Multi-GNSS Constellation Anomaly Detection and Performance Monitoring

Global Navigation Satellite System (GNSS) precise orbit, clock and observable-specific bias (OSB) products are the prerequisite for precise point positioning with ambiguity solution (PPP-AR). Under the condition of sparse or regional ground network, integrating low Earth orbit (LEO) onboard observations is a potential approach to supplementing GNSS global tracking measurements. In this contribution, a method of integrating ground and LEO onboard observations to generate GNSS precise satellite clock and phase OSB based on square root information filter (SRIF) estimation is proposed. Particularly, a method of phase OSB estimation with a 1.5 h sampling rate is established under regional ground stations and 8 LEO satellites. The results show that integrating 30 global ground stations and 8 LEO satellites, the standard deviation (STD) and convergence time of satellite clocks reach to 0.093 ns and 4.93 h, which reduced by 41.5% and 35.4% compared to ground-only solutions, respectively. Phase OSB accuracy improves from 0.092 to 0.078 cycles, and the product availability increases from 42.2% to 95.1%. The kinematic PPP-AR accuracy of integrated solutions improved by 26%, 17% and 18% on the E, N and U component, and convergence time accelerated by 17%. In addition, phase OSB products generated by 20 European regional ground stations integrating 8 LEO satellites achieve an STD of 0.086 cycles, with the product availability of more than 98%, and kinematic PPP-AR accuracy is close to the solutions with phase OSB generated by 70 ground stations.

<p><span><span>Global Navigation Satellite Systems (GNSS) play a critical role for providing real-time positioning and navigation services, and the precise satellite orbit and clock products are essential for the high-precision GNSS applications. The International GNSS Service (IGS) and its Analysis Centers (ACs) have been working on the study on precise GNSS data processing and provision of the precise products. The German Research Center for Geosciences (GFZ), as one of the ACs, also provides the multi-GNSS rapid products: the GBM product. We introduce the GBM data processing strategy, analyze the precision of GBM multi-GNSS orbits from 2015 to 2021, and present the impact of applying the undifferenced ambiguity resolution on satellite orbits. The GPS orbits of GBM products agree with the IGS final orbits at the level of 11-13 mm in the three directions, and the GPS orbit 6-hour prediction precision is around 6 cm. The 6-hour prediction precision of GLONASS is around 12 cm, slightly worse than that of GALILEO, which hold an average value of 10 cm in the same period but shows a significant improvement to around 5 cm after end of 2016. The prediction precision of BDS MEO satellites are around 10 cm, and that of the BDS GEO satellites and QZSS satellites are at the level of 1 to 3 meter. The Satellite Laser Ranging (SLR) residuals show that the orbit precision of GALILEO, GLONASS, and BD3-MEO is 23 mm, 41 mm, and 47 mm, respectively. Moreover, comparing the double-differenced ambiguity resolution, adopting the undifferenced ambiguity resolution improves the 6-hour orbit prediction precision by 9-15%</span></span>，<span><span>15-18%</span></span>，<span><span>11-13%</span></span>，<span><span>6-17%</span></span>和<span><span>14-25% for the GPS, GLONASS, GALILEO, BDS-2 and BDS-3 MEO satellites, respectively.</span></span></p>

Ground-based GNSS ZTD/IWV estimation system for numerical weather prediction in challenging weather conditions

The contribution of Multi-GNSS Experiment (MGEX) to precise point positioning

Acousto-optic devices using lead molybdate and related compounds: Bell Telephone Labs Inc US patent 3617931 (1971) filed 5 May 1969

Assessment of L5 frequency on real-time GPS orbit and clock estimation

Real-time precise orbit and clock products are crucial for real-time high-precision precise point positioning (PPP). The key for precise orbit prediction is to precisely determine the initial conditions (satellite position, velocity at the initial epoch) and the solar radiation pressure (SRP) parameters using an arc of known precise orbits. The optimal arc length will be investigated for precise orbit prediction, with which the high-accuracy satellite clocks can be estimated. The orbit prediction accuracies with different arc lengths and conditions over time are also investigated. The results indicate that the predicted orbit accuracy is governed by the SRP parameters. In addition, the predicted orbits and estimated clocks are evaluated quantitatively with respect to PPP solutions during in and out of eclipses seasons, based on which the optimal arc length is identified. The numerical results show that high-accuracy orbital prediction can be achieved using an arc length between 36 and 48 h and generally an arc length of 42 h is considered to be optimal. With this optimal arc length, the estimated clock accuracy is 0.048 and 0.122 ns for a prediction interval of 6 and 24 h, respectively. The average PPP ambiguity fix-rates with a prediction interval of 6 h are 80.2 and 84.7 % over an observation period of 30 min and 1 h, respectively.

Thanks to the high performance of the spaceborne GPS receiver and the availability of precise IGS orbit and clock products, Zero-difference kinematic precise orbit determination (POD) has turned out to be a new method in orbit determination for the LEO satellites. Zero-difference Kinematic POD, which is based on the GPS measurements only from the spaceborne GPS receiver, is independent of force models and orbit design. From that point of view, kinematic POD is well suited for the Earth Observation satellites at very low altitudes, such as CHAMP, GRACE and GOCE et al. This paper reviews the basic zero-difference GPS model, and the corrections in the model are discussed. A block-wise least squares algorithm, which firstly separates the parameters in to groups and then solves the parameter by elimination and back-substitution, is discussed and proposed for the kinematic orbit determination. The orbit solutions for one week of GRACE observations are calculated Comparisons with the published Rapid Science Orbit (RSO) indicate that the accuracy in radial, along-track and cross-track direction can achieve 5.5cm, 5.5cm and 6.6cm respectively, and the RMS in distance is better than 8.6cm.

Integrity monitoring (IM) is essential if GNSS positioning technologies are to be fully trusted by future intelligent transport systems. A tighter and conservative stochastic model can shrink protection levels in the position domain and therefore enhance the user-level integrity. In this study, the stochastic models for vehicle-based GNSS positioning are refined in three respects: (1) Gaussian bounds of precise orbit and clock error products from the International GNSS Service are used; (2) a variable standard deviation to characterize the residual tropospheric delay after model correction is adopted; and (3) an elevation-dependent model describing the receiver-related errors is adaptively refined using least-squares variance component estimation. The refined stochastic models are used for positioning and IM under the Advanced Receiver Autonomous Integrity Monitoring (ARAIM) framework, which is considered the basis for multi-constellation GNSS navigation to support air navigation in the future. These refinements are assessed via global simulations and real data experiments. Different schemes are designed and tested to evaluate the corresponding enhancements on ARAIM availability for both aviation and ground vehicle-based positioning applications.

Global navigation satellite system (GNSS) plays a crucial role in many fields, such as aerospace and transportation. Integrity is the measure of trust used in GNSS positioning especially in safety-critical applications. Advanced receiver autonomous integrity monitoring (ARAIM), taking full advantage of multi-constellation GNSS, shows huge potential to provide vertical navigation in civil aviation en route navigation and terminal approaches. However, the multi-constellation ARAIM also greatly exposes computational complexity and potential performance hazards in fault modes determination and fault-tolerant positioning. From the perspective of integrity risk control, rather than the pursuit of better positioning accuracy blindly for safety-critical applications, the concept of constellation dynamic selection is proposed and implemented in ARAIM and the performance analysis is discussed in this paper. Only the best two constellations which have the best vertical geometry performance are involved in ARAIM calculation anytime anywhere. The proposed method shows superiority in both integrity availability and computational complexity in both simulations and actual GNSS signal experiments. While the computational complexity is less than 10% of that using four constellations, 100% availability under LPV-200 criteria can be achieved in worldwide coverage experiment. The proposed method also overcomes the shortcomings of ARAIM with two fixed constellations and shows good robustness under depleted scenarios. Furthermore, the statistics results from observation stations proved the applicability and generality of the proposed method under current developing GNSS constellations.

GipsyX/RTGx is the Jet Propulsion Laboratory’s (JPL) next generation software package for positioning, navigation, timing, and Earth science using measurements from three geodetic techniques: Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS); with Very Long Baseline Interferometry (VLBI) under development. The software facilitates combined estimation of geodetic and geophysical parameters using a Kalman filter approach on real or simulated data in both post-processing and in real-time. The estimated parameters include station coordinates and velocities, satellite orbits and clocks, Earth orientation, ionospheric and tropospheric delays. The software is also capable of full realization of a dynamic terrestrial reference through analysis and combination of time series of ground station coordinates.Applying lessons learned from its predecessors, GIPSY-OASIS and Real Time GIPSY (RTG), GipsyX/RTGx was re-designed from the ground up to offer improved precision, accuracy, usability, and operational flexibility. We present some key aspects of its new architecture, and describe some of its major applications, including Real-time orbit determination and ephemeris predictions in the U.S. Air Force Next Generation GPS Operational Control Segment (OCX), as well as in JPL’s Global Differential GPS (GDGPS) System, supporting User Range Error (URE) of <5 cm RMS; precision post-processing GNSS orbit determination, including JPL’s contributions to the International GNSS Service (IGS) with URE in the 2 cm RMS range; Precise point positioning (PPP) with ambiguity resolution, both statically and kinematically, for geodetic applications with 2 mm horizontal, and 6.5 mm vertical repeatability for static positioning; Operational orbit and clock determination for Low Earth Orbiting (LEO) satellites, such as NASA’s Gravity Recovery and Climate Experiment (GRACE) mission with GRACE relative clock alignment at the 20 ps level; calibration of radio occultation data from LEO satellites for weather forecasting and climate studies; Satellite Laser Ranging (SLR) to GNSS and LEO satellites, DORIS-based and multi-technique orbit determination for LEO; production of terrestrial reference frames and Earth rotation parameters in support of JPL’s contribution to the International Terrestrial Reference Frame (ITRF).

Many sophisticated Global Navigation Satellite System (GNSS ) applications require high-precision satellite orbit and clock products. The GNSS orbits and clocks are usually derived from the analysis of tracking data collected by a globally distributed GNSS receiver network. The estimation process adjusts parameters for the satellite orbits, transmitter and receiver clocks, station positions, tropospheric delays, Earth orientation, intersystem and interfrequency biases, and carrier-phase ambiguities. The estimation requires detailed modeling of geophysical processes, atmospheric and relativistic effects, receiver tracking modes, antenna phase centers, spacecraft properties, and attitude control algorithms. This chapter describes precise orbit and clock determination of the GNSS constellations as performed by the analysis centers of the International GNSS Service, including models, estimation strategies, products, and the combination of orbit and clock solutions.

A 20-channel, dual-frequency GPS receiver collecting pseudorange and carrier phase observations was used as a stand-alone receiver to determine positional accuracy of 19 points under conifer tree canopies. The positions were determined utilizing precise satellite orbit and clock products from the International GNSS Service. The mean positional accuracy ranged from 0.27 to 0.88 m for an observation period of 120 minutes, and 0.95 to 3.48 m for 15 minutes. For the 15 minute observation period computed positions could not be found for 8 to 44 percent of the locations. Accuracy increased with decreasing forest stand density. Stand basal area (R 2 � 0.11, p � 0.001) and number of tree stems (R 2 � 0.07, p � 0.001) were significantly correlated with accuracy. The probability of determining a position increased with longer observation periods and decreasing number of tree stems. For natural resource applications where the costs associated with the length of the observation period on each site in field is a critical factor, differential GPS seems to be a more robust alternative than precise point positioning with GPS.

This paper investigates the impact of multi-GNSS integration on the convergence performance of Precise Point Positioning (PPP), using observations from GPS, Galileo, and BDS constellations. Dual-frequency data from two IGS stations, BRUX and PTBB, on October 2, 2023, were processed using CNES CLK93 precise orbit and clock products. Two system configurations were evaluated: GPS and GPS+Galileo+BDS. Results show that the inclusion of Galileo and BDS significantly improves satellite geometry and observability, effectively accelerating convergence. Compared to GPS-only, the tripleconstellation configuration reduced convergence epochs in the East and North components by more than 50%, while maintaining similar performance in the Up direction. Additionally, PPPderived clock differences between BRUX and PTBB exhibited faster stabilization and reduced noise under the multi-GNSS solution, confirming its advantage in both positioning and timing applications.

This paper will address issues related to Precise Point Positioning (PPP) and perform raw observation data analysis to evaluate the specific performance of PPP based its all solutions processed by CSRS-PPP using final precise orbit and clock products from IGS. For this purpose, daily sessions of observations, at 30 seconds intervals, of five different IGS stations are processed using aforementioned approach. The tests show that, the solution accuracy reaches averagely 0.0023 m, -0.0005 m, -0.0035 m for static mode and about 5 cm for kinematic mode, respectively; centimeter convergence is obtained after being processed for 10-20 min, and residuals are nearly close to true-value after 30 min or more longer; tropospheric zenith delay estimates, both for static and kinematic, are greatly consistent with the solutions from IGS, and fluctuate within 5 mm; the residuals for clock offset are approximate to 0.1 ns about 1 h later, and the final values are close to 0 for three stations. It also may be inferred that carrier-phase ambiguity, real or integer, can play a pivotal role in aforementioned solutions.