Real-time determination of the free electrons distribution in the ionosphere: Some motivations, collaborative activities and potential evolution
Authors: Hernández-Pajares Manuel , Liu Qi , Yang Heng , Monte-Moreno Enric , Roma-Dollase David , García-Rigo Alberto 
Published: August 2022

&lt;p&gt;Presently, the high and increasing number of worldwide permanent Global Navigation Satellite System (GNSS) receivers providing the raw measurements in real-time (RT) and openly from a large fleet of GNSS transmitters, has opened the door to an accurate real-time determination of the distribution of the ionospheric electron content, either at global scale.&lt;/p&gt;
&lt;p&gt;The motivations are diverse for such a scientific and technological challenging target, for instance: (1) to facilitate the fast carrier phase ambiguity resolution of roving users and corresponding fast decimeter-error level navigation at large distances (hundreds of kilometers) from supporting permanent GNSS receivers (see Hern&amp;#225;ndez-Pajares et al. 2000, Juan et al. 2012, Olivares-Pulido et al. 2021); and (2) the possibility of generating in real-time realistic global maps of ionospheric storm state and Vertical Total Electron Content (VTEC) gradients like it can be done with a latency of 1 day (Liu et al. 2021a, 2022 respectively) based on UQRG GIMs, one of the best behaving ones in the International GNSS Service (IGS), see Roma-Dollase et al. (2018). This might be feasible thanks the recent advances in individual and collaborative real-time global VTEC mapping (see Yang et al. 2021 and Liu et al. 2021b, respectively) and in global VTEC forecasting (Monte-Moreno et al. 2022). These potential new RT products might be useful as ionospheric storm semaphores and quantifiers for example for contributing to the integrity of single-frequency GNSS based navigation in civil aviation.&lt;/p&gt;
&lt;p&gt;One possible evolution in this field might include the consistent 4D combination of the already existing RT GNSS measurements with the potential RT additional geodetic measurements sensitive to the ionospheric delay, like Doppler measurements (DORIS), and like dual-frequency vertical delay measurements over the oceans provided by altimeters, such as JASON-3 among others (Hern&amp;#225;ndez-Pajares et al. 2021). This would significantly extend the coverage and accuracy at global scale, improving as well the vertical resolution.&lt;/p&gt;
&lt;p&gt;References&lt;/p&gt;
&lt;p&gt;Hern&amp;#225;ndez&amp;#8208;Pajares, M., Juan, J. M., Sanz, J., &amp; Colombo, O. L. (2000). Application of ionospheric tomography to real&amp;#8208;time GPS carrier&amp;#8208;phase ambiguities resolution, at scales of 400&amp;#8211;1000 km and with high geomagnetic activity. Geophysical Research Letters, 27(13), 2009-2012.&lt;/p&gt;
&lt;p&gt;Juan, J. M., Hern&amp;#225;ndez-Pajares, M., Sanz, J., Ramos-Bosch, P., Aragon-Angel, A., Orus, R., ... &amp; Tossaint, M. (2012). Enhanced precise point positioning for GNSS users. IEEE transactions on geoscience and remote sensing, 50(10), 4213-4222.&lt;/p&gt;
&lt;p&gt;Liu, Q., Hern&amp;#225;ndez&amp;#8208;Pajares, M., Lyu, H., Nishioka, M., Yang, H., Monte&amp;#8208;Moreno, E., ... &amp; Or&amp;#250;s&amp;#8208;P&amp;#233;rez, R. (2021a). Ionospheric storm scale index based on high time resolution UPC&amp;#8208;IonSAT global ionospheric maps (IsUG). Space Weather, 19(11), e2021SW002853.&lt;/p&gt;
&lt;p&gt;Liu, Q., Hern&amp;#225;ndez-Pajares, M., Yang, H., Monte-Moreno, E., Roma-Dollase, D., Garc&amp;#237;a-Rigo, A., ... &amp; Ghoddousi-Fard, R. (2021b). The cooperative IGS RT-GIMs: A reliable estimation of the global ionospheric electron content distribution in real time. Earth System Science Data, 13(9), 4567-4582.&lt;/p&gt;
&lt;p&gt;Liu, Q., Hern&amp;#225;ndez&amp;#8208;Pajares, M., Yang, H., Monte&amp;#8208;Moreno, E., Garc&amp;#237;a&amp;#8208;Rigo, A., Lyu, H., ... &amp; Or&amp;#250;s&amp;#8208;P&amp;#233;rez, R. (2022). A New Way of Estimating the Spatial and Temporal Components of the Vertical Total Electron Content Gradient Based on UPC&amp;#8208;IonSAT Global Ionosphere Maps. Space Weather, 20(2), e2021SW002926.&lt;/p&gt;
&lt;p&gt;Monte-Moreno, E., Yang, H., &amp; Hern&amp;#225;ndez-Pajares, M. (2022). Forecast of the Global TEC by Nearest neighbour technique. Remote Sensing, 14(6), 1361.&lt;/p&gt;
&lt;p&gt;Olivares-Pulido, G., Hern&amp;#225;ndez-Pajares, M., Lyu, H., Gu, S., Garc&amp;#237;a-Rigo, A., Graffigna, V., ... &amp; Or&amp;#250;s-P&amp;#233;rez, R. (2021). Ionospheric tomographic common clock model of undifferenced uncombined GNSS measurements. Journal of Geodesy, 95(11), 1-13.&lt;/p&gt;
&lt;p&gt;Roma-Dollase, D., Hern&amp;#225;ndez-Pajares, M., Krankowski, A., Kotulak, K., Ghoddousi-Fard, R., Yuan, Y., ... &amp; G&amp;#243;mez-Cama, J. M. (2018). Consistency of seven different GNSS global ionospheric mapping techniques during one solar cycle. Journal of Geodesy, 92(6), 691-706.&lt;/p&gt;
&lt;p&gt;Yang, H., Monte-Moreno, E., Hern&amp;#225;ndez-Pajares, M., &amp; Roma-Dollase, D. (2021). Real-time interpolation of global ionospheric maps by means of sparse representation. Journal of Geodesy, 95(6), 1-20.&lt;/p&gt;