GPS + BDS Network Real-Time Differential Positioning Using a Position Domain Estimation Method

The utilization of Global Navigation Satellite System (GNSS) is becoming an attractive navigation approach for geostationary orbit (GEO) satellites. A high-sensitivity receiver compatible with Global Position System (GPS) developed by the United States and BeiDou Navigation Satellite System (BDS) developed by China has been used in a GEO satellite named TJS-5 to demonstrate feasibility of real-time navigation. According to inflight data, the GNSS signal characteristics including availability, position dilution of precision (PDOP), carrier-to-noise ratio (C/N0), observations quantity and accuracy are analyzed. The mean number of GPS and GPS + BDS satellites tracked are 7.4 and 11.7 and the mean PDOP of GPS and GPS + BDS are 10.24 and 3.91, respectively. The use of BDS significantly increases the number of available navigation satellites and improves the PDOP. The number of observations with respect to C/N0 is illustrated in detail. The standard deviation of the pseudorange noises are less than 4 m, and the corresponding carrier phase noises are mostly less than 8 mm. We present the navigation performance using only GPS observations and GPS + BDS observations combination at different weights through comparisons with the precision reference orbits. When GPS combined with BDS observations, the root mean square (RMS) of the single-epoch least square position accuracy can improve from 32.1 m to 16.5 m and the corresponding velocity accuracy can improve from 0.238 m/s to 0.165 m/s. The RMS of real-time orbit determination position accuracy is 5.55 m and the corresponding velocity accuracy is 0.697 mm/s when using GPS and BDS combinations. Especially, the position accuracy in x-axis direction reduced from 7.24 m to 4.09 m when combined GPS with BDS observations.

In-orbit performance of GNOS on-board FY3-C and the enhancements for FY3-D satellite

Because of the special design of BeiDou navigation satellite system (BDS) constellation, the effects of ionospheric scintillation on operational BDS generally are more serious than on the global positioning system (GPS). As BDS is currently providing global services, it is increasingly important to seek strategies to mitigate the scintillation effects on BDS navigation and positioning services. In this study, an improved cycle-slip threshold model is proposed to decrease the high false-alarm rate of cycle-slips under scintillation conditions, thus avoiding the frequent unnecessary ambiguity resets in BDS precise point positioning (PPP) solution. We use one-year (from 23 March 2015 to 23 March 2016) BDS dataset from Hong Kong Sha Tin (HKST) station (22.4°N, 114.2°E; geomagnetic latitude: 15.4°N) to model the cycle-slip threshold and try to make it suitable for three types of BDS satellites and multiple scintillation levels. The availability of our mitigation strategy is validated by using three months (from 1 September 2015 to 30 November 2015) BDS dataset collected at 10 global navigation satellite system (GNSS) stations in Hong Kong. Positioning results demonstrate that our mitigated BDS PPP can prevent the sudden fluctuations of positioning errors induced by the ionospheric scintillation. Statistical results of BDS PPP experiments show that the mitigated solution can maintain an accuracy of about 0.08 m and 0.10 m in the horizontal and vertical components, respectively. Compared with standard BDS PPP, the accuracy of mitigated PPP can be improved by approximately 24.1%, 38.2%, and 47.9% in the east, north, and up directions, respectively. Our study demonstrates that considering different scintillation levels to establish appropriate cycle-slip threshold model in PPP processing can efficiently mitigate the ionospheric scintillation effects on BDS PPP.

Using Global Navigation Satellite System (GNSS) signals of BeiDou navigation satellite system (BDS) and global position system (GPS) for soil moisture estimation is an emerging technology in microwave remote sensing since soil moisture is an important index in regional water cycle research. Based on the developed forward multipath model, this study first investigates the feasibility of soil moisture estimation based on GNSS interference signals; then compares the difference between the Signal-to-Ratio (SNR) data and the signal strength (sn_rnx) data from GPS. After that, this paper analyzes the experimental data from BDS receivers and CORS stations. The analyzed results show that both fitted phase of BDS SNR data and GPS sn_rnx data will increase when the soil moisture increases, and the phase will decrease when soil moisture decreases, and the method of using sn_rnx data can obtain the change trend of soil moisture and can be used as an alternative to solve the problem that the SNR data is unavailable in many GNSS data sets.

PPP-RTK which takes full advantages of both Real-Time Kinematic (RTK) and Precise Point Positioning (PPP), is able to provide centimeter-level positioning accuracy with rapid integer Ambiguity Resolution (AR). In recent years, with the development of BeiDou Navigation Satellite System (BDS) and Galileo navigation satellite system (Galileo) as well as the modernization of Global Positioning System (GPS) and GLObal NAvigation Satellite System (GLONASS), more than 140 Global Navigation Satellite System (GNSS) satellites are available. Particularly, the new-generation GNSS satellites are capable of transmitting signals on three or more frequencies. Multi-GNSS and multi-frequency observations become available and can be used to enhance the performance of PPP-RTK. In this contribution, we develop a multi-GNSS and multi-frequency PPP-RTK model, which uses all the available GNSS observations, and comprehensively evaluate its performance in urban environments from the perspectives of positioning accuracy, convergence and fixing percentage. In this method, the precise atmospheric corrections are derived from the multi-frequency and multi-GNSS observations of a regional network, and then disseminated to users to achieve PPP rapid AR. Furthermore, a cascade ambiguity fixing strategy using Extra‐Wide‐Lane (EWL), Wide-Lane (WL) and L1 ambiguities is employed to improve the performance of ambiguity fixing in the urban environments. Vehicle experiments in different scenarios such as suburbs, overpasses, and tunnels are conducted to validate the proposed method. In suburbs, an accuracy of within 2 cm in the horizontal direction and 4 cm in the vertical direction, with the fixing percentage of 93.7% can be achieved. Compared to the GPS-only solution, the positioning accuracy is improved by 87.6%. In urban environments where signals are interrupted frequently, a fast ambiguity re-fixing can be achieved within 5 s. Moreover, multi-frequency GNSS signals can further improve the positioning performance of PPP-RTK, particularly in the case of small amount of observations. These results demonstrate that the multi-frequency and multi-GNSS PPP-RTK is a promising tool for supporting precise vehicle navigation.

The demand for more precise positioning method using GNSS (Global Navigation Satellite System) in Indonesia continue to rise. The accuracy of GNSS positioning depends on the length of baseline and the distribution of observed satellites. BeiDou Navigation Satellite System (BDS) is a positioning system owned by China that operating in Asia-Pacific region, including Indonesia. This research aims to find out the contribution of BDS in increasing the accuracy of long baseline static positioning in Indonesia. The contributions are assessed by comparing the accuracy of measurement using only GPS (Global Positioning System) and measurement using the combination of GPS and BDS. The data used is 5 days of GPS and BDS measurement data for baseline with 120 km in length. The software used is open-source RTKLIB and commercial software Compass Solution. This research will explain in detail the contribution of BDS to the accuracy of position in long baseline static GNSS measurement.

Ground‐based augmentation systems (GBASs) are widely used augmentation systems using satellite navigation. GBAS can improve the performance of BeiDou navigation satellite system (BDS) in civil aviation application. Evaluation is necessary before using a GBAS. On the basis of the flight trials conducted at the Dongying airport, evaluation results show that based on both the global positioning system (GPS) L1 signal and BDS B1I signal, ground accuracy designators (GADs) of four reference receivers are consistent with GAD‐C levels. The improvement in accuracy obtained with GBAS differential process is ∼60%, and the differential position error based on the BDS B1I signal is 40% greater than that of the GPS L1 signal, because currently incomplete BDS satellites distribution is worse than that of GPS. Airborne protection levels assessments show that the approach trials based on the BDS B1I signal exhibit several false alarm events. Further analysis indicates that BDS vertical protection levels were larger because of larger pseudoinverse S result from low elevation angle and unsatisfactory geometric distribution of BDS satellites. Besides, monitors show that BDS B1I signal is more affected by ionospheric delay than GPS L1 signal, which can also contribute to false alarm events.

The construction of the third generation of the Chinese BeiDou navigation satellite system (BDS), which provides global positioning, navigation, and timing (PNT) services, will be completed in 2020. As BDS satellites contain the geostationary orbit (GEO), inclined geosynchronous (IGSO), and medium earth orbit (MEO), orbital manoeuvres occur more frequently. Thus, orbit manoeuvre detection is important for continuous and reliable PNT services. A BDS orbit manoeuvre detection method based on the combination of global positioning system (GPS) and BDS observations is proposed in this study. The standard deviations of the observation residuals of the epoch-differenced velocity estimation of the single BDS system and combined GPS + BDS system were compared to determine the beginning and end of the orbit manoeuvre. The results show that the orbital manoeuvre leads to a position, velocity, and receiver clock error bias of approximately tens to hundreds of metres, several centimetres per second, and tens to hundreds of metres, respectively. Based on the proposed method, the start and end times can be detected in real time and the usable observation time can be extended by >157 min.

Monitoring and evaluation algorithm of GNSS signal in space availability

The world's first dual-system compatible Global Navigation Satellite System (GNSS) Occultation Sounder (GNOS) for Beidou Navigation Satellite System (BDS) and Global Positioning System (GPS) was successfully launched into orbit with FY-3C satellite on 23 September 2013, and a large number of ionospheric BDS Radio Occultation (BDSRO) data have been collected. The ionospheric BDSRO products observed by FY-3C GNOS are introduced firstly, then the distribution of ionospheric Electron Density Profiles (EDPs) in October 2013 is analyzed. Finally, the precision of ionospheric BDSRO data obtained by FY-3C GNOS is verified through F<sub>2</sub>-layer peak electron density (<i>N</i><sub>m</sub><i>F</i><sub>2</sub>) comparisons between BDSRO and ionosondes. The results show that the correlation coefficient of<i>N</i><sub>m</sub><i>F</i><sub>2</sub> data between BDSRO and ionosondes is 0.96, the bias is 10.21% and the standard deviation is 19.61%, which is comparable with other international ionospheric occultation products based on GPS. The <i>N</i><sub>m</sub><i>F</i><sub>2</sub> precision of BDSRO has the following characteristics. Its precision at daytime is higher than that at nighttime, and its precision in summer is higher than that in equinox seasons, while the precision in winter is lowest. Moreover, its precision at mid-latitude region is higher than that at low-latitude region, and the precision at high-latitude region is lowest. The precision of BDS satellites on Inclined Geosynchronous Satellite Orbit (IGSO) is higher than that on Geosynchronous Orbit (GEO) and Medium Earth Orbit (MEO). The precision consistency of the ionospheric occultation product between BDS-FY-3C GNOS and other internationl occultation projects is of great significance for comprehensively utilizing the GNSS occultation observation data. With the continued deployment of FY-3 satellites and Beidou navigation satellites, GNOS can provide more ionospheric occultation data in the future, which will be helpful to the ionospheric research and space weather forecasting based on GNSS occultation observations.

With the rapid development of the Beidou satellite navigation system (BDS), assessment of its signal quality and navigation performance becomes more and more important. The pseudo-range multipath, carrier-phase signal-to-noise ratio (SNR) and accuracy of standard point positioning are usually taken as the indicators for assessment. In this research, we compared the signal quality and navigation performance of BDS with other three Global Navigation Satellite System (GPS, GLONASS, Galileo). The results of the Asia-Pacific region show that the pseudo-range multipath error for BDS is similar with GPS whereas the GLONASS code measurements exhibit a higher level. The level of carrier-phase signal-to-noise ratio for Galileo is largest, but similar for BDS, GPS and GLONASS. The standard point navigation and positioning performance of BDS is slightly lower than GPS, but it is better than GLONASS, and the accuracy of BDS, GPS and GLONASS standard point positioning is about 10 m. The pseudo-range multipath error of BDS increases as the satellites elevation decreases, but the carrier-phase signal-to-noise ratio is opposite. The variation of pseudo-range multipath error of BDS GEO satellites is small except for low satellites elevation, but the variation of signal-to-noise ratio of GEO satellites is relatively stable. The variation of multipath and signal-to-noise ratio of BDS IGSO and MEO satellites is relatively stable when the satellites elevation is larger than a certain value.

Global navigation satellite systems (GNSSs) are widely utilized in various scientific domains such as agriculture, health monitoring, weather forecasting, and crop health detection. GNSS is a satellite network utilized in positioning and navigation including location-based services and precise orbit determination. The satellite navigation system offers various advantages such as real-time location and temporal synchronization. The Global Positioning System (GPS) is one of the well-known GNSSs, operated by the United States. Some of the other examples may include GPS; Galileo; Russia’s GLONASS, known as the Global Navigation Satellite System; and the BeiDou Navigation Satellite System (BDS). GNSS has found many applications in the field of precision agriculture, timing and synchronization in telecommunication, search and rescue operations, military and defense purposes, Earth observation, construction, and weather forecasting. However, GNSS has also been explored in the accurate and reliable positioning and timing information for climate change analysis and modeling. Therefore, it is essential to examine the potential of GNSS in climate monitoring and modeling. This chapter will present a review of GNSS types, GNSS remote sensing applications, and challenges faced by GNSS. Additionally, this chapter also explores the meta-analysis of the publications or research contributions made by different authors in the field of GNSSs.

The code-phase divergences, which are minimal for GPS, GLONASS, and Galileo satellites, are commonly found in BeiDou Navigation Satellite System (BDS) Geostationary Orbit (GEO), Inclined GeoSynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites. Several precise positioning applications which use code observations are severely affected by these code biases. We present an analysis of code bias based on multipath (MP) combination observations. To mitigate the effect of BDS code bias on precise positioning, we proposed a periodical correction method using a low-pass filter for BDS GEO, IGSO and MEO satellites. The auto-correlation of MP series over long periods is analyzed to obtain the periods of the dominant repeating components for three types of BDS satellites. The periods of the dominant daily repeating components are close to 86,160 s for BDS GEO and IGSO satellites while 603,120 s for MEO satellites. The zero phase-shift low-pass filter was used to extract the low-frequency components of MP series and then low-frequency components are applied to mitigate the code bias periodically. The developed correction methods can make a more remarkable improvement for the accuracy of MP series, compared to the current elevation-dependent correction models. Data sets collected at 50 Global Navigation Satellite System (GNSS) ground stations including 15 of the International GNSS Monitoring and Assessment System (iGMAS) and 35 of the Multi-GNSS Experiment (MGEX) stations are employed for this study. To analyze the influence of code bias on precise positioning and validate the effectiveness of the correction methods, some applications such as single point positioning (SPP), wide-lane (WL) ambiguity analysis and Uncalibrated Phase Delays (UPDs) estimation are conducted. After applying the proposal correction method to the code observations, SPP solutions outperform the uncorrected ones in term of positioning accuracy. The positioning accuracy decreased by 0.28 and 0.1 m in the north and east components and the improvements are more significant for the U components decreased by 0.42 m. In addition, the systematic variations of Melbourne-Wübbena (MW) combination are greatly removed and the convergence time of the MW series are decreased. Moreover, significant improvement is also achieved in terms of the usage rate and residuals of UPDs estimation.

Ground based augmentation system (GBAS) is the only global navigation satellite system (GNSS) with augmentation that can support category III precision approach and landing for aircrafts in the near future. The State Key Laboratory of Air Traffic Management System and Technology has developed a GBAS prototype that based on the Beidou Navigation Satellite System (BDS). Here, an assessment of BDS GBAS performance during position domain trials has been given with respect to requirements for the GBAS approach service type C (GAST C), which permits aircraft approach and landing down to the zero visibility conditions of category I approaches. The experimental results show that the accuracy of the airborne terminal is sub-meter level after the pseudorange has been corrected, and the availability of the BDS GBAS for GAST C is greater than 0.99999. The BDS GBAS is implemented within the required accuracy, integrity and availability standards for GAST C. However, due to the poor distribution of satellites geometry in some time periods, the availability of the BDS GBAS cannot meet the required criteria for GAST-D approaches. In the future, a dual-frequency and dual-constellation GBAS based on the BDS and GPS will be developed to improve the integrity and availability of the system.

The dual-system compatible Global Navigation Satellite System(GNSS) occultation sounder (GNOS) for Beidou Navigation Satellite System (BDS) and Global Positioning System(GPS) was launched into orbit with FY3C satellite on September 23, 2013, in which we can get an important ionospheric parameter: F2-layer peak electron density (NmF2), it can limit the maximum feasible frequency for ground signal propagation, as well as defining the minimum required frequency for transionospheric radio wave propagation, so it's a crucial parameter for charactering ionosphere and the space weather. In this presentation, we verify the precision of ionospheric BDS radio occultation (BDSRO) data obtained by FY3C/GNOS through NmF2 comparisons between BDSRO and ionosondes. The results show that the correlation coefficient of NmF2 data between BDSRO and ionosondes is 0.96, the bias is 10.21 % and the standard deviation (std) is 19.61 %. The NmF2 precision of BDSRO at daytime is higher than that of nighttime; When Beidou satellites are at Inclined Geosynchronous Satellite Orbit (IGSO), its precision is higher than that at Geosynchronous orbit (GEO) and Medium Earth Orbit (MEO).