S3

iag-comm4-2022
SC44: GNSS Integrity and Quality Control
Convener: Pawel Wielgosz | Co-conveners: Jianghui Geng, Grzegorz Krzan
Orals
| Mon, 05 Sep, 15:20–16:20 (CEST)|Wissenschaftsetage Potsdam

Orals: Mon, 05 Sep | Wissenschaftsetage Potsdam

Chairperson: Jungang Wang
15:20–15:40
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iag-comm4-2022-27
Ali Karimidoona and Steffen Schön

High accuracy positioning is an important part in the future applications of the Intelligent Transportation Systems (ITS) such as autonomous driving in urban environments. Global Navigation Satellite System (GNSS) exploiting differential techniques from reference stations namely Real Time Kinematic (RTK), is capable to provide the most precise and accurate absolute solution for a navigation system. Network RTK uses several reference stations to produce corrections which provide the availability of the service in a wider area. Integrity measures such as Protection Level (PL) and Position Error (PE) are parameters that can give an indicator about the quality of the positioning. In urban environments where the surrounding buildings obscure the signals coming from the satellites, the signal coming to the receiver may fall into four different situations: line-of-sight (LOS), non-line-of-sight (NLOS), multipath and blocked. In these areas the number of available signals for positioning decreases and also some of the available signals reach the receiver in multipath or NLOS conditions, which degrade the signal strength as well as inducing respective errors in code and phase observations.  

For this contribution a kinematic test was performed in urban environments of city Hannover, mounting RTK receiver on top of a van driving 1 km loop for twelve times. The reference trajectory is calculated by tightly coupling the GNSS data with a high grade IMU data. The GPS/GLONASS RTK corrections are provided by the local service provider (SAPOS). Having a level 2 of details (LoD2) 3D city model, it is possible to use the known ephemerides of the satellites to predict the signal availability for a specific receiver position considering NLOS and multipath situations. This prediction of the satellite visibility, is then used in an Extended Kalman Filter (EKF) to solve the positioning problem. As this solution is based on the predicted visible satellites, it works as a prediction of the positioning. Based on this solution, the Protection Levels (PL) are calculated. The Ray Tracing algorithm also provides us with the code and phase observation errors caused by NLOS and multipath cases. These errors (assuming that they are the main driving errors for deviation in the coordinates domain) are then feed to our positioning algorithm as the observation vector to have an estimate of the position error (PE). On the other hand, the receiver itself provides an estimate of the PL. We calculate the PE by comparing the RTK solution of the receiver and the reference trajectory. Finally, we compare the predicted PL and PE with the real observed PL and PE, considering a horizontal alert limit (HAL) of 10 cm. The results so far, show a partial agreement between the PLs of the predicted and the real data, but there is more space for improvement by considering the C/N0 degradation of the signals and large multipath or NLOS signals.  The route will be selected based on reliable solutions which are evaluated by the percentage of the nominal operation in each round.

How to cite: Karimidoona, A. and Schön, S.: Route Planning for Network RTK – Based Urban Navigation with High Integrity, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-27, https://doi.org/10.5194/iag-comm4-2022-27, 2022.

15:40–16:00
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iag-comm4-2022-10
Ningbo Wang, Yang Li, Zishen Li, Liang Wang, and Zongyi Li

The precise different code bias (DCB) correction information is basically required in the high-precision applications of multi-frequency Global Navigation Satellite Systems (GNSS). It is noted that the phase center offset (PCO) errors have not yet been properly handled in the generation of GNSS DCBs. In this paper, we first checked the variation characteristics of satellite PCOs of BeiDou global navigation satellite system (BDS-3), and analyzed the PCO effects on the generated BDS-3 DCBs. The empirical PCO correction model for DCBs (i.e., PCO-corrected-DCB) is then proposed, and the DCB estimation method with PCO correction applied (i.e., PCO-estimated-DCB) is also presented. Using BDS-3 observation data from the International GNSS Service (IGS) stations, the BDS-3 C2I/C1P/C1X-C6I DCBs with/without PCO corrections are estimated. The BDS-3 C2I/C1P single-frequency standard point positioning (SF-SPP) utilizing precise satellite orbits and clocks is performed to check the quality of the generated DCBs. Results show that the differences between DCBs estimated with and without PCO corrections reach 0.60 ns. The DCB discrepancy between different satellite types of BDS-3 is up to 1.16 ns, indicating the PCO errors in the generated DCBs can not be ignored in the associated positioning applications. Compared to the BDS-3 SF-SPP result applying DCBs without PCO corrections, the positioning accuracy improves by 1.0% and 9.6% in horizontal and vertical components for PCO-corrected-DCBs, which corresponds to 5.6% and 15.7% for PCO-estimated-DCBs. Since the temporal variation of PCO errors is properly handled in the estimated DCBs with PCO correction applied, the notable improvement in PCO-estimated-DCB based positioning can be foreseen, compared to the PCO-corrected-DCB based solution.

How to cite: Wang, N., Li, Y., Li, Z., Wang, L., and Li, Z.: Estimation of BDS-3 Difference Code Biases with Satellite Phase Center Offset Correction Applied, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-10, https://doi.org/10.5194/iag-comm4-2022-10, 2022.

16:00–16:20
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iag-comm4-2022-52
Longjiang Tang, Jungang Wang, Huizhong Zhu, Maorong Ge, Aigong Xu, and Harald Schuh

The current real-time precise point positioning (RT-PPP) service heavily relies on the network communication, as the real-time orbits and clocks are updated with seconds. However, the communication interruption cannot be avoided and thus degrades the RT-PPP performance. The new GNSS constellations such as Galileo and BDS or satellite types such as GPS III have stable satellite clock onboard, which facilitates the windows of using predicted orbits and clocks for RT PPP. In this study we investigate the performance of RT-PPP using half-hourly updated multi-GNSS orbits and clocks. An epoch-parallel processing strategy is proposed for efficient GNSS processing, which shortens the latency of multi-GNSS POD of 120 satellites and 90 stations from one-hour, state-of-the-art ultra-rapid POD solution of IGS, to 30-min. The orbits and clocks are further predicted for RT-PPP. We adopt a new weighting strategy based on the orbit and clock prediction precision to exploit the benefit of all satellites. Using this satellite-specific weighting strategy, the 3D accuracy of quad-constellation kinematic RT-PPP in 5-, 10-, 20-, and 30-min becomes 0.70 m, 0.49 m, 0.35 m and 0.29 m, respectively. The position accuracy after convergence, which is counted starting from three hours, is 0.10 m in horizontal and 0.14 m in vertical. The PPP performance without considering satellite-specific weighting strategy is about 45% worse. This study is serviceable for the real-time GNSS applications with dm-level accuracy requirement.

How to cite: Tang, L., Wang, J., Zhu, H., Ge, M., Xu, A., and Schuh, H.: Real-time precise point positioning using ultra-rapid orbits and predicted clocks, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-52, https://doi.org/10.5194/iag-comm4-2022-52, 2022.

Posters | Poster area

P3
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iag-comm4-2022-33
Saad Alqahtani, Abdulwasiu Salawu, Suliman AlJebreen, and Meshari AlShahrani

Many government agencies in Saudi Arabia had created their own CORS networks in order to meet their business requirements. The operation and maintenance of these CORS networks have resulted in redundant infrastructure, government financial burden, inconsistent geodetic references, and inconsistent real-time service solutions. If the situation is not unified, it is possible that more agencies will construct new CORS networks in the future. It is the responsibility of the General Authority for Survey and Geospatial Information (GASGI) to oversee and regulate the unification of networks. The authority has studied the networks design and proposed stations from several agencies in order to combine the existing CORS networks into a sustainable national CORS network. Currently, the GASGI KSA-CORS network of over 200 stations is offering services to users. The precision of the network's real-time service is 1 cm for grid coordinates and 3-5 cm for both ellipsoidal and orthometric heights. This article also examines the preliminary evaluation of the KSA-CORS network in order to evaluate the user-side services. The passive geodetic network was utilized for the real-time and static survey validation campaigns, as well as the integration of the geoid model in the real-time services. In addition, the centimeter-level precision acquired during the testing of the KSA-CORS network is highlighted. A unified nationwide CORS network will satisfy all positioning users and ensure users have confidence in the products and services. It will also provide users with real-time and post-processing access to the national spatial reference frame.

How to cite: Alqahtani, S., Salawu, A., AlJebreen, S., and AlShahrani, M.: KSA-CORS and unification of CORS networks in KSA, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-33, https://doi.org/10.5194/iag-comm4-2022-33, 2022.

P4
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iag-comm4-2022-43
Jungang Wang, Maorong Ge, Susanne Glaser, Robert Heinkelmann, and Harald Schuh

Global Geodetic Observing System (GGOS) requires an accuracy of 1 mm and a stability of 1 mm/decade for the Terrestrial Reference Frame (TRF), which is not achieved yet. Global Navigation Satellite Systems (GNSS) play a critical role in TRF determination, thanks to its continuous observations collected from hundreds of globally distributed stations. GNSS observations at the user side are affected by antenna, receiver, and the local environment. The impact of the antenna is more significant to carrier-phase observations and handled with the phase center offset and variation corrections, meanwhile that of the receiver is more significant to the pseudo-range observations. In this study, we investigate the long-term agreement of station coordinates at co-located GNSS stations, i.e., more than one GNSS stations within a few hundreds of meters. We demonstrate that (1) the coordinate difference between co-located GNSS stations could have seasonal variations and long-term trends, (2) discontinuities are often observed after instrument changes, especially the antenna change, and (3) different receiver types could cause station coordinate bias up to 1 mm. A preliminary investigation of the possible reasons is carried out and addressed as well.

How to cite: Wang, J., Ge, M., Glaser, S., Heinkelmann, R., and Schuh, H.: Investigating the long-term variations of coordinate time series at co-located GNSS stations, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-43, https://doi.org/10.5194/iag-comm4-2022-43, 2022.

P5
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iag-comm4-2022-53
Michael Bako, Basem Elsaka, Jürgen Kusche, and Luciana Fenoglio-Marc

Several high-resolution global geopotential models (GGMs) provide valuable information about the Earth’s gravity field products such as gravity anomalies and geoid heights. However, ground-based datasets are required to assess these products quality. This contribution investigates different validation strategies using geoid heights derived from GRACE- and GOCE-based GGMs as well as combination GGMs with the corresponding 136 well-distributed ground-based Global Navigation Satellite System (GNSS)/levelling points over the country of Nigeria within longitude 3º to 14º E and latitude 4º to 14º N. One of our validation strategies is to consider the spectral consistency by applying the spectral enhancement method (SEM) between the GGMs and the ground-based geoid heights. Accordingly, we incorporate high/very-high frequencies of gravity functional, i.e. the gravity signal beyond maximum d/o of GOCE-based GGMs, using the EGM2008 and the high-resolution digital terrain model provided by the SRTM (Shuttle Radar Topography Mission). Our findings show that using the SEM strategy helped much in assessing the quality of the GGMs solutions more unbiasedly. In particular, the fits of GNSS/levelling to EGM2008 geoid heights show improvement from 0.288 m without applying the SEM compared to 0.276 m after the SEM was applied. Finally, four types of transformation models, i.e. linear, four-, five- and seven-parameter transformations, are examined to mitigate reference system offsets between the studied GGMs and the GNSS/levelling data over Nigeria. With the SEM technique and the best-fitting transformation model, we find that GGM misfits to GNSS/levelling down to about 0.26 m.

Keywords: GNSS/levelling, geoid heights, Global Geopotential Models (GGMs), transformation models, Spectral Enhancement Method (SEM).

How to cite: Bako, M., Elsaka, B., Kusche, J., and Fenoglio-Marc, L.: Validating the Geoid Heights over Nigeria from Global Geopotential Models using ground-based GNSS/levelling Data, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-53, https://doi.org/10.5194/iag-comm4-2022-53, 2022.