G43A-0912
Response of TEC to solar flare: latitude and longitude features
The analysis of occurrence of ionospheric electron content to the large solar flare that occurred near 11 00 UT on November 28, 2003 (class X17) is presented. The TEC data were obtained using GPS observations carried by European Permanent Network (EPN) and International GNSS Service (IGS). The detail pictures of TEC response to the solar flare over Europe were obtained on the base of GPS measurements of EPN. Spatial and temporal changes of TEC during solar flare we examine using TEC maps. During creating TEC maps over Europe we used simultaneous measurements from 120-160 GPS stations. Time resolution of these maps were 5 min. TEC measurements along individual satellite passes were used to find out the effect of the solar flare in Antarctic and Arctic ionosphere. The changes of TEC value is dominate by the changes in F2 region as the lower part of ionosphere can also contribute for TEC changes . So for better understanding the plasma properties at the top side ionosphere the wave and particle in situ measurements were analysed. Sudden increases of TEC started about 11.00 UT over whole European region. TEC increased ranged from 8 to 20 TECU on time interval 10 min. The percentage TEC enhancement was about 10-30. The recovery stage lasted more than 2-3 hour. The larger enhancement took place at lower latitudes. At high latitudes the TEC enhancement didn't exceed 5 TECU. It was found out that TEC response to the solar flare has a complicated character depending on latitude, longitude and solar zenith angle. Features of TEC response occurrence at high and middle latitudes are discussed in this report.
G43A-0913
Influence of the Second-Order Ionospheric Delay on GNSS Geodetic Solutions
The level of accuracy reached today by reference GNSS technology is in great part a consequence of progress in modelling various contributions to the observables. At the same time, advances in modelling of a single effect will not necessarily lead to improvements in the final result if there exist greater mismodelled contributions from other sources. One of the effects, the importance of which has been recently understood, is the ionospheric effect of second order. The commonly accepted ionosphere-free linear phase combination eliminates the plasma-induced phase delay in GNSS measurements in the approximation of zero magnetic field. However, due to interaction of the GNSS carrier wave, propagating in an ionospheric plasma, with the geomagnetic field a small additional delay is introduced in the phase observable. The magnitude of such a delay is of the order of~1~cm, and it depends on the geographic coordinates of the observer, local time, season, and solar activity. We use International Reference Ionosphere (IRI2007) and International Geomagnetic Reference Field (IGRF2005) models to create worldwide maps of the second-order ionospheric delay and resulting geodetic displacements for different time, seasons, and phases of solar activity cycle. We show that, due to diurnal and seasonal variability of the second-order ionospheric delay, there are apparent oscillations of geocentre position. We demonstrate that the pattern of such oscillations changes with the phase of solar activity cycle. In applications to kinematic and sub-daily static positioning, we demonstrate that due to high horizontal gradients of electron total content during morning and evening hours, taking into account the second-order ionospheric effect becomes meaningful not only for the global, but also for regional networks. In the case of daily static solutions accounting for the second-order ionospheric delay can help improving the solution precision. Based on the results presented we recommend introducing the modelling of the second-order ionospheric delay in routine GNSS processing for global geodetic networks, especially in view of the forthcoming solar activity maximum.
G43A-0914
Analysis of the second and third order ionospheric effects for GNSS positioning in Brazil
The Global Positioning System (GPS) transmits two frequencies, wich allow to correct the first order ionospheric effects through the ionospheric-free observable. These effects, characterized by the phase advance and group delay, cause several errors in the GPS positioning. The second and third order ionospheric effects, generally neglected in the GPS data processing, also introduce sazonal variations of few millimeters in the station position, besides centimeter diurnal variations in the latitude and height. The first order effect is directly proportional to the Total Electron Content (TEC) in the atmosphere. The second and third order are also dependent of TEC, besides the Earth geomagnetic field and the maximum electron density, respectively. In this paper, the second and third order ionospheric effects were taken into account in the GPS data processing. This processing was accomplished using GPS data of the Brazilian region, involving relative and precise point positioning (PPP), either in static or kinematic mode. The analysis was accomplished considering the discrepancies between the solution with and without the cited corrections. For the PPP kinematic scenario, considering low ionospheric activities, the results showed effects of the order of 2 and 4 mm for the horizontal and height components, respectively. For more active ionosphere the discrepancies reached up to 1 cm. For relative processing using baselines larger than 100 km, the discrepancies reached the order of 2 and 7 mm in the height stations for low and more activity ionosphere, respectively. These results and new ones will take part of this presentation.
G43A-0915
Discrepancy in neutral atmospheric delay due to different models for ray path and atmosphere in raytracing
We quantify the discrepancy in neutral atmospheric delay due to different assumptions when raytracing in numerical weather models (NWM). We make the comparisons in a worst-case scenario, in the vicinity of hurricane Charley (August 14, 2004, 12h UTC), as represented by a 15-km (horizontal resolution) NWM. First, we compare the ray path models bended-2d and bended-3d. In the former the ray is confined to a plane of constant azimuth; bending changes the ray direction in elevation angle only. That assumption is embodied in the widely used Bougler's formula, strictly valid only in a spherical atmosphere. In the latter model we solve the original Eikonal equation, making no assumption about the direction of the refraction gradient vector, thus allowing for out-of-plane bending due to horizontal gradients. Employing the 3d NWM, we show that the discrepancy (bended-2d minus bended-3d) at 3-degree elevation reaches at most -10.6 mm, 9.5 mm, and 2.4 mm in geometric, hydrostatic, and non-hydrostatic partial delays, respectively. The discrepancy in total delay is always positive, because the bended-3d model follows more closely Fermat's least time principle. The discrepancy in the combined geometric plus hydrostatic delay is smaller, due to the reversed sign of the components involved. We conclude that the smallness of that discrepancy warrants the use of the simpler bended-2d ray path model with a 15-km 3d atmospheric model. Next, we keep the ray model fixed to bended-2d, and compare different atmospheric models, all derived from the same NWM. Those models are, from simplest to most realistic: spherical concentric, spherical osculating, graded, and 3d. In the spherical models we take a vertical profile for each atmospheric parameter and assume their values constant for different horizontal positions at the same height. The main difference between the two spherical models, concentric and osculating, is in the location of their centers: if it is at the geo-center, we call it spherical concentric; if it is along the ellipsoidal normal passing through the point of interest (e.g., the GPS receiver), we call it spherical osculating. In the graded model we add to the spherical osculating model a vertical profile of horizontal gradient for each atmospheric parameter, whose values are assumed constant for different horizontal positions at the same height. Finally, the 3d model is the original NWM, made available as a grid and then interpolated at each position making up the discretization of the ray path. The figures below are maximum absolute discrepancies w.r.t. spherical osculating model (which is azimuthally symmetric), in non-hydrostatic followed by hydrostatic delay at 3-degree elevation. The spherical concentric atmosphere shows a very large discrepancy (20 cm, 1.3 m) in the North-South direction and zero discrepancy in the East-West direction. That is a simple consequence of the tilting of the spherical horizon with respect to the ellipsoidal horizon. The 3d atmosphere shows discrepancies in non-hydrostatic delay much larger than those in hydrostatic delay (30 cm, 5 cm), as consequence of the higher variability of humidity. The graded atmosphere represents well the main direction of azimuthal asymmetry present in the 3d model, but it is unable to account for secondary directions. Their discrepancy is smallest along that main direction (1 cm, 5 mm), but reaches non-negligible values (15 cm, 2.5 cm) elsewhere. We conclude that, for the purpose of making mapping functions available (not necessarily so in the parametrization for estimation of residual delay), secondary directions of azimuthal symmetry should be considered.
G43A-0916
Correlation Between Mapping Functions and Atmospheric Pressure Loading in the Processing of GNSS Data
We investigate the correlation between mapping functions and atmospheric pressure loading in the processing of GNSS data. Previous work from the same authors showed that site dependent mapping functions (especially a total mapping function) based on the ray tracing of a numerical weather model give coordinate and ZTD times series that, when differentiated with time series obtained with a traditional mapping function like the NMF, show high level of correlation with the predicted atmosphere pressure loading signal.
G43A-0917
Numerical Simulation of Positioning Errors due to Tropospheric Delay- a Case Study With the Japanese GPS Network (GEONET)
In this study, we evaluate positioning errors due to tropospheric delay in the coordinate time series of GPS stations in the Japanese GPS network (GEONET) through numerical simulation and investigate their dependence on analysis software and strategies. For this purpose, we have developed simulation software to reproduce GPS observations under certain weather conditions. The software take numerical weather models as input, calculate tropospheric delays using the ray tracing method, and output simulated GPS observations in the RINEX format which are ready for analysis with general GPS analysis software. Here, we first demonstrate the ability of the simulation software by reproducing daily variations of observed positioning errors under severe weather conditions. Then we investigate the characteristics of long-term positioning errors due to tropospheric delays and their dependence on analysis software and strategies. First, we simulated the GPS observations at 71 points in and around Izu peninsula, central Japan from August 29 to September 2 in 2004 when we observed large positioning errors that are coincidental with bad weather conditions due to the combination of the weather front and the typhoon. We used tailored fine-scale numerical weather models which have spatial resolutions of 2km as input. We preliminary analyzed the simulated and observed GPS data with the GIPSY-OASIS software under the same analysis strategy (precise point positioning; NMF mapping function). As a result, we successfully reproduced the daily variations of the directions of observed horizontal errors as well as the patterns of observed vertical errors. This result shows the ability of the software to reproduce the realistic positioning errors of tropospheric origin. Then we investigated long-term positioning errors. We tentatively simulated the GPS observations at 101 points which are more or less evenly-spaced throughout Japan for 2004. We used the meso-scale objective analysis from the Japanese Meteorological Agency (JMA) with spatial resolutions of 10km as input. We analyzed the simulated and observed GPS data with the GIPSY-OASIS software under the same analysis strategy with the former case. As a result, we found clear spatial pattern in the annual signal in the simulated vertical time series. The annual amplitudes gradually increase toward north, reaching 4mm in the northern Japan. The phases are generally homogeneous, taking maximum at around DOY 240. This pattern is recognizable in the observed vertical time series as well, confirming that there still remain considerable unmodelled biases in the GEONET time series on the annual time scale. In this presentation, we will also discuss the spectrum characteristics of troposphere-related positioning errors as well as these seasonal characteristics. Then we will compare the time series obtained with different analysis software and strategy to investigate the dependence of these characteristics on analysis software and strategies.
G43A-0918
Potential contribution of GNSS data based tropospheric zenith delay to weather forecasting in alpine areas
High resolution meteorological analysis of the humidity field is an important precondition for a better monitoring of local and regional extreme precipitation events and for forecasts with improved spatial resolution. Errors in the analysis occur mainly in alpine areas where the predicted models do not reproduce the mountain atmosphere correctly. Due to the interest of meteorologists in the wet component of the troposphere as an addiditonal data source for Numerical Weather Prediction, several regional projects were initiated in Europe and abroad to derive the zenith wet delay from ground based GNSS observation data. While the hydrostatic zenit delay of GNSS microwave signals is usually well sizeable, the wet component, describing the rapid variable water vapour content of the troposphere (one of the limiting error sources in GNSS precise point positioning) has to be estimated from the observation data. In this presentation we present the project GNSS-MET which makes use of continuous measurements of a regional network consisting of 8 GPS/GLONASS reference stations, located in Carinthia, Austria. The network has been extended with surrounding stations of the IGS and EUREF-network. The aim of the project is to provide GNSS based measurements of the tropospheric water vapour content with a temporal delay of less than one hour to use them within the INCA (Integrated Nowcasting through Comprehensive Analysis) system, operated by the Austrian Meteorological Service (ZAMG).
G43A-0919
Tropospheric path delay corrections for vertical positioning precision by using GPS
Recent scientific interest on constantly enhancing positioning accuracy by the Global Positioning System (GPS) is a challenging task among a variety of research tasks. The positioning accuracy of the GPS surveying has been improved considerably during the past two decades. The main error sources have been reduced substantially, if not eliminated. However, troposhpeic influence with its high temporal and spatial variability appears to be one of the existing major error sources. It is hence an increased interest among GPS researchers to reduce the tropospheric influence. Two techniques have been commonly implemented to correct the tropospheric path delay in GPS data processing. The first technique, known as parameter estimation, characterizes the path delay with empirical models and the parameters of interest are determined from the GPS measurements. The second strategy, termed as external correction, involves independent path delay measurements. The present study is an integration of both techniques in which the parameter estimation and external correction are used to correct the path delay for 10~365 km range baselines. Twenty-four zenith path parameters and tropospheric gradients have been set in 24 hours solution for parameter estimation strategy. Measurements from meteorological instruments and water vapor radiometers are applied in the GPS data processing, separately, as an external strategy of present research work. Interesting results have been found, indicating more stable repeatability in baseline when the external correction strategy is applied especially with the inclusion of water vapor radiometer data. The offset on the order of centimeter is found in the baselines determined by the two strategies. On the other hand, parameter estimation exhibits more stable in terms of GPS height repeatability. The offset in the GPS height determined by the two strategies is on the order of few centimeters.
G43A-0920
Comparisons of regional water vapor measurements derived from AIRS soundings and GPS receivers in Japan and Southern California
Water vapor measurements from the Atmospheric InfraRed Sounder (AIRS) are compared with water vapor estimates derived from ground-based Global Positioning System (GPS) receivers at several sites in Japan and Southern California for January 2005. AIRS is a polar orbiting hyper-spectral infrared sounder flown aboard NASA's Earth Observing System (EOS) Aqua platform providing high resolution 3-dimensional observations of water vapor, temperature and trace gases from space. AIRS produces both daytime and night time water vapor retrievals and is able to generate some retrievals in the presence of clouds. GPS measurements from the Japan GEONET network and the Southern California Integrated GPS Network (SCIGN) of ground-based receivers are processed at the Jet Propulsion Laboratory (JPL) using the Global Mapping Function (GMF) described by Boehm et al. (2005). A robust method (Bevis et al. 1992) is used to derive water vapor from tropospheric path delay measured at several GPS receiver sites for January 2005. In order to compare GPS-derived water vapor with AIRS-retrieved water vapor, we use surface pressure estimates from the National Center for Environmental Prediction (NCEP) reanalysis to convert GPS nominal wet and dry components to real water vapor and hydrostatic estimates. GPS-derived water vapor measurements are compared with AIRS retrieved quantities, and both are used to estimate the spatial and temporal distribution of water vapor over the region.