Real-time GNSS Research Articles

Development of Visualization Tool for Real-time GNSS Time-series

Development of Visualization Tool for Real-time GNSS Time-series

Evaluation of the Possibility of Using the Predicted Tropospheric Delays in Real Time Gnss Positioning

ABSTRACT Among many sources of errors that influence Global Navigation Satellite System (GNSS) observations, tropospheric delay is one of the most significant. It causes nonrefractive systematic bias in the observations on the level of several meters, depending on the atmospheric conditions. Tropospheric delay modelling plays an important role in precise positioning. The current models use numerical weather data for precise estimation of the parameters that are provided as a part of the Global Geodetic Observation System (GGOS). The purpose of this paper is to analyze the tropospheric data provided by the GGOS Atmosphere Service conducted by the Vienna University of Technology. There are predicted and final delay data available at the Service. In real time tasks, only the predicted values can be used. Thus it is very useful to study accuracy of the forecast delays. Comparison of data sets based on predicted and real weather models allows for conclusions concerning possibility of using the former for real time positioning applications. The predicted values of the dry tropospheric delay component, both zenith and mapped, can be safely used in real time PPP applications, but on the other hand, while using the wet predicted values, one should be very careful.

Software-based Real-time GNSS Signal Generation and Processing Using a Graphic Processing Unit (GPU)

A graphic processing unit (GPU) can perform the same calculation on multiple data (SIMD: single instruction multiple data) using hundreds of to thousands of special purpose processors for graphic processing. Thus, high efficiency is expected when GPU is used for the generation and correlation of satellite navigation signals, which perform generation and processing by applying the same calculation procedure to tens of millions of discrete signal samples per second. In this study, the structure of a GPU-based GNSS simulator for the generation and processing of satellite navigation signals was designed, developed, and verified. To verify the developed satellite navigation signal generator, generated signals were applied to the OEM-V3 receiver of Novatel Inc., and the measured values were examined. To verify the satellite navigation signal processor, the performance was examined by collecting and processing actual GNSS intermediate frequency signals. The results of the verification indicated that satellite navigation signals could be generated and processed in real time using two GPUs.

Real-time GNSS seismology using a single receiver

High-rate GNSS has attracted increasing attention and numerous applications in geohazard monitoring and early warning. In this paper, we investigate three current existing single-receiver approaches for real-time GNSS seismology, comparing their observation models for equivalence and assessing the impact of main error components. We propose some refinements to the variometric approach and especially consider compensating the geometry error component by using the accurate initial coordinates before the earthquake to eliminate the drift trend in the integrated coseismic displacements. After careful corrections of satellite ephemeris, ionospheric delay, tropospheric delay and geometry errors, the refined variometric approach and the temporal point positioning (TPP) method have equivalent mathematical model with the converged precise point positioning (PPP). We evaluated the precision of the variometric and TPP approaches with various error correction schemes and duration time using numerous data sets and demonstrated that few centimetres accuracy of coseismic displacements is achievable even for 20 min interval. We applied these single-receiver approaches to process 1 Hz GPS data collected from the Tohoku-Oki earthquake (Mw 9.0, 2011 March 11) in Japan to capture coseismic displacement, and further, inverted the obtained displacement fields for fault slip distribution and moment magnitude. Comparisons of the results obtained using the refined variometric approach and TPP, as well as the converged PPP, displayed very good consistence both in coseismic displacements within few centimetres and in the slip distribution patterns and moment magnitudes.

Assessment of EGNOS Model over Asia Area Using IGS_ZTD

In this study, three years of the zenith tropospheric delay (ZTD) data observed from 46 International GNSS system (IGS) sites distributed in Asian area used to assess the effectiveness and accuracy of ZTD calculated from EGNOS model, and the application of the EGNOS model are also analyzed in Asian area. Relative to IGS observed ZTD, the bias and root mean square (RMS) for ZTD calculated from EGNOS model presents an obvious variation in temporal and spatial. These results provide a reference for the study of the tropospheric delay correction model, the real-time GNSS navigation and positioning.

Monitoring precipitable water vapor in real-time using global navigation satellite systems

This paper addresses real-time monitoring of the precipitable water vapor (PWV) from GNSS measurements and presents some results obtained from 6-month long GNSS PWV experiments using international and domestic GNSS networks. In the real-time GNSS PWV monitoring system a server/client structure is employed to facilitate formation of PWV networks and single-differenced GNSS measurements are utilized to mitigate errors in GNSS satellites’ orbits and clocks. An issue relating to baseline length between the server and clients is discussed in detail and as a result the PWV monitor is configured to perform in two modes depending on the baseline length. The server estimates sequentially the zenith wet delay of the individual stations, which is then converted into the PWV of the stations. We evaluate system performance by comparing the real-time PWV solution with reference solutions including meteorological measurements obtained with radiosondes and deferred-time precision GNSS PWV solutions. Results showed that the standard deviation of difference between the real-time PWV and the reference solutions ranged from 2.1 to 3.4 mm in PWV for a 6-month long comparison, which was improved to 1.4 to 2.9 mm by reducing comparison period to 20 days in winter.

실시간 GNSS 기만 시뮬레이터를 이용한 위성항법수신기에서의 기만 영향 분석

In this paper, spoofing effects on a GNSS receiver were analyzed. The spoofer (spoofing device) was classified to two categories. One is an active spoofer and the other is a passive spoofer. The active spoofer was considered for analysis. For the analysis of spoofing effects on a GNSS receiver, a real-time GNSS spoofing simulator was developed. The simulator was consisted with two parts which are a baseband signal generation part and a RF up-conversion part. The first GNSS baseband signal was generated according to spoofing parameters such as range, range rate, GNSS navigation data, spoofing to GNSS signal ratio, and etc. The generated baseband signal was up-converted to GNSS L1 band. Then the signal transmitted to a GNSS signal. For a perfect spoofing, a spoofer knew an accurate position and velocity of a spoofing target. But, in real world, that is not nearly possible. Although uncertainty of position and velocity of the target was existed, the spoofer was operated as an efficient jammer.

The GFZ real-time GNSS precise positioning service system and its adaption for COMPASS

Motivated by the IGS real-time Pilot Project, GFZ has been developing its own real-time precise positioning service for various applications. An operational system at GFZ is now broadcasting real-time orbits, clocks, global ionospheric model, uncalibrated phase delays and regional atmospheric corrections for standard PPP, PPP with ambiguity fixing, single-frequency PPP and regional augmented PPP. To avoid developing various algorithms for different applications, we proposed a uniform algorithm and implemented it into our real-time software. In the new processing scheme, we employed un-differenced raw observations with atmospheric delays as parameters, which are properly constrained by real-time derived global ionospheric model or regional atmospheric corrections and by the empirical characteristics of the atmospheric delay variation in time and space. The positioning performance in terms of convergence time and ambiguity fixing depends mainly on the quality of the received atmospheric information and the spatial and temporal constraints. The un-differenced raw observation model can not only integrate PPP and NRTK into a seamless positioning service, but also syncretize these two techniques into a unique model and algorithm. Furthermore, it is suitable for both dual-frequency and sing-frequency receivers. Based on the real-time data streams from IGS, EUREF and SAPOS reference networks, we can provide services of global precise point positioning (PPP) with 5–10cm accuracy, PPP with ambiguity-fixing of 2–5cm accuracy, PPP using single-frequency receiver with accuracy of better than 50cm and PPP with regional augmentation for instantaneous ambiguity resolution of 1–3cm accuracy. We adapted the system for current COMPASS to provide PPP service. COMPASS observations from a regional network of nine stations are used for precise orbit determination and clock estimation in simulated real-time mode, the orbit and clock products are applied for real-time precise point positioning. The simulated real-time PPP service confirms that real-time positioning services of accuracy at dm-level and even cm-level is achievable with COMPASS only.

An Analysis for the Accuracy of Tropospheric Zenith Delay Calculated From ECMWF/NCEP Data Over Asia

AbstractThe tropospheric delay is an important error source for the satellite navigation and positioning, and the numerical forecast data derived from meteorological instruments can be used to calculate the tropospheric delay. In this study, the zenith tropospheric delay (ZTD) observed from 49 Global Positioning System (GPS) sites distributed in Asia was applied to assess the availability and accuracy for ZTD calculated from the European Center for Medium‐Range Weather Forecasts (ECMWF) and the United States National Centers for Environmental Prediction (NCEP). The practicability of ECMWF/NCEP data in Asia and the relation between the resolution and the accuracy for ECMWF were also studied. The results show that relative to GPS observed ZTD (GPS ZTD), the bias and rms for the ECMWF data are –1.0 cm and 2.7 cm, respectively, which are better than that for the NCEP data and can be used for ZTD study and application with high accuracy. The bias and rms for the NCEP forecast data are 2.4 cm and 6.8 cm, which are sufficed for the tropospheric delay correction in the real‐time GNSS navigation and positioning. The bias and rms exhibit a seasonal variation, which are generally of larger values in summer months and smaller values in winter months; however, the relation between the bias and rms and the latitude is not obvious, but the rms is decreasing while the altitude increasing. In addition, the daily bias and rms in eastern Asia in summer and in southern Asia in winter have a relatively larger variation. The resolutions of 2.5 degree and 0.5 degree for the ECMWF data are compared; the rms for the latter is decreased by 1~5 mm than the former. These results provide a reference for the availability and accuracy of the ECMWF/NCEP data used for calculating the tropospheric delay in the space geodesy, InSAR, navigation and positioning.

GNSS precise point positioning in regional reference frames using real-time broadcast corrections

As a result of the Real-Time IGS Pilot Project (RT-IGS) and the EUREF Real-Time Analysis Project (EUREF-RT), real-time GNSS satellite orbit and clock cor- rections to broadcast ephemeris, the so-called real-time broadcast corrections (BC), have become available for the application in precise point positioning (PPP). The real- time BCs are currently available in global as well as in regional reference frames. In this contribution the PPP usage and performances of these global and regional BCs are analysed for the Australian continent. We identify the limitations of the current regional BCs, which results in scale-induced biases, and show how they can be improved. We also present an alternative, more user friendly, approach for regional BCs. It provides PPP users with simple means to correct for the biases of the regional approach.

Development and Assessment of a Multiple Rover Network Approach for Real-Time GNSS Networks

: In this paper multiple GNSS users are considered as a virtual network of stations that can operate autonomously and combine with the reference station network. The existing network infrastructure is densified through the shorter inter-receiver distances. A new network RTK methodology that encompasses the multiple users of network RTK services for instantaneous positioning is presented. Users are equipped with a means of two-way communication that enables the data to be transmitted to a central processing facility (CPF) on an epoch by epoch basis. Once the reference station ambiguities are resolved and the regional ionospheric corrections are estimated and filtered, the proposed Multiple Rover Network (MRN) solution is applied. The MRN solution increases the ambiguity success rates for all rovers for the designed testbed network compared to the single baseline solution (SBS) and the single rover network solution (SRN). The highest increment is observed for the rover operating outside the network.

A Wide Area Real-Time Differential GPS Prototype System in China and Result Analysis

This paper introduces the Wide Area Real-Time Differential GPS Prototype System on the basis of the PANDA software platform which is independently developed by Wuhan University. The system is a suite of algorithmic, infrastructure elements to enable the processing, distribution and archiving of Real-Time and Near Real-time GNSS data and results. Currently, it adopts observation data from nearly 100 real-time stations, including IGS global stations and regional stations of China, to continuously estimate real-time satellite clock offset second by second and to provide a Global Ionospheric Map product updated every 5 minutes. 1 HZ corrections to the GPS spacecraft position and clock to Broadcast Ephemeris and grid VTEC with 30 sec' interval are provided to users through a wireless communication network. In this research, real-time orbit and clock product, as well as single frequency and double frequency point positioning accuracy are investigated. Results of tests show the SISRE of real-time orbit and clock by the system is about 5cm; real-time double frequency positioning RMS are respectively better than 10cm and 20cm in the horizontal and vertical direction; single frequency positioning RMS(3D) is better than 1m .

Fast GNSS ambiguity resolution as an ill-posed problem

A linear observational equation system for real-time GNSS carrier phase ambiguity resolution (AR) is often severely ill-posed in the case of poor satellite geometry. An ill-posed system may result in unreliable or unsuccessful AR if no care is taken to mitigate this situation. In this paper, the GNSS AR model as an ill-posed problem is solved by regularizing its baseline and ambiguity parameters, respectively, with the threefold contributions: (i) The regularization parameter is reliably determined in context of minimizing mean square error of regularized solution where the covariance matrix of initial values of unknowns is used as an approximate smoothness term instead of the quadratic matrix of the true values of unknowns; (ii) The different models for computing initial values of unknowns are systematically discussed in order to address the potential schemes in real world applications; (iii) The superior performance of the regularized AR are demonstrated through the numerically random simulations as well as the real GPS experiments. The results show that the proposed regularization strategies can effectively mitigate the model’s ill-condition and improve the success AR probability of the observational system with a severely ill-posed problem.

Characterization of GPS/GIOVE sensor stations in the CONGO network

The Cooperative Network for GIOVE Observation (CONGO) is a global network of real-time capable multi-constellation GNSS receivers, which has been established by the German Aerospace Center (DLR) an...

Three Carrier Ambiguity Resolutions: Generalised Problems, Models and Solutions

In this paper, the problems of three carrier phase ambiguity resolution (TCAR) and position estimation (PE) are generalized as real time GNSS data processing problems for a continuously observing network on large scale. In order to describe these problems, a general linear equation system is presented to uniform various geometry-free, geometry-based and geometry-constrained TCAR models, along with state transition questions between observation times. With this general formulation, generalized TCAR solutions are given to cover different real time GNSS data processing scenarios, and various simplified integer solutions, such as geometry-free rounding and geometry-based LAMBDA solutions with single and multiple-epoch measurements. In fact, various ambiguity resolution (AR) solutions differ in the floating ambiguity estimation and integer ambiguity search processes, but their theoretical equivalence remains under the same observational systems models and statistical assumptions. TCAR performance benefits as outlined from the data analyses in some recent literatures are reviewed, showing profound implications for the future GNSS development from both technology and application perspectives.

맵 기반의 부분시간 공통 중간주파수 제거방식을 이용한 GNSS 신호의 상관 기법

In this paper, we propose the efficient Doppler removal method using map-based partial-time common intermediate frequency removal technique. In the proposed algorithm, the 2-stage carrier removal process was used. First, the component of common intermediate frequency is removed. Next the component of Doppler was removed with averaging and approximation. For the evaluation of the proposed algorithm, The real-time software GPS L1 C/A-code receiver was implemented. When the proposed algorithms are used, 12 tracking channels with 3 track arm(early, prompt, late) is operated real-time on PC using a Intel Pentium-III 1.0GHz CPU. Also, the requirement of memory was less than 2Mbytes. The real-time software GNSS receiver using the proposed algorithms provides the navigation solution with below 10 meter rms error. Especially, in spited of using the various approximations for implementing the algorithms, the high sensitivity capability (able to track the weak signal with -159dBm) was achieved.