J. Bosy

Near-real-time regional troposphere models for the GNSS precise point positioning technique

The GNSS precise point positioning (PPP) technique requires high quality product (orbits and clocks) application, since their error directly affects the quality of positioning. For real-time purposes it is possible to utilize ultra-rapid precise orbits and clocks which are disseminated through the Internet. In order to eliminate as many unknown parameters as possible, one may introduce external information on zenith troposphere delay (ZTD). It is desirable that the a priori model is accurate and reliable, especially for real-time application. One of the open problems in GNSS positioning is troposphere delay modelling on the basis of ground meteorological observations. Institute of Geodesy and Geoinformatics of Wroclaw University of Environmental and Life Sciences (IGG WUELS) has developed two independent regional troposphere models for the territory of Poland. The first one is estimated in near-real-time regime using GNSS data from a Polish ground-based augmentation system named ASG-EUPOS established by Polish Head Office of Geodesy and Cartography (GUGiK) in 2008. The second one is based on meteorological parameters (temperature, pressure and humidity) gathered from various meteorological networks operating over the area of Poland and surrounding countries. This paper describes the methodology of both model calculation and verification. It also presents results of applying various ZTD models into kinematic PPP in the post-processing mode using Bernese GPS Software. Positioning results were used to assess the quality of the developed models during changing weather conditions. Finally, the impact of model application to simulated real-time PPP on precision, accuracy and convergence time is discussed.

Impact and implementation of higher-order ionospheric effects on precise GNSS applications

High precision Global Navigation Satellite Systems (GNSS) positioning and time transfer require correcting signal delays, in particular higher-order ionospheric (I2+) terms. We present a consolidated model to correct second- and third-order terms, geometric bending and differential STEC bending effects in GNSS data. The model has been implemented in an online service correcting observations from submitted RINEX files for I2+ effects. We performed GNSS data processing with and without including I2+ corrections, in order to investigate the impact of I2+ corrections on GNSS products. We selected three time periods representing different ionospheric conditions. We used GPS and GLONASS observations from a global network and two regional networks in Poland and Brazil. We estimated satellite orbits, satellite clock corrections, Earth rotation parameters, troposphere delays, horizontal gradients, and receiver positions using a global GNSS solution, Real-Time Kinematic (RTK) and Precise Point Positioning (PPP) techniques. The satellite-related products captured most of the impact of I2+ corrections, with the magnitude up to 2 cm for clock corrections, 1 cm for the along- and cross-track orbit components, and below 5 mm for the radial component. The impact of I2+ on troposphere products turned out to be insignificant in general. I2+ corrections had limited influence on the performance of ambiguity resolution and the reliability of RTK positioning. Finally, we found that I2+ corrections caused a systematic shift in the coordinate domain that was time- and region-dependent, and reached up to -11 mm for the North component of the Brazilian stations during the most active ionospheric conditions. .

Optimum stochastic modeling for GNSS tropospheric delay estimation in real-time

In GNSS data processing, the station height, receiver clock and tropospheric delay (ZTD) are highly correlated to each other. Although the zenith hydrostatic delay of the troposphere can be provided with sufficient accuracy, zenith wet delay (ZWD) has to be estimated, which is usually done in a random walk process. Since ZWD temporal variation depends on the water vapor content in the atmosphere, it seems to be reasonable that ZWD constraints in GNSS processing should be geographically and/or time dependent. We propose to take benefit from numerical weather prediction models to define optimum random walk process noise. In the first approach, we used archived VMF1-G data to calculate a grid of yearly and monthly means of the difference of ZWD between two consecutive epochs divided by the root square of the time lapsed, which can be considered as a random walk process noise. Alternatively, we used the Global Forecast System model from National Centres for Environmental Prediction to calculate random walk process noise dynamically in real-time. We performed two representative experimental campaigns with 20 globally distributed International GNSS Service (IGS) stations and compared real-time ZTD estimates with the official ZTD product from the IGS. With both our approaches, we obtained an improvement of up to 10% in accuracy of the ZTD estimates compared to any uniformly fixed random walk process noise applied for all stations.

IGS/EPN Reference Frame Realization in Local GPS Networks

The modern geodetic reference systems (e.g. ITRS) are realized by reference frames, i.e. a set of stations with position coordinates at a reference epoch and station velocities (e.g. ITRF2000). IGS (International GNSS Service) global network and EPN (EUREF Permanent Network) regional network stations are parametrized in this way. The main IGS/EPN products (gained daily and weekly) are estimated station coordinates and velocities, as well as orbits’ and ionospheric and tropospheric parameters. The connection of local GPS networks with IGS/EPN stations enables to use the above products. In this publication the method of EPN/IGS reference stations selection for the purpose of local GPS networks is presented. Two approaches are applied: station velocity analysis and cluster analysis. It has also been suggested to process permanent and epoch observations in local GPS networks, which are based on IGS global networks and EPN regional networks’ solutions jointly.

Tropospheric delay modelling for the EGNOS augmentation system

Tropospheric delay is one of the deleterious factors limiting the accuracy of the precise Global Navigation Satellite Systems positioning. The value of delay depends on the path through which a signal has to follow in the subsurface layers of the atmosphere. Tropospheric delay models are developed to overcome this limitation. Among them one can find UNB, TropGrid or IGGtrop models. In this paper, we adjusted the UNB3m model to the actual meteorological parameters from Europe. A new model was called UNBe.eu covering the EGNOS augmentation system area. The use of meteorological observations helped us to decrease the bias for more than 70% of reference radio sounding locations. Still, 30% of reference sites depicted a lack of any improvements of the ZTD estimation with regard to the newly established model. Therefore, this study puts forward a need for a deeper investigation of the discussed issue.

GNSS tomography, from epoch local solution to country-wide near real time system

GNSS meteorology is the remote sensing of the atmosphere (particularly troposphere) using Global Navigation Satellite Systems (GNSS) to derive information about its state. Over past five years the studies on GNSS tomography were performed in the Wroclaw University of Environmental and Life Sciences on the GNSS tomography. The results for epoch wise researches, allows to develop stable tomography numerical model (its optimal size and shape), assess errors, define special constraints, include in the model results of the analysis of the air flow. This paper presents the issues of the Near Real Time (NRT) tomographic model construction, the building of the required IT infrastructure, developing algorithms for effective numerical processing, requirements for NRT products, means of integration with other measurement techniques, comparison with weather prediction models.