Jiexian Wang

Performance of Real-Time Precise Point Positioning

The IGS Real-time Pilot Project (IGS-RTPP) provides real-time precise orbits and clocks, which support real-time positioning for single stations over large areas using the Precise Point Positioning (PPP) technique. This paper investigates the impact of real-time orbits, network configuration, and analysis strategies on real-time PPP implementation and demonstrates the real-time PPP performance. One month of data from the IGS network is analyzed in a real-time simulation mode. Results reveal the following: (1) In clock estimation, differential approaches are much more efficient than the zero-differenced approach. (2) The precision of IGS Ultra rapid (IGU) orbits could meet the IGS-RTPP requirement for precise clock estimation and PPP positioning. (3) Considering efficiency and precision, a network with 50 stations is recommended for the IGS-RTPP. It is demonstrated that the real-time satellite clock precision is 0.1 ns supporting hourly static PPP with a mean precision of 2–3 cm in the North component and 3–4 cm in the other components. Kinematic PPP assessed with onboard GPS data collected from a buoy provided mean coordinate precision of 2.2, 4.2, 6.1 cm in the North, East and Up directions, compared to the RTK solutions.

Application of Inter-system Hardware Delay Bias in GPS/GLONASS PPP

GPS applies Code Division Multiple Access technique in signal coding, while GLONASS’s signal is produced with Frequency Division Multiple Access technique. The differences in signal frequency results in inter-system hardware delay bias for GPS/GLONASS receivers. Subjecting to these hardware delays, strategies and models of GPS/GLONASS PPP based positioning needs to be modified. We derived a GPS/GLONASS combined PPP positioning models by introducing inter-system hardware delay biases. Several scenarios were simulated to test the introduced models using the observation of IGS stations. The results show that: ① Adding a couple of GLONASS satellites can improve the positioning accuracy in the environment without enough GPS satellites being tracked; ② The inter-system hardware delay bias is stable on daily base, and it could be predicted and be fixed in the GPS/GLONASS combined PPP.

Surveying Colocated GNSS, VLBI, and SLR Stations in China

The local tie vectors between different space geodesy instruments in colocated sites, such as the global navigation satellite system (GNSS), very long baseline interferometry (VLBI), and satellite laser ranging (SLR), are essential for combination with the international terrestrial reference frame (ITRF). This paper introduces the surveying method and data processing model for determining the tie vectors in the seven colocated sites in Shanghai, Wuhan, Kunming, Beijing, Xian, Changchun, and Urumqi, and presents the values and full variance-covariance of these local ties. The surveying methodology and data processing method of the current work are rigorously determined to guarantee the relative positional precision of reference points (RPs) of different instruments in each colocation site to be a few millimeters. This paper compares the new tie vectors with those derived from ITRF 2008 products to consider the discrepancies at tie epoch. By comparing the new results to the previous results by other organizations, the new tie vector at the Wuhan site is in good agreement, but the vertical coordinate difference of the tie vector at the Shanghai site is as large as 2.24 cm. Therefore, the tie vector at the Shanghai site may have changed about 2 cm from 2003 to 2011.

GNSS Satellite Clock Real-Time Estimation and Analysis for Its Positioning

Real-time and high-precision Multi-GNSS positioning technical has been playing an important role in the determination of low earth orbiter (LEO) and monitoring of geologic hazards. The key concern should be on the achievement of the high-precision satellite orbit and clock products. In this paper, real-time clock estimation strategy was introduced. Based on the mean square root filtering method, dates via 35 global uniformly distributed IGS observations were used to estimate real-time satellite clock errors of GPS and CLONASS, which was proved 0.2 ns and 0.8 ns respectively. The outcomes were verified again via precise point positioning. Consequently, compared with the positioning accuracy via only GPS, that of GPS and GLONASS improved 26 % in X direction, 40 % in Y direction and 2 % in Z direction. The convergence time shorten 2 to 4 times as well.

Weekly inter-technique combination of SLR, VLBI, GPS and DORIS at the solution level

Constructing and maintaining a stable terrestrial reference frame (TRF) is one of the key objectives of fundamental astronomy and geodesy. The datumrealization for all the global TRF versions, such as ITRF2014 and its predecessor ITRF2008, assumes linear time evolution for transformation parameters and then imposes some conditions on these Helmert transformation parameters. In this paper, we investigate a new approach, which is based on weekly estimation of station positions and Helmert transformation parameters from a combination of the solutions of four space-geodetic techniques, i.e., Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Global Positioning System (GPS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). For this study, an interval of one week is chosen because the arc length of the SLR solutions is seven days. The major advantage of this weekly estimated reference frame is that both the non-linear station motions and the non-linear origin motion are implicitly taken into account. In order to study the non-linear behavior of station motions and physical parameters, ITRF2008 is used as a reference. As for datum definition of weekly reference frame, on one hand SLR is the unique technique to realize the origin and determine the scale together with VLBI, and on the other hand the orientation is realized via no net rotation with respect to ITRF2005 on a subset of core stations. Given the fact that without enough collocations an inter-technique combined TRF could not exist, the selection and relative weight of local ties surveyed at co-location sites are critical issues. To get stable results, we first assume that, if there were no events such as equipment changes between the measurement epoch of the local tie and that of the space-geodetic solution, the relative position between the two co-located stations should be invariant and this local tie could be used for computing the inter-technique combined reference frame in those weeks during the stable period of this tie. The resulting time series of both station positions and transformation parameters are studied in detail and are compared with ITRF2008. The residual station positions in the weekly combined reference frame are usually in the range of two millimeters without any periodic characteristic, but the residual station positions, when subtracting the regularized station position in ITRF2008, may reach a magnitude of a few centimeters and seem to have a significant annual signal. The physical parameter series between the weekly reference frame and ITRF2008 also show the obvious existence of an annual signal and reach a magnitude of one centimeter for origin motion and two parts per billion (ppb) for scale.

Comparison of Three Methods for Estimating GPS Multipath Repeat Time

Sidereal filtering is an effective method for mitigating multipath error in static GPS positioning. Using accurate estimates of multipath repeat time (MRT) in sidereal filtering can further improve the performance of the filter. There are three commonly used methods for estimating the MRT: Orbit Repeat Time Method (ORTM), Aspect Repeat Time Adjustment (ARTA), and Residual Correlation Method (RCM). This study utilizes advanced sidereal filtering (ASF) adopting the MRT estimates derived by the three methods to mitigate the multipath in observation domain, then evaluates the three methods in term of multipath reduction in both coordinate and observation domain. Normally, the differences between the MRT estimates from the three methods are less than 1.2 s on average. The three methods are basically identical in multipath reduction, with RCM being slightly better than the other two methods, whereas for a satellite affected by orbit maneuver (satellite number 13 in this study), the MRT estimated by the three methods differ by up to tens of seconds, and the RCM- and ARTA-derived MRT estimates are better than ORTM-derived ones for ASF multipath reduction. The RCM shows a slight advantage in multipath mitigation, while ORTM is the one of lowest computation and ARTA is the optimal one for real-time ASF. Thus, the best MRT estimation method for practical applications depends on which criterion overweighs the others.

Advanced Sidereal Filtering for Mitigating Multipath Effects in GNSS Short Baseline Positioning

Advanced sidereal filtering (ASF) is an observation-domain sidereal filtering that adopts the repeat time of each individual satellite separately rather than the mean repeat time, adopted by the modified sidereal filtering (MSF). To evaluate the performance of ASF, we apply the method to filter the multipath for a short baseline based on a dual-antenna Global Navigation Satellite System (GNSS) receiver. The errors from satellite and receiver clocks, satellite orbit, troposphere, ionosphere, and antenna phase center variations are greatly eliminated by single difference between the two antennas because they are connected to the same receiver clock. The performances of ASF are compared with MSF to evaluate the gain for multipath mitigation. Comparisons indicate that ASF slightly outperforms MSF when the repeat time values of all satellites incorporated in data processing are within the normal range (86,145–86,165 s), but the difference of variance reduction rate between ASF and MSF is statistically significant. When the data of a satellite with repeat time outside the normal range are included, the performances of MSF become much worse, but ASF is almost not affected. This advantage of ASF over MSF is important because the proportion of the days on which at least one satellite’s repeat time exceeds the normal range reaches 71.19% based on the statistics on the data of 2014 and 2015. After applying ASF multipath corrections on the test days, the averages of standard deviations of north, east, and up component are reduced from 3.8 to 2.1 mm, 3.2 to 1.7 mm, and 7.6 to 4.3 mm, respectively. Comparison between applying ASF with the single-day model and with the seven-day model indicates that the former is generally more effective in multipath reduction.

Assessment of the Contribution of QZSS Combined GPS/BeiDou Positioning in Asia-Pacific Areas

Three QZSS satellites are launched in 2017, which implies that a four satellites regional system is to be established in 2018. There is no doubt that QZSS will play a more important role in the future global GNSS constellations. So it is quite necessary to investigate the importance of current QZSS constellation in positioning. In this paper, the number of visible satellite and PDOP (Position Dilution of Precision) value improvement by combining QZSS with the existing GPS and BeiDou system is analyzed among Asia-Pacific areas. 9 IGS stations are selected to evaluate the performance of SPP (Single Point Positioning) and PPP (Precise Point Positioning) using GPS, BeiDou and GPS + QZSS, BeiDou + QZSS system. Analysis results show that QZSS improves SPP performance for both GPS and BeiDou at different level. Especially when the satellite number is reduced, such as in urban areas or when the elevation cutoff is high, the positioning error will reduce after adding QZSS satellite and the availability of other GNSS systems will also improves. For kinematic PPP users, QZSS could also reduce the convergence period. Meanwhile, the dual frequency and single frequency RTK (Real Time Kinematic) positioning performance is compared after adding QZSS satellite into GPS and BeiDou. Kinematic car test in urban environment shows that when combining QZSS satellite with GPS and BeiDou, the rate of instantaneous ambiguity resolution will increase for both single and dual-frequency users.

The Service Improvement of BDS Positioning Based on Advanced Equivalent Satellite Clock Calculation

Positioning precision of navigation satellite system can be measured by two indicators: the dilution of precision (DOP) and the user equivalent range error (UERE). As the DOP values are only related to the spatial distribution of navigation satellites, the reduction of UERE is the main approach to improve the positioning precision. Equivalent satellite clock (ESC) has been used by Beidou satellite system (BDS) to reduce the UERE and to improve the user’s positioning accuracy. In this contribution, both the pseudo-range and carrier-phase measurements of BDS are used to compute the ESC respectively, and the corresponding navigation positioning performance are also compared. It is shown that the UERE improvement based on phase observables is 50.1%, while 32.1% based on pseudo-range observables. Kinematic positioning experiments of 4 MEGX stations are performed respectively under the standard PNT service and wide area differential service (WADS). It is shown that horizontal, vertical and three-dimensional positioning results of WADS are better than that of the standard PNT service.

Real-Time Detection and Repair of Cycle-Slip Based on Pseudo-range Phase Combinations for Un-differenced GNSS Triple-Frequency Observations

For the detection and repair of cycle-slip for un-differenced GNSS triple-frequency observations, current algorithms have difficulties in efficiency, stability and even some special cycle-slip combinations cannot be detected. This paper investigates the strategies in real-time detection and repair of cycle-slip. Geometry free ionospheric free code-phase combinations together with phase combinations are used, where the selection criteria of combination coefficients is based on the principle of the minimal condition number. Advantage of the method is that each cycle-slip value can be calculated without searching, thus the efficiency is improved and success rate is still high. Experiment results show that even under the severe ionospheric conditions, cycle-slips of triple-frequency un-differenced observations can be detected and repaired.