Network RTK Research Articles

Statistical analysis of accuracy and precision of GNSS receivers used in network RTK

It is important to make accurate and precise measurements in surveying applications. The concepts of accuracy and precision are not synonymous, even though they are commonly used in place of each other. Nowadays, real-time kinematic (RTK) method is widely used at surveying. As the RTK method can be done depending on a reference station, in countries which establish Continuously Operating Reference Stations (CORS) Network, method known as Network-RTK (N-RTK) can be done depending on the CORS Network. Continuously Operating Reference Stations-Turkey Network (CORS-TR), which consists of 146 reference stations that allow positioning both real time and post-process, was established in 2009. In this study, accuracy and precision of Global Navigation Satellite Systems (GNSS) receivers are tried to determine depending on different correction techniques. For this purpose, 12-h GNSS observations were performed at SLCK-Turkish National Fundamental GPS Network (SLCK-TNFGN) point. The observations were adjusted based on CORS-TR. N-RTK measurements were performed with different GNSS receivers, and accuracies of the receivers were investigated. In order to determine precisions of the receivers, means of RTK measurements were calculated and precisions of the receivers were determined. As a result of investigation, it is seen that accuracy and precision of receivers at 2D positioning and height vary depending on correction technique.

Analysis of Positioning Accuracy of PPP, VRS, DGPS in Coast and Inland Water Area of South Korea

Jang, W.S.; Park, H.S, and Park, S.G., 2018. Analysis of Positioning Accuracy of PPP, VRS, DGPS in Coast and Inland Water Area of South Korea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1276–1280. Coconut Creek (Florida), ISSN 0749-0208.Various research activities are being conducted every year in the coastal and inland water areas of Korea. Among them, location accuracy is very important for coastline change survey, depth survey, and coastal route analysis. VRS and DGPS are commonly used in coastal and inland areas. VRS is a type of Network RTK that offers high location accuracy and ease of use. However, due to the limitations of the Network RTK method, VRS is not able to significantly degrade or use its location accuracy unless the survey area is located within the network. On the other hand, DGPS has a position accuracy of about 1m, which is lower than that of VRS. However, DGPS is widely used for location measurement in the coastal area due to its wide service range. PPP is the most recently developed satellite positioning method. PPP can be used anywhere in Korea, like DGPS, while providing accuracy similar to VRS's positioning accuracy. However, PPP is still an early stage positioning technology and has a long convergence time to show high positioning accuracy and high complexity of use. Therefore, it is not used for research activities in actual coastal and inland water. In this paper, we present our PPP positioning system and analyze the results of PPP using our PPP positioning system, VRS and DGPS positioning presented in coastal and inland areas of Korea. In this paper, we compare and analyze the accuracy of three positioning methods and present the results.

The Comparison and Analysis of BDS NRTK between DD and UD Models

The BeiDou Navigation Satellite System (BDS) Network Real Time Kinematic (NRTK) position algorithm with both Double-Differenced (DD) model and Un-Differenced (UD) model is investigated in this study. This algorithm is realized and validated with experimental BDS data collected at four regional stations. For the experiment within the regional network, a short convergence time for each model is considered. The results show that the BDS NRTK is able to provide positioning service with accuracy at the cm level. Theoretically, UD-NRTK and DD-NRTK are equivalent to each other. The small differences found between the DD and UD models in this study can be clarified in that the error correction for the DD model is more precise than that for the UD model, resulting in faster solution convergence in the DD model. However, the noise of the DD model is two times of the UD model, which usually leads to larger final RMS of positioning accuracy for the DD model. Copyright © 2018 Institute of Navigation

Check and calibration of a single GNSS receiver by using the VRS RTN positioning method

The constant evolution of the demands of geodetic applications requires instruments, products and services with certified specifications and measurement accuracy.Particularly Global Navigation Satellite Systems (GNSS) are now closely approaching the same uncertainty as measurements provided by total stations, in current geodetic applications. In addition, the small occupation time required to measure points, leads to the increased use of them.The technological advancement of GNSS receivers and the installation of many networks of permanent and continuously operated reference stations in the world have now resulted in the use of a single GNSS receiver to determine coordinates and geometric features worldwide.This paper proposes a methodology for both, the check and calibration of a single GNSS receiver, using the VRS method, which is subset of the network RTK positioning methods (from now on mentioned as RTN methods).In both procedures the appropriate statistical checks were carried out in order to conclude about the proper function of the receiver under check.The methodology succeeds the results by using efficient number and type of observations and simple mathematical models. So it is convenient to be used by professionals in order to improve and ensure their products. Moreover it may consists as a new supplement of the ISO, as the existing ISO 17,123-8 (ISO 17123-8, 2007) requires two GNSS receivers in order to be implemented by using the real time kinematic method. It is worth mentioned that ISO 17,123-8 informs the user only for the internal accuracy, namely the precision of the base receiver which is under check.Therefore the proposed methodology concludes not only about the precision of a single GNSS receiver being checked, but also about the provided accuracy due to the comparison with the true values.

Digital Topographic Mapping in Urban Obstructed Environment Based on Multi-GNSS Network RTK Technology

Digital topographic mapping experiments were carried out based on network RTK technology using GPS/BEIDOU/GLONASS multi-constellation compatible GNSS receivers in urban obstructed environment. Operation scheme and technique flow were discussed. Experimental results show that the horizontal position and elevation of the points measured by RTK can reach 2cm and 3cm precision level respectively in open environment. RTK initialization time needs about 3-5s. While in obstructed environment, such as high building and tree shanding, the RTK initialization time needs about dozens of seconds or tens of seconds, and sometimes floating solutions or even differential solutions were obtained. The impact of dense and tall building on RTK measurement is more seriously. It is more likely to get RTK fixed solution in the south side of high building than the north side of the building.

UAVS TO ASSESS THE EVOLUTION OF EMBRYO DUNES

Abstract. The balance of a coastal environment is particularly complex: the continuous formation of dunes, their destruction as a result of violent storms, the growth of vegetation and the consequent growth of the dunes themselves are phenomena that significantly affect this balance. This work presents an approach to the long-term monitoring of a complex dune system by means of Unmanned Aerial Vehicles (UAVs). Four different surveys were carried out between November 2015 and November 2016. Aerial photogrammetric data were acquired during flights by a DJI Phantom 2 and a DJI Phantom 3 with cameras in a nadiral arrangement. GNSS receivers in Network Real Time Kinematic (NRTK) mode were used to frame models in the European Terrestrial Reference System. Processing of the captured images consisted in reconstruction of a three-dimensional model using the principles of Structure from Motion (SfM). Particular care was necessary due to the vegetation: filtering of the dense cloud, mainly based on slope detection, was performed to minimize this issue. Final products of the SfM approach were represented by Digital Elevation Models (DEMs) of the sandy coastal environment. Each model was validated by comparison through specially surveyed points. Other analyses were also performed, such as cross sections and computing elevation variations over time. The use of digital photogrammetry by UAVs is particularly reliable: fast acquisition of the images, reconstruction of high-density point clouds, high resolution of final elevation models, as well as flexibility, low cost and accuracy comparable with other available techniques.

Accuracy Improvement of DGPS for Low-Cost Single-Frequency Receiver Using Modified Flächen Korrektur Parameter Correction

A differential global positioning system (DGPS) is one of the most widely used augmentation systems for a low-cost L1 (1575.42 MHz) single-frequency GPS receiver. The positioning accuracy of a low-cost GPS receiver decreases because of the spatial decorrelation between the reference station (RS) of the DGPS and the users. Hence, a network real-time kinematic (RTK) solution is used to reduce the decorrelation error in the current DGPS system. Among the various network RTK methods, the Flächen Korrektur parameter (FKP) is used to complement the current DGPS, because its concept and system configuration are simple and the size of additional data required for the network RTK is small. The FKP was originally developed for the carrier-phase measurements of high-cost GPS receivers; thus, it should be modified to be used in the DGPS of low-cost GPS receivers. We propose an FKP-DGPS algorithm as a new augmentation method for the low-cost GPS receivers by integrating the conventional DGPS correction with the modified FKP correction to mitigate the positioning error due to the spatial decorrelation. A real-time FKP-DGPS software was developed and several real-time tests were conducted. The test results show that the positioning accuracy of the DGPS was improved by a maximum of 40%.

Precision Farming in Hilly Areas: The Use of Network RTK in GNSS Technology

The number of GNSS satellites has greatly increased over the last few decades, which has led to increased interest in developing self-propelled vehicles. Even agricultural vehicles have a great potential for use of these systems. In fact, it is possible to improve the efficiency of machining in terms of their uniformity, reduction of fertilizers, pesticides, etc. with the aim of (i) reducing the timeframes of cultivation operations with significant economic benefits and, above all, (ii) decreasing environmental impact. These systems face some perplexity in hilly environments but, with specific devices, it is possible to overcome any signal deficiencies. In hilly areas then, the satellite-based system can also be used to safeguard operators’ safety from the risk of rollover. This paper reports the results obtained from a rural development program (RDP) in the Lazio Region 2007/2013 (measure project 1.2.4) for the introduction and diffusion of GNSS satellites systems in hilly areas.

Research into GNSS levelling using network RTK in Taiwan

Modern height surveying obtains orthometric heights through geoid undulations and ellipsoidal heights derived from global navigation satellite system (GNSS), a process referred to as ‘GNSS levelling’. In this study, an efficient survey technology, the e-global navigation satellite system (e-GNSS), was used instead of static satellite surveying to execute GNSS levelling. The e-GNSS in Taiwan is based on Virtual Reference Station-Real-Time Kinematic technology and networks. The orthometric heights were calculated by subtracting the geoid undulations, derived from three geoid models (gravimetric, hybrid, and geometric models), from the ellipsoidal heights, derived from the e-GNSS. Additionally, two corrector methods, namely the difference and the corrector surface method, were adopted to improve the accuracies and the systematic errors. The experiment showed that the two corrector methods improved the accuracies of the orthometric heights in the gravimetric model by approximately 66.3 and 80.0% and mitigated the systematic errors in the gravity model by approximately 96.2 and 99.9%.

Determining the Coordinates of Control Points in Hydrographic Surveying by the Precise Point Positioning Method

After Global Navigation Satellite Systems (GNSS) were first used in the field of hydrography in 1980, developments in hydrographic surveying accelerated. Survey precision in hydrography has been improved for both horizontal and vertical positioning and seafloor acoustic measurement by means of these new developments. Differential Global Positioning System (DGPS), Real Time Kinematic (RTK) and Network RTK (NRTK) techniques are the satellite-based positioning techniques that are commonly used in shallow water surveys and shoreline measurements. In line with these developments, the newer Precise Point Positioning (PPP) has been introduced. Combining precise satellite positions and clocks with dual-frequency GNSS data, PPP can provide position solutions from the centimetre to decimetre level. In this study, the coordinates of control points were determined by using the Post-Process PPP (PP-PPP) technique. Seven test points, which are the points of the Continuously Operating Reference Station - Turkey (CORS-TR) network, are selected near the shorelines within Turkey. The 24-hour data was split from one to six hours by one hour periods. Automatic Point Positioning Service (APPS) was selected to process the data. The poisoning error of the test points were given and compared with International Hydrographic Organization (IHO) S44 hydrographic survey standards.

The study and realization of BDS un-differenced network-RTK based on raw observations

A BeiDou Navigation Satellite System (BDS) Un-Differenced (UD) Network Real Time Kinematic (URTK) positioning algorithm, which is based on raw observations, is developed in this study. Given an integer ambiguity datum, the UD integer ambiguity can be recovered from Double-Differenced (DD) integer ambiguities, thus the UD observation corrections can be calculated and interpolated for the rover station to achieve the fast positioning. As this URTK model uses raw observations instead of the ionospheric-free combinations, it is applicable for both dual- and single-frequency users to realize the URTK service. The algorithm was validated with the experimental BDS data collected at four regional stations from day of year 080 to 083 in 2016. The achieved results confirmed the high efficiency of the proposed URTK for providing the rover users a rapid and precise positioning service compared to the standard NRTK. In our test, the BDS URTK can provide a positioning service with cm level accuracy, i.e., 1cm in the horizontal components, and 2–3cm in the vertical component. Within the regional network, the mean convergence time for the users to fix the UD ambiguities is 2.7s for the dual-frequency observations and of 6.3s for the single-frequency observations after the DD ambiguity resolution. Furthermore, due to the feature of realizing URTK technology under the UD processing mode, it is possible to integrate the global Precise Point Positioning (PPP) and the local NRTK into a seamless positioning service.

A Unified Model for BDS Wide Area and Local Area Augmentation Positioning Based on Raw Observations.

In this study, a unified model for BeiDou Navigation Satellite System (BDS) wide area and local area augmentation positioning based on raw observations has been proposed. Applying this model, both the Real-Time Kinematic (RTK) and Precise Point Positioning (PPP) service can be realized by performing different corrections at the user end. This algorithm was assessed and validated with the BDS data collected at four regional stations from Day of Year (DOY) 080 to 083 of 2016. When the users are located within the local reference network, the fast and high precision RTK service can be achieved using the regional observation corrections, revealing a convergence time of about several seconds and a precision of about 2–3 cm. For the users out of the regional reference network, the global broadcast State-Space Represented (SSR) corrections can be utilized to realize the global PPP service which shows a convergence time of about 25 min for achieving an accuracy of 10 cm. With this unified model, it can not only integrate the Network RTK (NRTK) and PPP into a seamless positioning service, but also recover the ionosphere Vertical Total Electronic Content (VTEC) and Differential Code Bias (DCB) values that are useful for the ionosphere monitoring and modeling.

Effect of tropospheric models on derived precipitable water vapor over Southeast Asia

An interesting subject in the field of GPS technology is estimating variation of precipitable water vapor (PWV). This estimation can be used as a data source to assess and monitor rapid changes in meteorological conditions. So far, numerous GPS stations are distributed across the world and the number of GPS networks is increasing. Despite these developments, a challenging aspect of estimating PWV through GPS networks is the need of tropospheric parameters such as temperature, pressure, and relative humidity (Liu et al., 2015). To estimate the tropospheric parameters, global pressure temperature (GPT) model developed by Boehm et al. (2007) is widely used in geodetic analysis for GPS observations. To improve the accuracy, Lagler et al. (2013) introduced GPT2 model by adding annual and semi-annual variation effects to GPT model. Furthermore, Boehm et al. (2015) proposed the GPT2 wet (GPT2w) model which uses water vapor pressure to improve the calculations. The global accuracy of GPT2 and GPT2w models has been evaluated by previous researches (Fund et al., 2011; Munekane and Boehm, 2010); however, investigations to assess the accuracy of global tropospheric models in tropical regions such as Southeast Asia is not sufficient. This study tests and examines the accuracy of GPT2w as one of the most recent versions of tropospheric models (Boehm et al., 2015). We developed a new regional model called Malaysian Pressure Temperature (MPT) model, and compared this model with GPT2w model. The compared results at one international GNSS service (IGS) station located in the south of Peninsula Malaysia shows that MPT model has a better performance than GPT2w model to produce PWV during monsoon season. According to the results, MPT has improved the accuracy of estimated pressure and temperature by 30% and 10%, respectively, in comparison with GPT2w model. These results indicate that MPT model can be a good alternative tool in the absence of meteorological sensors at GPS stations in Peninsula Malaysia. Therefore, for GPS-based studies, we recommend MPT model to be used as a complementary tool for the Malaysia Real-Time Kinematic Network to develop a real-time PWV monitoring system.

Hydrographic survey of a south Florida canal using a hydrolite

Abstract In this study, a hydrographic map of a South Florida canal is prepared using RTK GPS (Real Time Kinematic Global Positioning System) measurements combined with a hydrolite (single beam echo sounder). RTK GPS measurements are made using both single RTK and Network RTK systems. Digital sounding measurements taken by hydrolite are compared to manual lead line measurements. Single RTK and Network RTK results differed in the order of tenth of a foot accuracy for horizontal coordinates and for height measurements variations can go up to several tenths. Sounding results indicate a best-fit trend line with a slope of 0.993, and an R squared value of 0.972, demonstrating that hydrolite measurements and manually collected depths at this site are well correlated.

Method and assessment of BDS triple-frequency ambiguity resolution for long-baseline network RTK

Fast and reliable ambiguity resolution (AR) over long baselines is one of the major challenges for large-scale GNSS network real-time kinematic (RTK) positioning. In the dual-frequency case, wide-lane (WL) ambiguities are usually fixed firstly, followed by the procedure of determination of the narrow-lane (NL) ambiguity integers based on the ionosphere-free model. For long-baseline network RTK, there are still some problems faced, for instance the WL AR still needs to take over tens of epochs due to the influence of pseudorange noises and multipath effects; the NL AR model is weak since more parameters are introduced, thus much time is needed to guarantee the precision of float ambiguity solutions. Along with the deployment of modernized GPS, BDS and other GNSS, the triple-frequency AR with real data gradually becomes feasible. Many literatures have proven the third signal will make AR faster and more reliable. In this paper based on the characteristics of network RTK, a modified ionosphere-free model was proposed to improve EWL/WL AR performance for long baselines, followed by the comparison with several commonly-used EWL/WL AR models. The NL AR performance with triple-frequency and dual-frequency observations was also tested. In NL AR, a partial AR strategy is adopted to weaken the negative influence of new-rising or low-elevation satellites. Experiments were conducted using real BDS triple-frequency data on 74km and 241km baselines. Results show that the modified ionosphere-free model can balance the ambiguity precision and ambiguity bias better for the EWL/WL AR. The NL AR with triple-frequency observations also performs significantly better than that in dual-frequency mode, with 29.9% and 32.9% improvement in time-to-first-fix (TTFF) for the 74km and 241km baselines respectively.

A new study of describing the reliability of GNSS Network RTK positioning with the use of quality indicators

The method of precise GNSS positioning using corrections from a network of reference stations, the so-called Network RTK, is currently the most accurate real time kinematic positioning method. The reliability of this method is largely dependent on the accuracy of determination of network ionospheric and geometric corrections (taking into account the tropospheric refraction and orbit errors). There are many indexes describing the reliability of Network RTK positioning in the aspect of the accuracy of modelling these errors. The so-called solution quality indicators are used for this purpose. They are parameters determined in the central network pre-processing which provide quantitative information regarding the predicted reliability of the positioning in an area encompassed by a network of reference stations. Unfortunately, their interpretation is hindered due to the lack of connection with basic parameters describing precise positioning quality, i.e. the correctness of the carrier phase ambiguity resolution and the accuracy of the rover position. This study presents a new approach to the design of quality indicators for the Network RTK method, based on a quantitative description of two parameters—solution accuracy and availability. The presented method is based on the existing parameters describing the probability of the correct fixing of ambiguities and the estimation of the fixed baseline solution accuracy. However, a stochastic model of observation, taking into account the accuracy of network corrections is used for these calculations. The proposed method enables a full account of all parameters affecting the reliability of positioning, which is not possible with the currently applied methods. Numerical tests of the new indicators carried out for a part of the regional reference stations network confirmed the effectiveness of this approach. The proposed indicators provide much more clear identification of the time periods for which the reliability of the solution was lower compared to the existing quality indexes which indications were unclear and inaccurate.

Tropospheric Anomaly Detection in Multi-reference Stations Environment during Localized Atmosphere Conditions-(1) : Basic Concept of Anomaly Detection Algorithm

Extreme tropospheric anomalies such as typhoons or regional torrential rain can degrade positioning accuracy of the GPS signal. It becomes one of the main error terms affecting high-precision positioning solutions in network RTK. This paper proposed a detection algorithm to be used during atmospheric anomalies in order to detect the tropospheric irregularities that can degrade the quality of correction data due to network errors caused by inhomogeneous atmospheric conditions between multi-reference stations. It uses an atmospheric grid that consists of four meteorological stations and estimates the troposphere zenith total delay difference at a low performance point in an atmospheric grid. AWS (automatic weather station) meteorological data can be applied to the proposed tropospheric anomaly detection algorithm when there are different atmospheric conditions between the stations. The concept of probability density distribution of the delta troposphere slant delay was proposed for the threshold determination.

A REVIEW ON LEGAL TRACEABILITY OF GNSS MEASUREMENTS IN THE MALAYSIAN CADASTRAL PRACTICE

Abstract. As the dependency on Global Navigation Satellite System (GNSS) in surveying has been growing over the years, the need for legal traceability of GNSS measurements has become a significant matter. In Malaysia, with the advent of the Malaysia Real-time Kinematic Network (MyRTKnet), GNSS surveying has revolutionised land survey and mapping. Correspondingly, the Department of Survey and Mapping Malaysia (DSMM) amended and published standard regulations and guidelines concerning cadastral survey, i.e., Cadastral Survey Regulations 2009, to include GNSS measurements. However, these regulations and guidelines has not comprehensively incorporated legal traceability of GNSS measurements; which is a prerequisite for cadastral surveys as it requires reliable and conclusive evidence for issues such as boundary disputes. The first objective of this paper is to review and discuss the legal traceability of GNSS measurements. Secondly, it will highlight the current practice and issues, i.e., with regard to legal traceability, within the present Malaysian cadastral regulation and guidelines, in relation to the prevalently adopted Network RTK (N-RTK) technique, GNSS instrument calibrations, and reference stations’ accuracy. Lastly, a rudimentary best practice guideline for GNSS surveying in cadastral survey for Malaysia is proposed. It is expected that this paper will contribute to the implementation of a best practice guideline, which is inclusive of legal traceability of GNSS measurements, for the Malaysian cadastral practice.

KALİBRASYON BAZINDA FARKLI YÖNTEMLERİN MESAFE ÖLÇME HASSASİYETLERİNİN KARŞILAŞTIRILMASI

Satellite-based measurements have been performed on EDM Calibration Base by Classic GPS, Network RTK and Static GPS methods and terrestrial measurements have been conducted with Total Stations in this study. FKP, VRS and MAC correction methods were used in the Network RTK method. The objective of this study was to make statistical comparisons of distances obtained by using all combinations between different measurement methods. Analysis of Variance (ANOVA) was utilized for this purpose. It was determined that there was a statistically significant difference between the measuring methods used in the one-way ANOVA test. The differences and standard deviations obtaind by the comparison of distances based on different measurement methods between benchmarks have been varied from mm to cm. The results obtained by the static GNSS surveying method are better than the standard deviations obtained by other methods of measurement and equipments when compared. The methods according to the distance measurement precision have been listed from the highest precision to low as Static GNSS, terrestrial surveying, classical RTK, FKP, VRS and MAC respectively. The distance measurement precision of Terrestrial measurement was found to be better than the classical RTK and NRTK measurement methods. The distance measurement precision of the classic RTK method was better than NRTK methods obtained by the different correction methods. In comparison, of NRTK correction methods in itself, the best results were obtained by FKP method. Keywords : GNSS, NRTK, MAC, FKP, VRS, Total Station, precision

Improved ambiguity resolution for URTK with dynamic atmosphere constraints

Raw observation processing method with prior knowledge of ionospheric delay could strengthen the ambiguity resolution (AR), but it does not make full use of the relatively longer wavelength of wide-lane (WL) observation. Furthermore, the accuracy of calculated atmospheric delays from the regional augmentation information has quite different in quality, while the atmospheric constraint used in the current methods is usually set to an empirical value. A proper constraint, which matches the accuracy of calculated atmospheric delays, can most effectively compensate the residual systematic biases caused by large inter-station distances. Therefore, the standard deviation of the residual atmospheric parameters should be fine-tuned. This paper presents an atmosphere-constrained AR method for undifferenced network RTK (URTK) rover, whose ambiguities are sequentially fixed according to their wavelengths. Furthermore, this research systematically analyzes the residual atmospheric error and finds that it mainly varies along the positional relationship between the rover and the chosen reference stations. More importantly, its ionospheric part of certain location will also be cyclically influenced every day. Therefore, the standard deviation of residual ionospheric error can be modeled by a daily repeated cosine or other functions with the help of data one day before, and applied by rovers as pseudo-observation. With the data collected at 29 stations from a continuously operating reference station network in Guangdong Province (GDCORS) in China, the efficiency of the proposed approach is confirmed by improving the success and error rates of AR for 10-20 % compared to that of the WL-L1-IF one, as well as making much better positioning accuracy.