Weimin Zhen

Ionospheric Correction Based on Ingestion of Global Ionospheric Maps into the NeQuick 2 Model

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.

A preliminary study of the NeQuick model over China using GPS TEC and ionosonde data

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.

Ionospheric TEC disturbances based on GPS observations

To indicate the ionospheric TEC variability, it is introduced in this paper an ionospheric disturbance index DI, which is defined as the relative deviation of the vertical total electron content. Based on the GPS observations at Qingdao and 5 stations along 116 degree longitude, specifications of the TEC variations are discussed, particularly for the upper and lower 5% values of DI. The criteria for identifying TEC storm event are proposed by combining the intensity and the duration of the DI variations. Moreover, the characters of TEC storm events in China region are presented.

On the occurrence of F region irregularities over Haikou retrieved from COSMIC GPS radio occultation and ground‐based ionospheric scintillation monitor observations

In this paper, the amplitude scintillation index (s4) derived from COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) Radio Occultation (RO) technique and ground-based Ionospheric Scintillation Monitor (ISM) at Haikou station (Geo. Lat.: 20.0°N, Geo. Lon.: 110.3°E, Mag. Lat.: 10.02°N) is used to investigate the morphology of F-region irregularities in the low latitudes of China. The RO events of tangent point within the range of 10-30°N latitude, 70-160°E longitude, and 150-500 km altitude are adopted to analyze the ionospheric scintillation characteristics. The percentage of ionospheric scintillation occurrence is computed to obtain its diurnal variations, seasonal trends and the dependence on solar and geomagnetic activities. Based on a statistical analysis of a long-term period dataset (year 2007 to 2013), we found that the ionospheric scintillation occurrence from both techniques show similar variations. After sunset (18 LT), the scintillation occurrence increases rapidly and reaches the maximum 3 hours later. Then it decreases rapidly till 04 LT and remains low level during the daytime. The ionospheric scintillation tends to occur more frequently during vernal and autumnal equinoxes, especially in March-April and September-October. The equinoctial asymmetry could be seen clearly from the ground-based ISM observations. The peak ionospheric scintillation occurrence time varies with seasons. It is reached latest in summer while in spring it is very close to that in autumn. The nighttime ionospheric scintillation occurrence tends to increase with increasing solar activities. The increasing tendency is more prominent in vernal and autumnal equinox than that in summer and winter. In general, the control of geomagnetic activities is apt to inhibit ionospheric scintillation at equinox nighttime. In summer and winter, the geomagnetic activities could either trigger or inhibit the generation of ionospheric irregularities in a much more complicated way. Thus it can be concluded that the tangent point location does accurately represent the scattering region, at least in an average sense. The RO technique is demonstrated to be a useful tool for remotely sensing the terrestrial ionosphere on a global scale down to the regional scale in terms of scintillation occurrence.

GPS-based ionospheric tomography with constrained IRI as a regularization

Recent developments in ionosphere remote sensing, in particular techniques using the global positioning system (GPS) provide an unprecedented capability for monitoring the state of the ionosphere, its reaction to solar-terrestrial events as well as ionospheric wave phenomena. There have been many methods that were suggested to study the feasibility of using the International Reference Ionosphere (IRI) model as a regularization profile for ionospheric tomography experiments. Due to the limited-angle geometry, the vertical precision of ground-based GPS ionospheric tomography still needs to be improved to meet the requirements of some applications, such as monitoring the ionospheric distributions during the ionospheric storm periods. HmF2 (peak height of F2 layer of ionosphere) obtained by vertical soundings as a constrained parameter to IRI will improve the vertical resolution of the ground-base GPS ionospheric tomography. Vertical and oblique sounding data obtained from CRIRP while TEC data obtained from a meridional chain of 6 GPS satellite receivers located in China are used for reconstruction of the mid and low latitude ionosphere, maximum entropy regularization method was used to solve the discrete ill-posed problems, numerical simulations show that the constrained IRI has the potential of becoming a effective regularization profile for ground-based GPS ionospheric tomographic reconstruction of the ionosphere. It has also been observed that the foF2 values obtained from reconstructed images that use constrained IRI as a regularization profile are closer to measured values than those obtained from the images reconstructed with the original IRI regularization profile.

Comparison of TEC measurements from DORIS with the NeQuick model

Doppler Orbitography and Radio positioning Integrated by Satellite(DORIS) is a high precision satellite orbit determination and ranging system based on Doppler shift measurements from dual frequency signals propagated between ground stations and a satellite. The Doppler shift measured on the 2.036GHz signal and an additional 401.25MHz is a very potential means for scientific researches on ionosphere. In this paper, global Total Electron Content(TEC) measuring principle of the DORIS system is analyzed, and an algorithm is presented to derive the space variations of TEC from the DORIS ionospheric correction data. Finally comparison of TEC latitudinal profiles are done to with the NeQuick model. And some advices on the improvement of the DORIS system are put forward for advantaging ionospheric sounding and tropospheric sounding.

A Study on Construction of Ionospheric Spatial Threat Model for China SBAS

Finite sampling of the ionosphere from the limited number of observations will threat the integrity of SBAS (Space-Based Augmentation System). Ionospheric spatial threat model is introduced to over-bound residual errors in delay estimation for this under-sampled scenario. Ionospheric anomaly existing in low latitude areas exerts considerable influence on threat model realization. Methods are presented to construct spatial threat model for China area with data during periods of severe storms, including the modeling of variogram, selection of disturbance detection threshold and design of data-deprivation strategy. Verification is also conducted to show its applicability in SBAS with GNSS data from China and around areas from typical storm events.

Scintillation Modeling and Its Application in GNSS

A method to analysis GNSS effects flexibly under various scintillation conditions is presented with a combination of irregularity strength modeling and signal propagating model. Irregularity strength model describes the spatial and temporal variation of strength of scintillation-producing irregularities. The model could be represented in terms of solar activity, magnetic latitudes, season and local time, and the coefficients are derived with measurements from China areas. Irregularity strength distribution, combined with propagating model, could produce the simulated time series of scintillation affected signals received by users. The combination also could give the scintillation index at user receivers under various geometry configurations, which could be further used to GNSS availability analysis under different scintillation conditions.

Ionospheric correction based on NeQuick 2 model adaptation to Global Ionospheric Maps

Global Ionospheric Maps (GIMs), computed by Center for Orbit Determination in Europe (CODE) during a period longer than an entire 11 years solar activity cycle, have been used as the primary source of data to provide global ionospheric corrections for possible single frequency positioning applications. The aim of the investigation is to assess the performance of NeQuick 2 model in providing global total electron content (TEC) prediction after ingesting GIMs data from previous day(s). The results show good performance of the GIMs-driven-NeQuick results with average cumulative distribution function (CDF) of vTEC error not to exceed 5 TECU or 20% (zi 20 ) near 76.70%. The performance of GIMs-driven-NeQuick also presents variability with solar activity, and behaves better during low solar activity years. There are also seasonal trends of the performance. Generally, zi 20 values are bigger in summer and winter and lower in spring and autumn. The differences between in summer and in winter are quite small. And as more measurements from earlier days are used, the accuracies may decrease.

A new function-based computerized ionospheric tomography algorithm

A new function-based computerized ionospheric tomography (CIT) algorithm for beacon measurements from satellite is proposed in this paper. It employs relative TEC measurements as input source data, and spherical harmonic function (SHF) and empirical orthogonal functions (EOF) as the basis function to represent spatial and temporal variation of ionosphere. Truncated singular value decomposition (TSVD) regularization is proposed for solving the ill-posed matrix inversion problem. The accuracy and reliability of this algorithm has been verified by simulated results.