The ionosphere has always been one of the important error sources in GNSS navigation and positioning applications. The refined real-time monitoring of the ionosphere can provide more accurate information for calibration of ionospheric errors in high-precision positioning. Based on over 2,000 GNSS monitoring stations of the Beidou Ground-Based Augmentation System, this paper proposed a framework for the development of a high-precision ionospheric monitoring system in China, planning to provide a group of ionospheric products. Taking ROTI map as an example, the post-processing version has a spatial resolution of 0.2° in both longitude and latitude, with a temporal resolution of 30 s. The ROTI map with high-precision can not only indicate the area where the GNSS signal is severely affected by the ionosphere, but also provide an optimal strategy for satellite data selection in high-precision positioning applications.

Ionospheric total electron content (TEC) is an important parameter in ionospheric researches and applications. However, the determination of the absolute value of TEC can be greatly influenced by the differential code biases (DCBs) estimation. Nowadays, there are more and more Global Navigation Satellite System (GNSS) signals available all over the world, which allow us to solve TEC and DCBs with the hypothesis of local spherical symmetry (LSS) imposed on the dual-frequency observations from only one individual station. For comparison, the results based on the global ionospheric map (GIM) act as a reference in this article. On the one hand, different combinations of Global Positioning System (GPS), GLONASS, and BeiDou Navigation Satellite System (BDS) are considered to illustrate the significance of multi-GNSS observations, with the mixed GPS, GLONASS, and BDS combination performing best when compared to the referenced results. On the other hand, different parameters in LSS condition are taken into account to investigate the suitable geometric constraint of LSS, with the differential longitude, latitude, and epoch suggested to be 3.0 & x00B0;, 0.6 & x00B0;, and 4 min, respectively. Moreover, a group of detailed comparisons from several different stations also show that the combined DCBs and ionospheric TEC derived from our method are compatible with those from the GIM-aided method, especially in the low-latitude area. In summary, with the advantage of the multi-GNSS signals from an individual station, our method can estimate the ionospheric TEC and DCBs independently, which could provide a potential tool in the future real-time applications.

Based on the international reference ionosphere (IRI) model, digital ionosonde and GNSS TEC data, we proposed a method which uses the empirical orthogonal function to estimate the topside ionospheric electron density profile and applied it to the Millstone Hill station. We then compared the estimated critical frequency and peak height with the digital ionosonde observations, and compared the estimated electron density above 400 km with density observed by incoherent scatter radar (ISR). Statistical results show that the estimated critical frequency and peak height are in consistent with the digital ionosonde data, the absolute error between modeled and ISR measured density above 400 km reduces 50% in comparison with that between digital ionosonde derivations and ISR. In summary, adding GNSS TEC in addition to digisonde could improve the topside ionospheric electron density estimation by our method accurately.

With the development of Global Navigation Satellite Systems (GNSS), the signal from more than 30 satellites of GNSS can be caught by a receiver in China at a time, which will provide conveniences for estimating instrumental biases of GNSS system. In this paper, instrumental biases of GNSS system have been firstly analyzed under different temperature conditions. The results show that instrumental biases are changed with the rapid change of temperature and the variation of instrumental biases can reach 12. 53 total electron content unit (TECU, 1 TECU= 10(16) el.m(2)). Furthermore, the change of instrumental biases is slow and about 1. 00 TECU under constant or room temperature. Based on the above experiments of instrumental biases, we have proposed a method of estimating GNSS instrumental biases for a receiver of multisystem. The method is named Triangle Decomposition and Difference Elimination (TDDE) method for single station and applied in analyzing GNSS data in Baoding during 2015-2017. Our analysis shows that TDDE is advanced in speed and independence for solving instrumental biases. The result obtained by TDDE in BeiDou Navigation Satellite System is better than that in GPS and GLONASS. Meanwhile, the instrumental biases corrected by TDDE are 2. 50-3. 00 TECU larger than those adjusted by Global Ionosphere Maps from Center for Obit Determination in Europe. In addition, the vertical TEC corrected by TDDE can clearly present diurnal variation, sunrise enhancement, semi-annual and annual variations, and equinoctial asymmetry of ionosphere TEC.

We developed a parameterized ionospheric electron density model based on the IRI-2012 model by spherical harmonic expansions in the horizontal and empirical orthogonal functions in the vertical. Then, after assimilating the monthly multisource total electron content (TEC) data from ground-based GPS, LEO radio occultation (RO), and the oceanic altimeter during magnetically quiet time into the model, we reanalyzed the monthly global ionospheric electron density TEC and other key parameters such as foF2 and NmF2. Both the reanalyzed and IRI-2012 model results were compared to the TEC measurements, the monthly median foF2 in a middle-latitude ionosonde station, and the global TEC map from CODE. The comparisons showed that both the reanalyzed and IRI results are consistent with those observations and the reanalyzed results perform better than the IRI model. Furthermore, the reanalyzed results are also consistent with the retrieved maps of HmF2, NmF2, and TEC from COSMIC RO observations. In summary, our method can reanalyze the global TEC and electron density using multisource TEC data assimilated into our model and improve the performance of IRI model.

On the basis of multiple observations of ionosondes, meteor radars, magnetometers and GNSS receivers, we present the response of the equatorial and low latitude ionosphere over the West Pacific Ocean Sector to an X1.2 solar flare that peaked at 1:48 UT on 15 May 2013. The geomagnetic H component observations indicate the equatorial electrojet strength over the East Asia region is obviously enhancement during the flare. After the end time of solar flare, the ionosonde observations at Guam, an ionosonde station near the geomagnetic equatorial region, show the decrease of the peak height of ionospheric F2 layer which is related to the decrease of the eastward electric field. Simultaneous strong southern wind is observed by meteor radar over Sanya, a geomagnetic low latitude station, which probably product the westward dynamo electric field and further result in the decrease of vertical drift velocity over the geomagnetic equatorial region. In addition, GNSS total electron content (TEC) observations from six stations in the researching region show the TEC enhancement only appears nearby the geomagnetic equator region.

The global ionospheric maps (GIMs), generated by Jet Propulsion Laboratory (JPL) and Center for Orbit Determination in Europe (CODE) during a period over 13 years, have been adopted as the primary source of data to provide global ionospheric correction for possible single frequency positioning applications. The investigation aims to assess the performance of new NeQuick model, NeQuick 2, in predicting global total electron content (TEC) through ingesting the GIMs data from the previous day(s). The results show good performance of the GIMs-driven-NeQuick model with average 86% of vertical TEC error less than 10 TECU, when the global daily effective ionization indices (Az) versus modified dip latitude (MODIP) are constructed as a second order polynomial. The performance of GIMs-driven-NeQuick model presents variability with solar activity and behaves better during low solar activity years. The accuracy of TEC prediction can be improved further through performing a four-coefficient function expression of Az versus MODIP. As more measurements from earlier days are involved in the Az optimization procedure, the accuracy may decrease. The results also reveal that more efforts are needed to improve the NeQuick 2 model capabilities to represent the ionosphere in the equatorial and high-latitude regions.

In this paper, the Global Ionospheric Maps (GIMs) during 1998–2012 have been adopted as the primary source of data to generate the Galileo-like ionospheric correction parameters. Taking the previous day GIMs as ingested dataset and the current day GIMs as reference, we have computed the cumulative distribution function (CDF) of TEC (Total Electron Content) error less than 5 TECU (TEC Unit, 1TECU=1016electron/m2) or 20% (denoted as zi20) to study the performance of NeQuick 2 in providing global ionospheric correction. The statistical analysis is carried out to study how the performance changes with seasons, solar activities and geographic latitudes. Our studies showed that: (1) in general, the NeQuick 2 model driven by the previous day GIMs agrees well with the current day GIMs, with monthly averaged zi20 exceeds 60%. (2) The performance of GIMs-driven-NeQuick model varies with geographic latitudes. The model behaves better in mid-latitudes than in low-latitudes, which is more prominent in northern hemisphere. (3) There is a nonlinear relationship between zi20 and F10.7. When the solar activity level is low or medium, zi20 tends to decrease with increasing solar activities. However, when F10.7 increases further to exceed a certain threshold, zi20 generally shows either saturation or an increase for high solar epoch. The zi20 decreases with F10.7 more sharply in mid-latitudes than in low-latitudes, in the southern hemisphere than in the northern hemisphere. (4) The seasonal behavior of zi20 in different latitudinal belts are quite different. In the southern hemisphere, zi20 mainly presents annual variation, and it is largest in winter and smallest in summer. In the northern hemisphere, the semi-annual variation is dominant, and zi20 is larger in solstice than in equinox. During the High Solar Activity years, the zi20 is much larger in summer than in winter in the low-latitude regions.

This paper presents a preliminary study of the NeQuick model over 3 stations in China, Changchun, Beijing and Chongqing for geomagnetic quiet days. In order to understand its weaknesses and validate its results, we chose to uncouple NeQuick formulation from its underlying data. It is found that NeQuick represents NmF2 and VTEC quite well on monthly average, except for some underestimate. After replacing the CCIR maps of NmF2 and M(3000)F2 by its DGS measurements, the model behaves even better. Based on these results, we conclude that: (1) NeQuick represents ionosphere quite well over China on monthly average. (2) NeQuick has the ability to accommodate other sources of data for its crucial parameters, such as DGS measurement. (3) NeQuick has the potential to represent ionosphere on daily or sub-daily average.

The ionosphere is one of the errors in satellite navigation applications. An ionospheric model is usually adopted to compensate this error in the systems. With a comparison between GPS and GALILEO global ionospheric models in their performance and realization, we make a thorough understanding of the models development and its possible improvements in the future. It will be a good foundation for our effort in the system model development.

In the presence of one dimension electron density disturbance,the model results of nonlinear evolution of the plasma instabilities are presented for ionospheric F region in equator and the low and mid-latitudes,according to different background electric fields conditions during high solar activity spring periods.We found that with the absence of electric field,the density disturbance can evolve into plasma bubbles only in equatorial region which rise beyond the height of maximal electron density in F layer,while eastward electric field is necessary for bubble evolution at low and mid-latitudes.The fact that the necessary eastward electric field tends to become larger for the higher latitudes indicts that,the predominant plasma instabilities mechanisms are different between the equatorial and the low and mid-latitudinal ionosphere.

The TEC data obtained from GPS is actually the slant TEC, i.e., the integration of electronic density along the GPS signal path.In many applications the slant TEC should be converted into vertical TEC.This is done by the so called mapping function which is usually determined by certain models of electron density profile.In this paper we proposed a new method to obtain mapping functions from TEC observation of GPS network.We first discuss the dependence of the mapping function on the'ionospheric height', which is the layer height for Single Layer Model (SLM), or the peak electron density height for the Chapman Model, and found that the value of the mapping function decreases obviously with the increase of the ionospheric height.We then estimated the'experimental mapping function'from the observation of vertical TEC (from JPL GIMs) and slant TEC (from IGS GPS data) during the whole year of 2006.After the comparison between both results, we find the values of the mapping functions from experimental data are much larger than that from the model with reasonable ionospheric height (e.g., 400 km SLM height) , when the zenith angle is large enough.We attribute this to the effect of the plasmasphere which exists above about 1000 km altitude hence may increase the effective'ionospheric height'.It is concluded that the present experimental results may be used to model or improve the vertical TEC mapping function.This might be helpful for retrieving TEC from GPS network.

Ionospheric imaging usually involves solving an underdetermined inversion problem. The inversion involving additional constraints is performed to enforce realistic profiles in the vertical. A new algorithm combined with empirical orthonormal functions (EOFs) analysis and multiplicative algebraic reconstruction technique (MART) for GPS-based ionospheric tomography has been given in this paper. EOFs can be used to reconstruct the background ionospheric electron density effectively with lower distortion, while the MART can lead to more accuracy by iteration. Numerical simulations and its application on actual GPS-based TEC observations proved that this algorithm offers better performance in physical study of the anomaly, particularly under storm events when its morphology deviates from normal.

In this paper, we calculate the ionospheric global electron content (GEC) from the GPS TEC data along the geographic longitude 120°E during the period of 1996–2004, and investigate the relationship between GEC and 10.7 cm solar radio flux F10.7 and its seasonal dependence with partial correlation analysis. Our results show that GEC is closely correlated with solar activity index F10.7 and is also related with annual and semiannual variations. An empirical GEC model driven by those factors is then to examine the influences of different solar activity proxies for the model input. The results suggest that GEC mainly depends on solar activity and the seasonal variations; the latter is also modulated by solar activity. Furthermore, the magnitude of semiannual variation is a little greater than that of annual variation. Our empirical GEC model is proved to be better than the model proposed by Afraimovich et al.

This paper reports a study on the relationship between ionospheric total electron content (TEC) over East Asia and the tropospheric circulation around the Qinghai-Tibet Plateau. Ionospheric TEC over East Asia are obtained from 25 observatories during 1996–2004. By applying a partial correlation method which can eliminate the influences of solar and geomagnetic activities, we find no significant correlation between TEC and the Asian zonal circulation index (Iz), but find a positive correlation between the day-to-day variability of TEC and Iz. We suggest that the positive correlation is closely related with the topography of the Qinghai-Tibet Plateau. The dynamical effect on airflow of the plateau can generate vortexes, and the vortexes may continuously excite internal gravity waves (IGWs) which transmit up to the ionosphere and cause regional wave disturbances. This study gives evidence for the dynamical mechanism of ionosphere–troposphere coupling and shows the importance of the Qinghai-Tibet Plateau in the ionosphere–troposphere coupling over East Asia.

We present 9‐ and 7‐day periodic oscillations in the global mean Total Electron Content (TEC) from 1 January 2005 to 31 December 2006. Spectral analysis indicates that the pronounced periodicities of 9 and 7 days observed in TEC are associated with variations in solar wind high‐speed streams and geomagnetic activity. Neutral temperature and winds near 250 km, measured by a Fabry‐Perot Interferometer at Resolute Bay, also exhibit 9‐ and 7‐day periodicities. These pronounced periodicities support simultaneous observations of 9‐ and 7‐day periodicities in thermosphere neutral density (Lei et al., 2008a; Thayer et al., 2008). It is anticipated that the ionospheric response at 9 and 7 days represents some combination of effects due to chemical loss, neutral winds, and disturbance dynamo‐driven electric fields.

利用120°E子午线上的GPS TEC数据, 计算了1996～2004年间的等效电离层全球电子总含量(GEC). 应用偏相关方法分析了GEC与太阳活动指数F10.7, 以年为基频的季节因子之间的相关性, 发现GEC与F10.7, 年变化和半年变化因子之间均存在较强的相关性. 据此, 构建了一个由太阳活动指数和季节因子共同驱动的GEC经验模式, 并考察了太阳活动指数的修正对建模效果的影响. 结果表明GEC的变化主要由太阳活动周变化, 年变化和半年变化3种分量构成, 其中太阳活动周变化由太阳活动指数控制, 其变化幅度最大; 年变化与半年变化的幅度也受太阳活动指数调制, 半年变化的幅度略大于年变化幅度. 此外, 经过对比分析, 文中构建的GEC经验模型优于Afraimovich等人提出的GEC经验模型.