Effects of stochastic models in GPS positioning

GPS ölçmelerindeki en önemli hata kaynakları multipath etkisi ve gecikme etkisidir. Son yıllarda bu hataları en aza indirme amacıyla yapılan araştırmalar ağırlıklı olarak, yeni troposferik gecikme modellerin üretilmesi, multipath etkisinin modellenmesi ve yeni stokastik modellerin geliştirilmesi üzerinde yoğunlaşmıştır. Bu çalışmada sinyal kalitesine dayalı bir stokastik model ve bu modelin nokta konumlarının belirlenmesi üzerindeki etkileri incelenmektedir. Temel sinyal kalitesi ölçütü olan sinyal-noise oranına dayalı olan bu model GPS ölçüleri üzerinde uygulanmış ve geleneksel eş-ağırlıklı değerlendirme modeli ile karşılaştırmalı olarak analiz edilmiştir. Bu modelin uygulanmasıyla daha iyi koordinat çözümleri elde edilmiştir. Sonuçlar, ayrıca multipath etkisinin birbirini izleyen günlerdeki tekrarlanabilirliğini ortaya koymaktadır.Anahtar Kelimeler: GPS, stokastik model, sigma-ε.The most important error sources in GPS measurements are the multipath effect, taking place when the electromagnetic wave reflects from the reflecting surfaces around the receiver, and the delay effect that occurs during the propagation of the wave through the troposphere because of the refraction effect. In recent years, many scientists have been doing research for minimising the effects of these error sources. These researches mainly focus on the development of the mapping functions produced for modelling the tropospheric delay with respect to the meteorological data or mathematical methods, modelling of the multipath effect and development of new stochastic or weighting models, which enable us to process the low elevated GPS observations too. In this study a signal quality based stochastic model and its impact on the point positioning are investigated. This model is the Sigma-? model, which is based on the signal-to-noise ratio, which is the basic signal quality measure. This model has been applied on some GPS measurements. It has been shown that through the application of this model better coordinate solutions are obtained. The results also show that this model characterises the day-to-day repeatability of the multipath effect.Keywords: GPS, stochastic model, sigma-?

A Variance Model in NRTK-Based Geodetic Positioning as a Function of Baseline Length

This study examines the effect of baseline length on accuracy and precision in Network Real-Time Kinematic (NRTK) positioning and develops an experimental mathematical model to express this effect. The study also measures the performances of the Flaechen Korrektur Parameter (FKP) and Virtual Reference Stations (VRS) methods at different baseline lengths. The study makes use of the stations that form two Continuously Operating Reference Station (CORS) networks, one of which is local and the other national. Calculations were made to perform various geodetic operations, such as datum transformations between the two networks, identifications of positional velocities, and epoch shifting. BERNESE (v5.2) software was used to identify coordinate values assumed to be true based on International GNSS Service (IGS) products. No significant changes were observed in the RMSE values in baseline lengths of up to 40 km. In contrast, an average linear correlation of 69.2% was determined between precision and baseline length. Measurements were evaluated and tested using the variance model created as a function of the baseline length, in line with the aims of the study, and the results were found to be consistent. Moreover, in an examination of the Root Mean Square Error (RMSE) and precision values of the FKP and VRS measurements, no significant differences were observed. The mean differences were at the millimetre level

Investigation of post-earthquake displacements in viaducts using Geodetic and finite element methods

WOS: 000285577800012This paper presents the results of research into the post-earthquake displacements of the partially constructed road viaducts in Bolu, Turkey after the Izmit/Kocaeli, (M-w = 7.4), and Duzce (M-w = 7.1) earthquakes on 17 August and 12 November 1999, respectively. The investigations on the viaducts were carried out using both Geodetic and Finite Element Methods (FEM). Firstly, all the geodetic network stations selected for the project were checked because of the recent deformation in the area. Then, new control stations were placed between the piers of the viaducts. 28 object points were placed and measured on each pier to determine their displacements. In the second stage, the behaviours of the viaducts were modelled using the FEM, and the Duzce earthquake acceleration record was analysed to observe the response of the viaducts in a time history domain. The modelled displacement response of the viaducts was compared with the geodetic measurements in order to interpret the sensitivity of the design calculation of the engineering model. The pier displacements that were geodetically measured and calculated using FEM peak pier displacements showed an increase in the piers located closer to the surface rupture from the Izmit/Kocaeli and Duzce earthquakes. The agreement between the observed and modelled displacements decreases with the increase in the distance from the fault line. Since, near the fault trace the horizontal displacement field is discontinuous and large inelastic deformation is expected, the behaviour of the part of the structure located near the fault line cannot be easily reproduced by FEM simulations. This is because the applied model loads derived from the source acceleration spectra cannot be included in the localized finite deformation effects. In order to obtain an improved engineering analysis, it is necessary to utilise more parameters in the numerical analysis

Quality assessment of global gravity field models in coastal zones: A case study using astrogeodetic vertical deflections in Istanbul, Turkey

We present the first high-precision astrogeodetic vertical deflection (VD) observations collected in Istanbul, Turkey, using a novel lightweight total station integrated with a charge-coupled device (CCD) camera, the QDaedalus system. The observed VDs are unique in that, they were measured for the first time in Istanbul, and they form Turkey’s first dense astrogeodetic network. To establish the Istanbul Astrogeodetic Network (IAN), we selected 30 benchmarks (BMs) with known geodetic coordinates. A total of 21 of these BMs are located in the coastal zone allowing us to investigate the quality of global gravity field models (GGFMs) along the coast of Istanbul. The standard deviations for our VDs are approximately ±0.20″which is commensurate with the VD accuracy of early studies assessing the QDaedalus observations. In particular, dedicated comparison measurements were conducted in two geographic regions—Munich and Istanbul—to control the accuracy of the VD measurements. Our new VD data set within the IAN was compared with predicted VDs from the Global Gravity Model plus (GGMplus) and the Earth Gravitational Model 2008 (EGM2008). The VD residuals between the QDaedalus observations, and predicted values from GGMplus and EGM2008 models tend to increase towards the coastlines, where discrepancies of several arcseconds were found. At 15 coastal BMs, the residuals in the N-S components exceed 2″ and reach values as large as 6″ while residuals in the E-W components exceeded 2″ at 3 BMs. We interpret these large differences as an indication of the current weaknesses in the GGF Ms, most likely reflecting errors in the altimetry-derived marine gravity measurements, which have been incorporated in the EGM2008 and GGMplus models, or the lack of coastal terrestrial gravity measurements, or both. We conclude that the astrogeodetic VDs observed by the QDaedalus are invaluable for independently assessing the quality of coastal-zone terrestrial gravity data sets and GGFMs