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Uncovering common misconceptions in GNSS Precise Point Positioning and its future prospect / Suelynn Choy in GPS solutions, vol 21 n° 1 (January 2017)  

Public [article]  inGPS solutions > vol 21 n° 1 (January 2017) . - pp 13 - 22 Titre : Uncovering common misconceptions in GNSS Precise Point Positioning and its future prospect Type de document : Article/Communication Auteurs : Suelynn Choy, Auteur ; Sunil Bisnath, Auteur ; Chris Rizos, Auteur Année de publication : 2017 Article en page(s) : pp 13 - 22 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes IGN] positionnement ponctuel précis [Termes IGN] précision centimétrique [Termes IGN] prospective [Termes IGN] récepteur GNSS [Termes IGN] test de performance Résumé : (Auteur) Within the last decade, GNSS Precise Point Positioning (PPP) has generated unprecedented interest among the GNSS community and is being used for a number of scientific and commercial applications today. Similar to the conventional relative positioning technique, PPP could provide positioning solutions at centimeter-level precision by making use of the precise carrier phase measurements and high-accuracy satellite orbits and clock corrections provided by, for example, the International GNSS Service. The PPP technique is attractive as it is computationally efficient; it eliminates the need for simultaneous observations at both the reference and rover receivers; it also eliminates the needs for the rover receiver to operate within the vicinity of the reference receiver; and it provides homogenous positioning quality within a consistent global frame anywhere in the world with a single GNSS receiver. Although PPP has definite advantages for many applications, its merits and widespread adoption are significantly limited by the long convergence time, which restricts the use of the PPP technique for many real-time GNSS applications. We provide an overview of the current performance of PPP as well as attempt to address some of the common misconceptions of this positioning technique—considered by many as the future of satellite positioning and navigation. Given the upcoming modernization and deployment of GNSS satellites over the next few years, it would be appropriate to address the potential impacts of these signals and constellations on the future prospect of PPP. Numéro de notice : A2017-012 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s10291-016-0545-x En ligne : http://dx.doi.org/10.1007/s10291-016-0545-x Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=83934 [article]

Spatial–temporal variations of water vapor content over Ethiopia: a study using GPS observations and the ECMWF model / Kibrom Ebuy Abraha in GPS solutions, vol 21 n° 1 (January 2017)  

Public [article]  inGPS solutions > vol 21 n° 1 (January 2017) . - pp 89 - 99 Titre : Spatial–temporal variations of water vapor content over Ethiopia: a study using GPS observations and the ECMWF model Type de document : Article/Communication Auteurs : Kibrom Ebuy Abraha, Auteur ; Elias Lewi, Auteur ; Frédéric Masson, Auteur ; et al., Auteur Année de publication : 2017 Article en page(s) : pp 89 - 99 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes IGN] analyse spatio-temporelle [Termes IGN] données GPS [Termes IGN] Ethiopie [Termes IGN] humidité de l'air [Termes IGN] modèle météorologique [Termes IGN] sécheresse [Termes IGN] station GPS [Termes IGN] teneur en vapeur d'eau [Termes IGN] teneur intégrée en vapeur d'eau [Termes IGN] variation saisonnière Résumé : (Auteur) We characterize the spatial–temporal variability of integrated water vapor (IWV) in Ethiopia from a network of global positioning system (GPS) stations and the European Center for Medium range Weather Forecasting (ECMWF) model. The IWV computed from the ECMWF model is integrated from the height of the GPS stations on 60 pressure levels to take both the actual earth surface and the model orography discrepancies into account. First, we compare the IWV estimated from GPS and from the model. The bias varies from site to site, and the correlation coefficients between the two datasets exceed 0.85 at different time scales. The results of this study show that the general ECMWF IWV trend is underestimation over highlands and overestimation over lowlands for wet periods, and overestimation over highlands and underestimation over lowlands for dry periods with very few exceptional stations. Second, we observe the spatial variation of the IWV. High values are obtained in those stations that are located in the north-eastern (Afar depression) sites and the south-western part of the country. This distribution is related to the spatial variability of the climate in Ethiopia. Finally, we study the seasonal cycle and inter-annual variability of IWV for all stations over Ethiopia. The main result is the strong inter-annual variability observed for the dry seasons. Numéro de notice : A2017-013 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s10291-015-0508-7 En ligne : http://dx.doi.org/10.1007/s10291-015-0508-7 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=83935 [article]

Ionosphere probing with simultaneous GNSS radio occultations / Viet-Cuong Pham in GPS solutions, vol 21 n° 1 (January 2017)  

Public [article]  inGPS solutions > vol 21 n° 1 (January 2017) . - pp 101 - 109 Titre : Ionosphere probing with simultaneous GNSS radio occultations Type de document : Article/Communication Auteurs : Viet-Cuong Pham, Auteur ; Jyh-Ching Juang, Auteur Année de publication : 2017 Article en page(s) : pp 101 - 109 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes IGN] données GNSS [Termes IGN] gradient ionosphèrique [Termes IGN] inversion [Termes IGN] occultation du signal [Termes IGN] orbite basse [Termes IGN] régularisation de Tychonoff [Termes IGN] teneur totale en électrons Résumé : (Auteur) Radio occultation (RO) is a powerful technique for providing vertical profiles of refractivity, temperature, pressure, and water vapor of the neutral atmosphere and electron density of the ionosphere. The Abel inversion method which is based on the spherical symmetry assumption has been widely utilized to retrieve electron density profiles (EDPs) from RO measurements, which are available by observing Global Navigation Satellite System (GNSS) satellites from low-earth-orbit satellites. It is well known that the Abel inversion is subject to errors in the presence of ionospheric horizontal gradients. With the arrival of new navigation systems, the opportunities of establishing simultaneous GNSS RO events are increasing. We develop an improved Abel inversion technique that accounts for pairs of simultaneous RO events to relax the spherical symmetry assumption. Through the use of Tikhonov regularization, the problem is formulated so that numerical conditioning is improved and a priori information such as expected electron density, asymmetric factor, and vertical total electron content can be incorporated. Appropriate weighting can be determined to reflect the availability and quality of information. By balancing the reference data and measurements, the method thus paves a way for ionospheric probing in challenging geomagnetic conditions as both the EDP at the intersection and the horizontal gradients are retrieved. Simulation and experimental results are provided to show the effectiveness of the proposed method. The robustness and sensitivity of the proposed method are also assessed. Numéro de notice : A2017-014 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s10291-015-0501-1 Date de publication en ligne : 02/01/2016 En ligne : http://dx.doi./org/10.1007/s10291-015-0501-1 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=83936 [article]

GPS/BDS short-term ISB modelling and prediction / Nan Jiang in GPS solutions, vol 21 n° 1 (January 2017)  

Public [article]  inGPS solutions > vol 21 n° 1 (January 2017) . - pp 163 - 175 Titre : GPS/BDS short-term ISB modelling and prediction Type de document : Article/Communication Auteurs : Nan Jiang, Auteur ; Yan Xu, Auteur ; Tianhe Xu, Auteur ; et al., Auteur Année de publication : 2017 Article en page(s) : pp 163 - 175 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie spatiale [Termes IGN] analyse comparative [Termes IGN] données BeiDou [Termes IGN] données GNSS [Termes IGN] erreur systématique [Termes IGN] erreur systématique inter-systèmes [Termes IGN] méthode des moindres carrés [Termes IGN] modèle d'erreur [Termes IGN] orbitographie [Termes IGN] positionnement par BeiDou [Termes IGN] positionnement par GNSS [Termes IGN] positionnement ponctuel précis Résumé : (Auteur) The Chinese BeiDou Navigation Satellite System (BDS) has completed its first milestone by providing coverage of the Asia–Pacific area navigation service since December 27, 2012. With the combination of BDS, the GNSS precise point positioning (PPP) can improve its positioning accuracy, availability and reliability. However, in order to achieve the best positioning solutions, the inter-system bias (ISB) between GPS and BDS must be resolved as precisely as possible. In this study, a 1-week period (GPS week 1810) of GPS/BDS observations for 18 distributed stations from the International GNSS Service Multi-GNSS Experiment are processed. Primarily, the ISB is estimated by an extended Kalman filter as a piece-wise parameter every 30 min. Then we generate a smoothed ISB series (ISB_s) with a sliding window median filter to reject the outliers from the original estimated ISB series (ISB_o). After analysing the characteristics of the ISB_s, a short-term station-dependent ISB model based on a 1-week period is proposed in this study. This model consists of a quadratic polynomial in time and two or three periodic functions with diurnal and semi-diurnal periods. Frequency spectrum analysis is used to determine the periods of the periodic functions, and the coefficients of the quadratic function and the periodic functions are estimated by least squares. For model verification, we compare the ISB derived from the model (ISB_m) with ISB_s (assumed the true values). The comparisons indicate an almost normal distribution. It is found that the proposed model is consistent with the true values: the root-mean-square (RMS) values being about 0.7 ns, and some stations are even better. This means that the short-term ISB model proposed has a high fitting accuracy. Hence, it can be used for ISB prediction. Comparing the prediction ISB series (ISB_p) with ISB_s in the following week (GPS week 1811), we can draw the conclusion that the accuracy of the prediction declines with an increase in the time period. The 1-day period precision can achieve 0.57–1.21 ns, while the accuracy of the 2-day prediction decreases to 0.77–1.72 ns. Hence, we recommend a predicting duration of 1 day. The proposed model will be beneficial for subsequent GPS/BDS PPP or precise orbit determination (POD) since the ISB derived from this model can be considered as a priori constraint in the PPP/POD solutions. With this a priori constraint, the convergence time can be shortened by 19.6, 16.1 and 2.4 % in N, E and U components, respectively. The accuracy of result in the E component is remarkably improved by 11.9 %. Numéro de notice : A2017-015 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s10291-015-0513-x Date de publication en ligne : 09/01/2016 En ligne : https://doi.org/10.1007/s10291-015-0513-x Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=83946 [article]

Optimal Doppler-aided smoothing strategy for GNSS navigation / Zebo Zhou in GPS solutions, vol 21 n° 1 (January 2017)  

Public [article]  inGPS solutions > vol 21 n° 1 (January 2017) . - pp 197 - 210 Titre : Optimal Doppler-aided smoothing strategy for GNSS navigation Type de document : Article/Communication Auteurs : Zebo Zhou, Auteur ; Bofeng Li, Auteur Année de publication : 2017 Article en page(s) : pp 197 - 210 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Traitement du signal [Termes IGN] code GNSS [Termes IGN] données Doppler [Termes IGN] glissement de cycle [Termes IGN] onde porteuse [Termes IGN] phase Résumé : (Auteur) Carrier-phase-smoothed code (CPSC), i.e., smoothing of the code using carrier phases, has widely been used to reduce the code noise in GNSS applications. However, the efficiency of CPSC suffers significantly from cycle slips, interruptions and jitters. The GNSS Doppler, as an instantaneous measurement, is robust and immune to cycle slips and proven useful in GNSS-challenged environments. We develop optimal Doppler-smoothed code based on the principle of minimum variance using the Hatch filter for two typical applications, which are called pure Doppler-smoothed code (PDSC) and continued Doppler-smoothed code. PDSC results from smoothing the code using only Doppler, whereas in case of continued Doppler-smoothed code, the smoothing continues using Doppler once the carrier phase becomes unavailable. Furthermore, in order to refine the Doppler-smoothed code model, a balance factor is introduced for adjusting the contributions of raw code and Doppler measurements to the smoothed code in case the Doppler noise is relatively large. Finally, experiments are carried out to demonstrate the performance of the theory, which verifies its validity and efficiency. Numéro de notice : A2017-016 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s10291-015-0512-y Date de publication en ligne : 20/01/2016 En ligne : https://doi.org/10.1007/s10291-015-0512-y Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=83947 [article]

Modeling tropospheric wet delays with dense and sparse network configurations for PPP-RTK / Paulo S. de Oliveira in GPS solutions, vol 21 n° 1 (January 2017)  

Public [article]  inGPS solutions > vol 21 n° 1 (January 2017) . - pp 237 - 250 Titre : Modeling tropospheric wet delays with dense and sparse network configurations for PPP-RTK Type de document : Article/Communication Auteurs : Paulo S. de Oliveira, Auteur ; Laurent Morel, Auteur ; François Fund, Auteur ; Romain Legros, Auteur ; et al., Auteur Année de publication : 2017 Article en page(s) : pp 237 - 250 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes IGN] France d'outre-mer [Termes IGN] orbite précise [Termes IGN] Orphéon [Termes IGN] positionnement cinématique en temps réel [Termes IGN] positionnement par GLONASS [Termes IGN] positionnement par GNSS [Termes IGN] positionnement par GPS [Termes IGN] positionnement ponctuel précis [Termes IGN] récepteur bifréquence [Termes IGN] réseau géodésique clairsemé [Termes IGN] réseau géodésique dense [Termes IGN] retard troposphérique [Termes IGN] retard troposphérique zénithal Résumé : (Auteur) Precise Point Positioning (PPP) is a well-known technique of positioning by Global Navigation Satellite Systems (GNSS) that provides accurate solutions. With the availability of real-time precise orbit and clock products provided by the International GNSS Service (IGS) and by individual analysis centers such as Centre National d’Etudes Spatiales through the IGS Real-Time Project, PPP in real time is achievable. With such orbit and clock products and using dual-frequency receivers, first-order ionospheric effects can be eliminated by the ionospheric-free combination. Concerning the tropospheric delays, the Zenith Hydrostatic Delays can be quite well modeled, although the Zenith Wet Delays (ZWDs) have to be estimated because they cannot be mitigated by, for instance, observable combinations. However, adding ZWD estimates in PPP processing increases the time to achieve accurate positions. In order to reduce this convergence time, we (1) model the behavior of troposphere over France using ZWD estimates at Orphéon GNSS reference network stations and (2) send the modeling parameters to the GNSS users to be introduced as a priori ZWDs, with an appropriate uncertainty. At the user level, float PPP-RTK is achieved; that is, GNSS data are performed in kinematic mode and ambiguities are kept float. The quality of the modeling is assessed by comparison with tropospheric products published by Institut National de l’Information Géographique et Forestière. Finally, the improvements in terms of required time to achieve 10-cm accuracy for the rover position (simulated float PPP-RTK) are quantified and discussed. Results for 68 % quantiles of absolute errors convergence show that gains for GPS-only positioning with ZWDs derived from the assessed tropospheric modeling are about: 1 % (East), 20 % (North), and 5 % (Up). Since ZWD estimation is correlated with satellite geometry, we also investigated the positioning when processing GPS + GLONASS data, which increases significantly the number of available satellites. The improvements achieved by adding tropospheric corrections in this case are about: 2 % (East), 5 % (North), and 13 % (Up). Finally, a reduction in the number of reference stations by using a sparser network configuration to perform the tropospheric modeling does not degrade the generated tropospheric corrections, and similar performances are achieved. Numéro de notice : A2017-017 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : 10.1007/s10291-016-0518-0 En ligne : http://dx.doi.org/10.1007/s10291-016-0518-0 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=83948 [article]