Wide Area Differential GPS Research Articles

Test Results of WADGPS System using Satellite-based Ionospheric Delay Model for Improving Positioning Accuracy

Most existing studies on the wide-area differential global positioning system (WADGPS) employed a grid ionosphere model for error correction in the ionospheric delay. The present study discusses the application of satellite-based ionospheric delay model that provides an error model as a plane function with regard to individual satellites in order to improve accuracy in the WADGPS. The satellite-based ionospheric delay model was developed by Stanford University in the USA. In the present study, the algorithm in the model is applied to the WADGPS system and experimental results using measurements in the Korean Peninsula are presented. Around 1 m horizontal accuracy was exhibited in the existing planar fit grid model but when the satellite-based model was applied, correction performance within 1 m was verified.

Performance Analysis of WADGPS System for Improving Positioning Accuracy

The Wide Area Differential Global Positioning System (WADGPS) that uses a number of Global Navigation Satellite System (GNSS) reference stations are implemented with various types and provide services as it can service a wide range of areas relatively. This paper discusses a constellation design of reference stations and performance analysis of the WADGPS. It presented performance results of static and dynamic users when wide area correction algorithm was applied using eight reference stations.

Precise Point Positioning Technique with IGS Real-Time Service (RTS) for Maritime Applications

The maritime navigation accuracy requirements for radionavigation systems such as GPS are specified by the International Maritime Organization (IMO). Maritime navigation usually consists of three major phases identified as Ocean/Coastal/Port approach/Inland waterway, in port navigation and automatic docking with an accuracy requirement that ranges from 10 m to 0.1 m. With the advancement in autonomous GPS positioning techniques such as Precise Point Positioning (PPP) and with the advent of the new IGS-Real-Time-Service (RTS), it is necessary to assess the possibility of a wider role of the PPP-based positioning technique in maritime applications. This paper investigates the performance of an autonomous real-time PPP-positioning solution by using the IGS- RTS service for maritime applications that require an accurate positioning system. To examine the performance of the real-time IGS-RTS PPP-based technique for maritime applications, kinematic data from a dual frequency GPS receiver is investigated. It is shown that the real-time IGS-RTS PPP-based GPS positioning technique fulfills IMO requirements for maritime applications with an accuracy requirement ranges from 10 m for Ocean/Coastal/Port approach/Inland waterways navigation to 1.0 m for in port navigation but cannot fulfill the automatic docking application with an accuracy requirement of 0.10 m. To further investigate the real-time PPP-based GPS positioning technique, a comparison is made between the real-time IGS-RTS PPP-based positioning technique and the real-time PPP-based positioning by using the predicted part of the IGS Ultra-Rapid products and the real-time GPS positioning technique with the Wide Area Differential GPS service (WADGPS). It is shown that the IGS-RTS PPP-based positioning technique is superior to the IGS-Ultra-Rapid PPP-based and WADGPS-based positioning techniques.

의사위성 기반 광역보정시스템(WA-DGPS) 구축 기술개발 및 성과

본 논문에서는 현재 국내에서 개발 중인 NDGPS 기준국을 활용한 의사위성 기반 광역보정시스템의 주요 알고리즘 및 성과에 대해서 설명한다. 광역보정시스템은 지상의 기준국 네트워크를 활용하여 전 국토에 적용 가능한 위성 및 전리층 지연 오차의 보정정보 및 무결성 정보를 생성하여 서비스 지역 내의 모든 사용자에 균일한 수준의 보강 서비스를 제공하는 시스템이다. 본 연구에서는 광역보정시스템의 지상 시스템을 개발하고 이의 성능을 지상에 설치된 의사위성을 활용하여 검증할 계획이다. 또한 2014년부터는 위성기반 광역보정시스템 (SBAS)의 개발이 시작될 예정이고, 본 연구에서 개발된 연구 결과들이 직 간접적으로 활용되어 연구개발이 이루어질 예정이다. This paper describes the progress and the plan of 'Wide Area Differential GPS (WA-DGPS) Development' project supported by Korean Ministry of Oceans and Fisheries. The project develops the main algorithms of the WA-DGPS which guarantees the improved accuracy, availability, and integrity all over the Korean peninsula. After the establishment of WA-DGPS ground infrastructure system, a real-time demonstration using pseudolite installed on the ground will be conducted in the final year. Also, the development of Korean Satellite-based Augmentation System (SBAS) is expected to be started from 2014, and the algorithms and the results in the WA-DGPS project will be used in the SBAS development.

SBAS orbit and satellite clock corrections for precise point positioning

The quality of real-time GPS positions based on the method of precise point positioning (PPP) heavily depends on the availability and accuracy of GPS satellite orbits and satellite clock corrections. Satellite-based augmentation systems (SBAS) provide such corrections but they are actually intended to be used for wide area differential GPS with positioning results on the 1-m accuracy level. Nevertheless, carrier phase-based PPP is able to achieve much more accurate results with the same correction values. We applied SBAS corrections for dual-frequency PPP and compared the results with PPP obtained using other real-time correction data streams, for example, the GPS broadcast message and precise corrections from the French Centre National d'Etudes Spatiales and the German Deutsches Zentrum fur Luft- und Raumfahrt. Among the three existing SBAS, the best results were achieved for the North American wide area augmentation system (WAAS): horizontal and vertical position accuracies were considerably smaller than 10 cm for static 24-h observation data sets and smaller than 30 cm for epoch-by-epoch solutions with 2 h of continuous observations. The European geostationary navigation overlay service and the Japanese multi-functional satellite augmentation system yield positioning results with biases of several tens of centimeters and variations larger by factors of 2---4 as compared to WAAS.

Implementation and Evaluation of the WADGPS System in the Taipei Flight Information Region

This paper describes the implementation of the Wide Area Differential Global Positioning System (WADGPS) system in order to evaluate the operational performance of a satellite based aviation navigation system within Taipei Flight Information Region (FIR). The main objective of the WADGPS is to provide real time integrity information regarding the use of GPS for civil aviation applications. This paper uses the e-GPS observation stations operated by the Taiwan Ministry of Interior (MOI) as the WADGPS reference stations to collect the L1-L2 dual-frequency GPS measurements. A WADGPS master station is also implemented to process all GPS measurements sent from each reference station, and then generate the vector corrections. These vector corrections consist of the satellite ephemeris and clock errors, and a grid of ionospheric delays. The data stream also includes confidence bounds for the corrections and “Use/Do Not Use” messages to provide integrity. These messages are then passed to the WADGPS user through the Internet. This paper discusses the WADGPS system architecture and the system performance analysis. A five-day operation performance in Taipei Flight Information Region (FIR) is presented in this paper. The results show that the WADGPS can improve the accuracy performance of GPS positioning and fulfill the integrity performance required by Non-Precision Approach (NPA) defined by the International Civil Aviation Organization (ICAO).

Potential for issuing ionospheric warnings to Canadian users of marine DGPS

Under normal operating conditions marine DGPS horizontal positioning accuracies on the order of several meters are achieved in North America. Degradations in positioning accuracy can occur during enhanced ionospheric activity. An ionospheric phenomenon known as storm enhanced density (SED) is observed to develop in the middle to high latitudes during ionospheric storm events. Very large gradients in total electron content are observed in the vicinity of this feature with DGPS positioning errors increased by a factor of 10–30 versus quiet conditions. The specific evolution of a given SED event and the magnitude of expected impact are not generally predictable. A method to monitor development of SED is to compute ionospheric maps in real time. Local gradients can then be computed for various geographic regions from North American maps of ionospheric delay. Sources of real‐time ionospheric information include the Wide Area Augmentation System (WAAS) and the Canadian GPS•C service. These are wide area differential GPS systems. In this paper, a real‐time ionospheric warning system is investigated for North American (primarily Canadian) DGPS users based on available real‐time data. The WAAS and GPS•C ionospheric models are inadequate to resolve ionospheric gradients for 100–200 km scale sizes. Raw GPS data from GPS•C reference sites can be used, however, to observe large ionospheric gradients and interpret the expected impact on DGPS users. Potential exists to issue marine user warnings based on this method. Results of this work can readily be extended to land DGPS applications, such as the NDGPS service in the United States.

Dual-Frequency GPS Precise Point Positioning with WADGPS Corrections

ABSTRACT: The goal of the research described in this paper is the design of a GPS dual-frequency data processing technique capable of producing high-accuracy positioning results with wide area differential GPS (WADGPS) corrections. The main issues in using WADGPS corrections for dual-frequency GPS point positioning are the satellite clock referencing issue and how to handle the increased noise level by use of the ionosphere-free dual-frequency combination. To address these concerns, a sequential forward carrier-phase smoothing technique which utilizes the fully combined uncertainty for both systematic and random errors in the smoothing process has been designed. To account for the satellite clock referencing issue, the effects of the satellite instrumental biases have been precisely investigated and the observation equations for the different observables assuming the source of corrections is WADGPS have been developed. Results determined via developed software indicate that positioning accuracy at the few decimeter-level is attainable at a 95% confidence level.

Performance of satellite‐based navigation for marine users during ionospheric disturbances

The Global Positioning System (GPS) is used worldwide for marine navigation in support of hydrographic surveying operations, where horizontal positioning requirements are typically better than 10 m (95%). Differential techniques are used to reduce errors associated with signal propagation through the dispersive ionosphere and to achieve such accuracies. Two such methods are differential GPS (DGPS), in which range corrections are derived for a nearby reference station and are applied at the remote user location, and wide‐area differential GPS (WADGPS), in which ionosphere range errors are modeled over a large area using a sparse network of GPS ground stations. Both DGPS and WADGPS (Wide Area Augmentation Service) positioning accuracies are investigated here, throughout North America, for a severe geomagnetic storm event in 2003. DGPS horizontal positioning errors of 10–15 m (95%) are observed during this event, and WADGPS positioning errors generally exceed those for DGPS. This is attributed to the sparse WADGPS reference network and associated limitations in resolving the severe ionosphere gradients.

Performance Analysis of Korean WADGPS Algorithms with NDGPS Data

To provide more accurate and reliable positioning and timing services to Korean nationwide users, the Ministry of Maritime Affairs and Fisheries of Korea is implementing Korean NDGPS (Nationwide DGPS), which is operational partly. And it also has a plan to construct WADGPS (Wide Area Differential GPS) system using sites and equipments of the NDGPS reference stations. For that, Seoul National University GNSS Laboratory is implementing and testing prototypes of WRS (Wide-area Reference Station) and WMS (Wide-area Master Station). Until now, because there are not enough installed WRSs to be used for computing wide area correction information, we cannot test algorithms of WMS with the data processed actually in WRSs. Therefore to evaluate the performance of the algorithms, we made a MATLAB program which can process RINEX (Receiver INdependent Exchange) format data with WADGPS algorithm. Using that program which consists of WRS, WMS and USER modules, we processed the data collected at NDGPS reference stations, which are saved in RINEX format. In WRS module, we eliminate the atmospheric delay error from the pseudorange measurement, smooth the measurement by hatch filter and calculate pseudorange corrections for each satellite. WMS module collects the processed data from each reference stations to generate the wide area correction information including estimated satellite ephemeris errors, ionospheric delays at each grid point, UDRE (User Differential Range Error), GIVE (Grid Ionosphere Vertical Error) and so on. In USER part, we use the measurements of reference stations as those of users and estimate the corrected users' positions and protection levels (HPL, VPL). With the results of estimation, we analyzed the performance of the algorithms. We assured the estimated UDRE /GIVE values and the protection levels bound the corresponding errors effectively. In this research, we can expect the possible performance of WADGPS in Korea, and the developed modules will be useful to implementation and improvement of the algorithms.

The Development and Testing of a Prototype Mobile Mapping System for Real-time Forest Fire Hot Spot Detection

The development of a mobile mapping system (MMS) to test the feasibility of automatic real-time identification and location of forest fire hot spots is discussed in this paper. The system combines an integrated inertial navigation system (INS) and Wide Area Differential Global Positioning System (WADGPS) with a thermal video imager and computer system. The overall objective of the system is to achieve an overall positioning accuracy of 2 m or better in real-time for the location of forest fire hot spots. The direct georeferencing model used in this system is briefly presented as well as how it is partitioned for implementing the real-time georeferencing of the thermal images and the final hot spot georeferencing. The paper then presents some specific issues affecting the identification, extraction, and tracking of forest fire hot spots from thermal images. Some of the hardware and software issues considered in designing the overall process flow for implementation as a real-time system are discussed next. Finally, an evaluation of the system’s potential real-time operation and georeferencing accuracy is presented.

The ionospheric impact of the October 2003 storm event on Wide Area Augmentation System

The United States Federal Aviation Administration’s (FAA) Wide-Area Augmentation System (WAAS) for civil aircraft navigation is focused primarily on the Conterminous United States (CONUS). Other Satellite-Based Augmentation Systems (SBAS) include the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-transport Satellite-based Augmentation System (MSAS). Navigation using WAAS requires accurate calibration of ionospheric delays. To provide delay corrections for single frequency global positioning system (GPS) users, the wide-area differential GPS systems depend upon accurate determination of ionospheric total electron content (TEC) along radio links. Dual-frequency transmissions from GPS satellites have been used for many years to measure and map ionospheric TEC on regional and global scales. The October 2003 solar-terrestrial events are significant not only for their dramatic scale, but also for their unique phasing of solar irradiance and geomagnetic events. During 28 October, the solar X-ray and EUV irradiances were exceptionally high while the geomagnetic activity was relatively normal. Conversely, 29–31 October was geomagnetically active while solar irradiances were relatively low. These events had the most severe impact in recent history on the CONUS region and therefore had a significant effect on the WAAS performance. To help better understand the event and its impact on WAAS, we examine in detail the WAAS reference site (WRS) data consisting of triple redundant dual-frequency GPS receivers at 25 different locations within the US. To provide ground-truth, we take advantage of the three co-located GPS receivers at each WAAS reference site. To generate ground-truth and calibrate GPS receiver and transmitter inter-frequency biases, we process the GPS data using the Global Ionospheric Mapping (GIM) software developed at the Jet Propulsion Laboratory. This software allows us to compute calibrated high resolution observations of TEC. We found ionospheric range delays up to 35 m for the day-time CONUS during quiet conditions and up to 100 m during storm time conditions. For a quiet day, we obtained WAAS planar fit slant residuals less than 2 m (0.4 m root mean square (RMS)) and less than 25 m (3.4 m RMS) for the storm day. We also investigated ionospheric gradients, averaged over distances of a few hundred kilometers. The gradients were no larger than 0.5 m over 100 km for a quiet day. For the storm day, we found gradients at the 4 m level over 100 km. Similar level gradients are typically observed in the low-latitude region for quiet or storm conditions.

Modified Tomographic Estimation of the lonosphereusing Fewer Coefficients

Ionospheric time delay is the biggest error source for single-frequency DGPSapplications, including time transfer and Wide Area Differential GPS (WADGPS).Currently, there are many attempts to develop real-time ionospheric time delayestimation techniques to reduce positioning error due to the ionospheric time delay.Klobuchar model is now widely used for ionosphehc time delay calculation forsingle-frequency users. It uses flat surface at night time and cosine surface atdaytime[1], However, the model was developed for worldwide ionosphere fit, it isnot adequate for local area single-frequency users who want to estimateionospheric time delay accurate1y[2]. Therefore, 3-D ionosphere model usingtomographic estimation has been developed. 3-D tomographic inversion modelshows better accuracy compared with prior a1gorithms[3]. But that existing 3-Dmodel still has problem that it requires many coefficients and measurements forgood accuracy. So, that algorithm has Umitation with many coefficients incontinuous estimation at the small region which is obliged to have fewermeasurements.In this paper, we developed an modified 3-D ionosphehc time delay modelusing tomography, which requires only fewer coefficients. Because the combinationsof our base coefficients correspond to the full coefficients of the existing model, ourmodel has equivalent accuracy to the existing. We confirmed our algorithm bysimulations. The results proved that our modified algohthm can perform continuousestimation with fewer coefficients.

Application of Multipath Template Technique to Real-Time Ionospheric Delay Estimation Using Global Positioning System Measurements

When global positioning system (GPS) signals propagate through the ionosphere, they experience an extra time delay. The ionospheric delay can be estimated using L1/L2 GPS measurement. In order to mitigate the pseudorange multipath effect on real-time ionospheric delay estimation, the multiday multipath template technique has been developed. The algorithm permits the pseudorange multipath error for a specific day to be corrected using the generated multipath template. Data from National Chengchi University, in Taiwan was used to test the application of the multipath template to real-time ionospheric delay estimation. Test results indicate that: (1) The performance of the various 5 day multipath templates is better than that of other multiday multipath templates and (2) the multipath template technique is an effective method to mitigate the GPS pseudorange multipath and should be implemented at reference stations within local-area differential GPS, wide-area differential GPS, and wide-area augmentation system networks, GPS deformation monitoring networks, etc.

Modelling of differential single difference receiver clock bias for precise positioning

A receiver hardware delay should be seriously considered for time-transfer and determination of ionospheric delay corrections for wide area differential GPS positioning. A receiver hardware delay does not generally effect the common geo-position application, as suitable differences of observations are used, or equivalently, clock error parameters are introduced, epoch-wise, that also absorb the delays. This paper investigates the behavior of inter-frequency (or observation-type) receiver hardware delays by using a single difference (SD) model, which estimates the receiver delay along with the receiver clock error (and SD ambiguities of a reference satellite with carrier phase observations) for zero and short baselines. The purpose of this paper is to model the between-observation-type delays for the purpose of precise positioning, under practical circumstances. The focus is on data series of differential SD receiver clock biases, since they reflect the behavior of receiver hardware delays with time. A simple linear regression of the data series is employed to study the behavior, and test statistics are employed to assess both the significance of the parameters and the observations’ fit for the linear regression. The statistical analysis results indicate that almost all inter-observation type receiver delays can be modeled as the sum of a constant (offset) and a constant rate of change (slope). The analysis shows that the differential receiver delay is generally at the mm- to cm-level on phase, while at the dm-level on code for the equipment used in the experiments.

Continuous localization via wide-area Differential Global Positioning System for outdoor navigation of mobile robots

Mobile robots must have some effective method for determining their position in the working environment in order to operate appropriately. In this paper, precise outdoor positioning of a vehicle is achieved by continuously fusing odometry with wide-area Differential Global Positioning System (DGPS) data through a Kalman filter. The propagation and reduction of spatial uncertainty is computed together with the mobile robot's coordinates and heading. Two distinct sources of differential corrections have been applied to autonomous and teleoperated navigation of the Aurora mobile robot in a road network. Path teaching and path tracking have been successfully tested in several experiments over a relatively flat surface.

A MIXED-MODE GPS NETWORK PROCESSING APPROACH FOR DEFORMATION MONITORING APPLICATIONS

The Global Positioning System (GPS) can be utilised in a wide range of deformation monitoring applications, such as the monitoring of active volcanoes, tectonic fault lines, landslides, ground subsidence, bridges, dams, high-rise buildings, etc. During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent, ‘fiducial’ GPS stations with dual-frequency, geodetic-grade receivers surrounding an ‘inner’ network of low-cost, single-frequency GPS receivers. Such a configuration offers considerable flexibility and cost savings for deformation monitoring applications, which require a dense spatial coverage of GPS stations, and where it is not possible, nor appropriate, to establish permanent GPS networks using dual-frequency instrumentation.The basis of the processing methodology is to separate the dual-frequency, ‘fiducial’ station data processing from the baseline processing involving the inner (single-frequency) receivers located in the deformation zone. The data processing for the former is carried out using a modified version of the Bernese software, to generate a file of ‘corrections’ (analogous to Wide Area DGPS correction models for the distance dependent biases – primarily due to atmospheric refraction). These ‘corrections’ are then applied to the double-differenced phase observations from the inner receivers to improve the single-frequency baseline accuracies (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected).This configuration has been tested by simulating such a two-stage network using data collected by part of the Southern California Integrated GPS Network (SCIGN). Six sitesforming an inner network of three GPS receivers surrounded by three ‘fiducial’ GPS receivers are used in this study. All sites are equipped with dual-frequency instrumentation, however, for most of the analysis the inner sites are treated as single-frequency stations by ignoring the observations made on L2. A description of the processing strategy, together with a discussion of the results is presented.

A MIXED-MODE GPS NETWORK PROCESSING APPROACH FOR DEFORMATION MONITORING APPLICATIONS

The Global Positioning System (GPS) can be utilised in a wide range of deformation monitoring applications, such as the monitoring of active volcanoes, tectonic fault lines, landslides, ground subsidence, bridges, dams, high-rise buildings, etc. During the past few years a methodology has been developed for processing data collected by GPS networks consisting of a mixed set of single-frequency and dual-frequency receivers. The strategy is to deploy a few permanent, ‘fiducial’ GPS stations with dual-frequency, geodetic-grade receivers surrounding an ‘inner’ network of low-cost, single-frequency GPS receivers. Such a configuration offers considerable flexibility and cost savings for deformation monitoring applications, which require a dense spatial coverage of GPS stations, and where it is not possible, nor appropriate, to establish permanent GPS networks using dual-frequency instrumentation.The basis of the processing methodology is to separate the dual-frequency, ‘fiducial’ station data processing from the baseline processing involving the inner (single-frequency) receivers located in the deformation zone. The data processing for the former is carried out using a modified version of the Bernese software, to generate a file of ‘corrections’ (analogous to Wide Area DGPS correction models for the distance dependent biases – primarily due to atmospheric refraction). These ‘corrections’ are then applied to the double-differenced phase observations from the inner receivers to improve the single-frequency baseline accuracies (primarily through empirical modelling of the residual atmospheric biases that otherwise would be neglected).This configuration has been tested by simulating such a two-stage network using data collected by part of the Southern California Integrated GPS Network (SCIGN). Six sitesforming an inner network of three GPS receivers surrounded by three ‘fiducial’ GPS receivers are used in this study. All sites are equipped with dual-frequency instrumentation, however, for most of the analysis the inner sites are treated as single-frequency stations by ignoring the observations made on L2. A description of the processing strategy, together with a discussion of the results is presented.

Global Differential GPS Positioning without Using a Base Station

Stand-alone GPS is currently capable of providing positioning solutions at accuracy from several meters up to several tens of meters. To obtain a better positioning accuracy, differential GPS techniques must be used including wide area differential GPS networks. Significant efforts are currently underway to develop new processing methods to allow stand-alone point positioning to achieve accuracy at a decimeter to centimeter level. This paper describes the concept of global DGPS positioning without the use of base stations. In addition to the conventional code-based data processing method, a carrier phase based data processing technique has been described in this paper along with test result.

Limitations in DGPS positioning accuracies at low latitudes during solar maximum

While it is possible to mitigate the impact of ionospheric effects on Global Positioning System (GPS) positioning applications through differential techniques, residual errors may persist in regions of steep TEC gradients. An enhancement of absolute TEC and large‐scale gradients is observed at low latitudes near the equatorial anomaly. This effect is significant in the equinoctial months during periods of solar maximum. In this paper, differential GPS (DGPS) positioning accuracies in the anomaly region are investigated during the period of solar maximum 1999–2000, using as data the L1 pseudoranges. TEC gradients of 30 TECU per 100 km are observed during March 2000, with corresponding horizontal and vertical position errors of approximately 25–30 m (95% confidence level) for single baseline processing. Positioning accuracies are improved by a factor of 5 for a wide area DGPS approach.