The core performance elements of global navigation satellite system include availability, continuity, integrity and accuracy, all of which are particularly important for the developing BeiDou global navigation satellite system (BDS-3). This paper describes the basic performance of BDS-3 and suggests some methods to improve the positioning, navigation and timing (PNT) service. The precision of the BDS-3 post-processing orbit can reach centimeter level, the average satellite clock offset uncertainty of 18 medium circular orbit satellites is 1.55 ns and the average signal-in-space ranging error is approximately 0.474 m. The future possible improvements for the BeiDou navigation system are also discussed. It is suggested to increase the orbital inclination of the inclined geostationary orbit (IGSO) satellites to improve the PNT service in the Arctic region. The IGSO satellite can perform part of the geostationary orbit (GEO) satellite’s functions to solve the southern occlusion problem of the GEO satellite service in the northern hemisphere (namely the “south wall effect”). The space-borne inertial navigation system could be used to realize continuous orbit determination during satellite maneuver. In addition, high-accuracy space-borne hydrogen clock or cesium clock can be used to maintain the time system in the autonomous navigation mode, and stability of spatial datum. Furthermore, the ionospheric delay correction model of BDS-3 for all signals should be unified to avoid user confusion and improve positioning accuracy. Finally, to overcome the vulnerability of satellite navigation system, the comprehensive and resilient PNT infrastructures are proposed for the future seamless PNT services.

/pdf/basic-performance-and-future-developments-of-beidou-global-18ysrcaj1o.pdf

Basic performance and future developments of BeiDou global navigation satellite system

In order to better understand the differential code biases (DCBs) of global navigation satellite system, the IGGDCB method is extended to estimate the intra- and inter-frequency biases of the global positioning system (GPS), GLONASS, BeiDou navigation satellite system (BDS), and Galileo based on observations collected by the multi-GNSS experiment (MGEX) of the international GNSS service (IGS). In the approach of IGGDCB, the local ionospheric total electronic content is modeled with generalized triangular series (GTS) function rather than using a global ionosphere model or a priori ionospheric information. The DCB estimated by the IGGDCB method is compared with the DCB products from the Center for Orbit Determination in Europe (CODE) and German Aerospace Center (DLR), as well as the broadcast timing group delay (TGD) parameters over a 2-year span (2013 and 2014). The results indicate that GPS and GLONASS intra-frequency biases obtained in this work show the same precision levels as those estimated by DLR (about 0.1 and 0.2–0.4 ns for the two constellations, respectively, with respect to the products of CODE). The precision levels of IGGDCB-based inter-frequency biases estimated over the 24-month period are about 0.29 ns for GPS, 0.56 ns for GLONASS, 0.36 ns for BDS, and 0.24 ns for Galileo, respectively. Here, the accuracies of GPS and GLONASS biases are assessed relative to the products of CODE, while those of BDS and Galileo are compared with the estimates of DLR. In addition, the monthly stability indices of IGGDCB-based DCBs are 0.11 (GPS), 0.18 (GLONASS), 0.17 (BDS), and 0.14 (Galileo) ns for the individual constellation.

Determination of differential code biases with multi-GNSS observations

Measurements of Global Navigation Satellite System
(GNSS) receivers are affected
by systematic offsets re-
lated to group and phase delays of the signal generation
and processing chain. The resulting code and phase biases
depend on the transmission frequency and the employed
signal modulation. Within this study differential code
biases (DCBs) of legacy an
d modernized GNSS signals
are derived from pseudodrange observations of a global
multi-GNSS receiver network.
Global ionosphere maps
(GIMs) are employed for th
e correction of ionospheric
path delays. Satellite and receiver-specific contributions
are separated based on the a
ssumption of additive biases
and a zero-mean condition for the satellite biases within a
constellation. Based on 6 months of data collected within
the Multi-GNSS Experiment (M
GEX) of the International
GNSS Service (IGS), DCBs for the publicly available
signals of GPS, Galileo and BeiDou have been deter-
mined. The quality of the resulting DCB estimates is as-
sessed and compared against group delay parameters
transmitted by the GNSS providers as part of the broad-
cast ephemeris data.

Differential Code Bias Estimation using Multi-GNSS Observations and Global Ionosphere Maps

This contribution summarizes the strategy used by Wuhan University (WHU) to determine precise orbit and clock products for Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS). In particular, the satellite attitude, phase center corrections, solar radiation pressure model developed and used for BDS satellites are addressed. In addition, this contribution analyzes the orbit and clock quality of the quad-constellation products from MGEX Analysis Centers (ACs) for a common time period of 1 year (2014). With IGS final GPS and GLONASS products as the reference, Multi-GNSS products of WHU (indicated by WUM) show the best agreement among these products from all MGEX ACs in both accuracy and stability. 3D Day Boundary Discontinuities (DBDs) range from 8 to 27 cm for Galileo-IOV satellites among all ACs’ products, whereas WUM ones are the largest (about 26.2 cm). Among three types of BDS satellites, MEOs show the smallest DBDs from 10 to 27 cm, whereas the DBDs for all ACs products are at decimeter to meter level for GEOs and one to three decimeter for IGSOs, respectively. As to the satellite laser ranging (SLR) validation for Galileo-IOV satellites, the accuracy evaluated by SLR residuals is at the one decimeter level with the well-known systematic bias of about $$-5$$
  cm for all ACs. For BDS satellites, the accuracy could reach decimeter level, one decimeter level, and centimeter level for GEOs, IGSOs, and MEOs, respectively. However, there is a noticeable bias in GEO SLR residuals. In addition, systematic errors dependent on orbit angle related to mismodeled solar radiation pressure (SRP) are present for BDS GEOs and IGSOs. The results of Multi-GNSS combined kinematic PPP demonstrate that the best accuracy of position and fastest convergence speed have been achieved using WUM products, particularly in the Up direction. Furthermore, the accuracy of static BDS only PPP degrades when the BDS IGSO and MEO satellites switches to orbit-normal orientation, particularly for COM products, whereas the WUM show the slightest degradation.

Precise orbit determination for quad-constellation satellites at Wuhan University: strategy, result validation, and comparison

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

A new technique for mapping of total electron content using GPS network in Japan

Ionospheric correction using GPS Klobuchar coefficients with an empirical night-time delay model

The ionospheric delay is one of the most important error sources affecting the accuracy of Global Navigation Satellite Systems (GNSS), especially for single-frequency users. It is important to investigate various methods to mitigate the ionospheric effects on GNSS single-point single-epoch (SPSE) positioning when using single-frequency receivers. Based on the GPS data gathered at 5 International GNSS Service (IGS) stations at the mid-low latitude regions in China, we accessed the performances of different ionospheric corrections in terms of ionospheric Total Electron Content (TEC) correction accuracy and SPSE positioning during periods of high (2002) and low (2009) solar activity respectively. Meanwhile, the impacts of ionospheric corrections on GNSS positioning (especially on horizontal accuracy) were analyzed, and the relationship between TEC and positioning residual errors was also discussed. The results show that all ionospheric corrections can obviously improve the vertical accuracy, while the horizontal improvement is less than the vertical one, and even worse, the horizontal error may become larger with ionospheric corrections. Furthermore, the north error plays an important role in limiting the horizontal accuracy. If north error was not properly corrected with ionospheric models, it would worsen the horizontal positioning performance.

Impact of Ionospheric Correction on Single-Frequency GNSS Positioning

A method, which can invert the ionospheric electron content above the CHAMP satellite, is presented. The daytime behavior of the equatorial ionospheric anomaly (EIA) is monitored using the CHAMP global positioning system data and the ionosphere map exchange format data supplied by the Center for Orbit Determination in Europe during the period at quiet geomagnetic conditions between November 7 and December 12, 2003. Results show that the plasma congregates and rises above the geomagnetic equator due to the daytime upward vertical drift during the early variations of EIA. After being lifted to higher altitudes, the topside ionospheric equatorial plasma above the CHAMP orbit (over 400 km above the ground) controlled mainly by the electromagnetic field starts to diffuse downward to higher latitudes along magnetic field lines, and the double-humped structure begins to appear in the topside ionosphere above the CHAMP orbit. However, a single crest is still observed for the entire ionosphere above the ground, and this single peak moves slightly to the winter hemisphere as a result of the transequatorial neutral wind. With the continuous diffusion of the plasma, the two crests also occur for the entire ionosphere at both sides of the geomagnetic equator.

Monitoring the Daytime Variations of Equatorial Ionospheric Anomaly Using IONEX Data and CHAMP GPS Data

The International Reference Ionosphere (IRI) is an empirical model widely used to describe ionospheric characteristics. In the previous research, high-precision total ionospheric electron content (TEC) data derived from global navigation satellite system (GNSS) data were used to adjust the ionospheric global index IG12 used as a driving parameter in the standard IRI model; thus, the errors between IRI-TEC and GNSS-TEC were minimized, and IRI-TEC was calibrated by modifying IRI with the updated IG12 index (IG-up). This paper investigates various interpolation strategies for IG-up values calculated from GNSS reference stations and the calibrated TEC accuracy achieved using the modified IRI-2016 model with the interpolated IG-up values as driving parameters. Experimental results from 2015 and 2019 show that interpolating IG-up with a 2.5° × 5° spatial grid and a 1-h time resolution drives IRI-2016 to generate ionospheric TEC values consistent with GNSS-TEC. For 2015 and 2019, the mean absolute error (MAE) of the modified IRI-TEC is improved by 78.57% and 77.42%, respectively, and the root mean square error (RMSE) is improved by 78.79% and 77.14%, respectively. The corresponding correlations of the linear regression between GNSS-TEC and the modified IRI-TEC are 0.986 and 0.966, more than 0.2 higher than with the standard IRI-TEC.

Algorithm Research Using GNSS-TEC Data to Calibrate TEC Calculated by the IRI-2016 Model over China

Ionospheric total electron content (TEC) is important data for ionospheric morphology, and also an important parameter for ionospheric correction in Global Navigation Satellite System (GNSS) precise positioning, navigation, and radio science. In this study, we present a data assimilation model for regional ionosphere based on a local ensemble Kalman filter (LEnKF) with the International Reference Ionosphere 2016 (IRI-2016) model as the background, to assimilate ionospheric TEC observations from GNSS. To demonstrate the results, the TEC estimates from the Crustal Movement Observation Network of China (CMONOC), which is about 260 stations in China, are applied as observation. The assessments are performed against the TEC estimates from BeiDou Navigation Satellite System (BDS) geostationary earth orbit (GEO) and against the final products from the Center for Orbit Determination in Europe (CODE). The assimilation results are in good agreement with BDS GEO TEC and the CODE TEC on a quiet or disturbed day. The correlation coefficient after assimilation is increased by about 17% compared with that before assimilation, and the RMSE after assimilation is decreased by about 42% compared with that before assimilation. Furthermore, the assimilated method is also evaluated in the single-frequency precise point positioning (PPP). The experimental results indicate that the PPP/Assimilated method can improve the GNSS positioning accuracy more effectively in comparison to the PPP/CODE. These results reveal that the LEnKF method can be considered as a useful tool for ionospheric assimilation.

/pdf/ionospheric-assimilation-of-gnss-tec-into-iri-model-using-a-1f4371h1.pdf

Xingliang Huo

Papers

Ionospheric correction using GPS Klobuchar coefficients with an empirical night-time delay model

Impact of Ionospheric Correction on Single-Frequency GNSS Positioning

Monitoring the Daytime Variations of Equatorial Ionospheric Anomaly Using IONEX Data and CHAMP GPS Data

Algorithm Research Using GNSS-TEC Data to Calibrate TEC Calculated by the IRI-2016 Model over China

Ionospheric Assimilation of GNSS TEC into IRI Model Using a Local Ensemble Kalman Filter