CHN/97/2019 and WPC1/ARTEMIS/2019).

This paper was edited by Christian Voigt and reviewed by two anonymous referees.