A53C-01
TEC Behavior in the Low Latitude Brazilian Area Using GPS Observables
GPS has definitely established as an important tool for the determination observation of atmosphere parameters. After the turn off the SA (Selective Availability) the ionosphere influence became the largest error source on the band L signals broadcasted by the GPS satellites. The signal that travels across the ionosphere allows obtaining information of the ionosphere layers. The code delay and phase advance on propagation of GPS signals reflect the behavior of the ionosphere as function of TEC (Total Electron Content). It is influenced by several variables, such as: solar cycle, time of the year, hour of the day, geographic location and geomagnetic activity. Lately, the TEC determination has also been made with double frequency GPS data measurements. In Brazil, the RMBC (Network Brazilian for Continuous Monitoring of GPS) is equipped with dual frequency GPS receivers that register GPS data continually. Therefore, this network can be considered a significant database of atmosphere information to the ionosphere activity. In this paper it is presented and discussed the seasonal behavior and diurnal effects of TEC in low latitude, specifically in the Brazilian area. This study is based on the VTEC (Vertical Total Electron Content) obtained from the linear combination of dual frequency GPS observables. In the experiments were used RBMC's data in the high activity period, in the solar cycle 23 (2000), and in the period of decline of the solar activity (2006). The results show the behavior of the VTEC in period of high and low solar activity.
A53C-02 INVITED
Accurate real-time ionospheric corrections as the key to extend the centimeter-error-level GNSS navigation at continental scale (WARTK)
The main focus of this presentation is to show the recent improvements in real-time GNSS ionospheric determination extending the service area of the so called "Wide Area Real Time Kinematic" technique (WARTK), which allow centimeter-error-level navigation up to hundreds of kilometers far from the nearest GNSS reference site.[-4mm] The real-time GNSS navigation with centimeters of error has been feasible since the nineties thanks to the so- called "Real-Time Kinematic" technique (RTK), by exactly solving the integer values of the double-differenced carrier phase ambiguities. This was possible thanks to dual-frequency carrier phase data acquired simultaneously with data from a close (less than 10-20 km) reference GNSS site, under the assumption of common atmospheric effects on the satellite signal. This technique has been improved by different authors with the consideration of a network of reference sites. However the differential ionospheric refraction has remained as the main limiting factor in the extension of the applicability distance regarding to the reference site.[-4mm] In this context the authors have been developing the Wide Area RTK technique (WARTK) in different works and projects since 1998, overworking the mentioned limitations. In this way the RTK applicability with the existing sparse (Wide Area) networks of reference GPS stations, separated hundreds of kilometers, is feasible. And such networks are presently deployed in the context of other projects, such as SBAS support, over Europe and North America (EGNOS and WAAS respectively) among other regions.[-4mm] In particular WARTK is based on computing very accurate differential ionospheric corrections from a Wide Area network of permanent GNSS receivers, and providing them in real-time to the users. The key points addressed by the technique are an accurate real-time ionospheric modeling -combined with the corresponding geodetic model- by means of:[-4mm] a) A tomographic voxel model of the ionosphere, estimated from the GNSS data only, and which reduces the errors associated to non-tomographic approaches, taking care of electron density vertical distribution.[-4mm] b) A simple GNSS model for the "Medium Scale Traveling Ionospheric Disturbances", being this one of the main ionospheric perturbations affecting the carrier phase ambiguity fixing, and hence the capability of accurate navigation.[-4mm] The corresponding models and performances will be illustrated with several experiments involving actual data. And finally the existing and new WARTK applications will be briefly mentioned, in the context of different projects. References: Hernandez-Pajares, M.; Juan, J. M.; Sanz, J.; Colombo, O. L., Improving the real-time ionospheric determination from GPS sites at very long distances over the equator, Journal of Geophysical Research (Space Physics), Volume 107, Issue A10, pp. SIA 10-1, CiteID 1296, DOI 10.1029/2001JA009203, 10/2002. Hernandez-Pajares, M.; Juan, J. M.; Sanz, J., Medium-scale traveling ionospheric disturbances affecting GPS measurements: Spatial and temporal analysis, Journal of Geophysical Research, Volume 111, Issue A7, CiteID A07S11, DOI 10.1029/2005JA011474, 06/2006.
A53C-03 INVITED
Improved Abel transform inversion: First application to COSMIC/FORMOSAT-3
In this paper the first results of Ionospheric Tomographic inversion are presented, using the Improved Abel Transform on the COSMIC/FORMOSAT-3 constellation of 6 LEO satellites, carrying on-board GPS receivers.[- 4mm] The Abel transform inversion is a wide used technique which in the ionospheric context makes it possible to retrieve electron densities as a function of height based of STEC (Slant Total Electron Content) data gathered from GPS receivers on board of LEO (Low Earth Orbit) satellites. Within this precise use, the classical approach of the Abel inversion is based on the assumption of spherical symmetry of the electron density in the vicinity of an occultation, meaning that the electron content varies in height but not horizontally. In particular, one implication of this assumption is that the VTEC (Vertical Total Electron Content) is a constant value for the occultation region. This assumption may not always be valid since horizontal ionospheric gradients (a very frequent feature in some ionosphere problematic areas such as the Equatorial region) could significantly affect the electron profiles. [- 4mm] In order to overcome this limitation/problem of the classical Abel inversion, a studied improvement of this technique can be obtained by assuming separability in the electron density (see Hernández-Pajares et al. 2000). This means that the electron density can be expressed by the multiplication of VTEC data and a shape function which assumes all the height dependency in it while the VTEC data keeps the horizontal dependency. Actually, it is more realistic to assume that this shape fuction depends only on the height and to use VTEC information to take into account the horizontal variation rather than considering spherical symmetry in the electron density function as it has been carried out in the classical approach of the Abel inversion.[-4mm] Since the above mentioned improved Abel inversion technique has already been tested and proven to be a useful tool to obtain a vertical description of the ionospheric electron density (see García-Fernández et al. 2003), a natural following step would be to extend the use of this technique to the recently available COSMIC data. The COSMIC satellite constellation, formed by 6 micro-satellites, is being deployed since April 2006 in circular orbit around the Earth, with a final altitude of about 700-800 kilometers. Its global and almost uniform coverage will overcome one of the main limitations of this technique which is the sparcity of data, related to lack of GPS receivers in some regions. This can significantly stimulate the development of radio occultation techniques with the use of the huge volume of data provided by the COSMIC constellation to be processed and analysed updating the current knowledge of the Ionospheres nature and behaviour. In this context a summary of the Improvel Abel transform inversion technique and the first results based on COSMIC constellation data will be presented. Moreover, future improvements, taking into account the higher temporal and global spatial coverage, will be discussed. [-4mm] References:M. Hernández-Pajares, J. M. Juan and J. Sanz, Improving the Abel inversion by adding ground GPS data to LEO radio occultations in ionospheric sounding, GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 16, PAGES 2473-2476, AUGUST 15, 2000.M. Garcia-Fernández, M. Hernández-Pajares, M. Juan, and J. Sanz, Improvement of ionospheric electron density estimation with GPSMET occultations using Abel inversion and VTEC Information, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A9, 1338, doi:10.1029/2003JA009952, 2003
A53C-04 INVITED
Seasonal Variability Study of the Tropospheric Zenithal Delay in the South America using regional Numerical Weather Prediction model
In 2010 a new navigation and administration system of the air traffic, denominated CNS-ATM (Communication
Navigation Surveillance - Air Traffic Management) should be running operationally in South America. This new
system will basically employ the positioning techniques by satellites to the management and air traffic control.
However, the efficiency of this new system demands the knowledge of the behavior of the atmosphere,
consequently, an appropriated Zenithal Tropospheric Delay (ZTD) modeling in a regional scale. The predictions
of ZTD values from Numeric Weather Prediction (NWP), denominated here dynamic modeling, is an alternative to
model the atmospheric gases effects in the radio-frequency signals in real time. Brazilian Center for Weather
Forecasting and Climate Studies (CPTEC) of the National Institute for Space Research (INPE), jointly with
researchers from UNESP (Sao Paulo State University), has generated operationally prediction of ZTD values to
South America Continent (available in the electronic address
http:satelite.cptec.inpe.br/htmldocs/ztd/zenithal.htm). The available regional version is obtained using ETA model
(NWP model with horizontal resolution of 20 km and 42 levels in the vertical). The application of NWP permit
assess the temporal and spatial variation of ZTD values, which is an important characteristic of this techniques.
The aim of the present paper is to investigate the ZTD seasonal variability over South America continent. A
variability analysis of the ZTD components [hydrostatic(ZHD) and wet(ZWD)] is also presented, as such as
discussion of main factors that influence this variation in this region. The hydrostatic component variation is
related with atmospheric pressure oscillation, which is influenced by relief and high pressure centers that prevail
over different region of the South America continent. The wet component oscillation is due to the temperature and
humidity variability, which is also influenced by relief and by synoptic events like: the penetration the cold front
from Antarctic pole into the continent and occurrence of humidity convergence zones. In South America there are
two main convergence zones that has strong influence in the troposphere variability, the ITCZ (Inter Tropical
Convergence Zone) and the SACZ (South Atlantic Convergence Zone) zones. These convergence zones are
characterized by an extensive precipitation band and high nebulosity almost stationary. The physical processes
associated with these convergence zones present strong impacts in the variability of ZWD values. This work aims
to contribute with ZTD modeling over South America continent using NWP to identify where and when the ZTD
values present lower predictability in this region, and consequently, minimizing the error in the GNSS positioning
that apply this technique.
http:satelite.cptec.inpe.br/htmldocs/ztd/zenithal.htm
A53C-05
Evaluating the Performance of the CPTEC-INPE/FCT-UNESP Troposphere Dynamic Model Using the VRS Concept
Nowadays, the use of Zenithal Tropospheric Delay (ZTD) prediction from Numeric Weather Prediction (NWP) models is a good alternative to minimize the effects of the troposphere in the radio frequency signs for real time and/or pos-processed applications. This process is denominated here after ZTD dynamic modeling. In Brazil, the procedure used to compute the ZTD by the NWP model was jointly developed by researchers from UNESP (São Paulo State University) and CPTEC (Center for Weather Forecasting and Climate Studies) of the INPE (National Institute for Space Research). The ZTD values are provided for all South America twice a day with predictions for a period of 66 hours. The database and the quality analysis are available by CPTEC-INPE in http:satelite.cptec.inpe.br/htmldocs/ztd/zenital.htm. In order to test the performance of the ZTD dynamic modeling in positioning applications, some experiments were carried out. Besides, the results obtained with dynamic modeling were compared with those obtained by Hopfield empirical model. These two tropospheric models were used to generate a VRS (Virtual Reference Station). In the VRS concept developed in this research, a reference station is generated near the user using data provided by a reference network station and atmospheric models. Therefore, the VRS data are not provided by a real receiver. But, the idea is that the VRS data resemble as much as possible a real receiver data at the same location. Therefore, the user has the possibility of using the VRS as if it were a real reference station in your proximities, and to accomplish the relative positioning with a single frequency receiver. This method was implemented in an software which has been developed at UNESP. In order to test the discussed method it was accomplished two experiments using data from two different networks: (a) Brazilian Continuous GPS Network (RBMC) and some extra stations; (b) GPS Active Network of West of São Paulo State. Using the first network it was processed 10 days of data collected in 2005 May. The results obtained by the dynamic modeling were better than those obtained by Hopfield model. It was obtained, on average, an improvement of 19% using the dynamic modeling instead of the Hopfield one. Using the second network it was processed 4 days of data collected in 2006 December. But, in this case, the results obtained were quite similar. This probably happened because in this period occurred a problem with the dynamic modeling. Some data from radiosondes launched over South America were not assimilated during this period deteriorating the ZTD predictions. This fact is important because shows how the assimilation process is important to the ZTD dynamic modeling and revels that this relationship between assimilation data and ZTD prediction quality must be better investigated. But, in spite of this problem, in general the results from dynamic modeling are still good if compared with empirical model.
A53C-06
ZTD Dynamic Modeling Versus Hopfield Model: Evaluation in DGPS Positioning
A positioning technique that has been received a great attention in the last years it is the Differential GPS (DGPS). This method has been used in several applications such as: navigation, surveying, precision agriculture and others. In the basic concept of DGPS it is assumed a high correlation of errors involved between base and rover stations, considering that the two stations are close together. This way, it is possible to generate corrections for the pseudorange. DGPS provides a reasonable accuracy for short baselines, which is degraded with distance growth due to spatial decorrelation of the errors (ionosphere effect, troposphere refraction and satellites orbit errors), consequently, the method efficiency is reduced. Therefore, to obtain a better positioning quality, an adequate modeling of these errors is indispensable. The aim of this paper is to evaluate the performance of the Numerical Weather Prediction (NWP) model, denominated here dynamic modeling, and the Hopfield empirical model for reducing Zenithal Tropospheric Delay (ZTD) in the DGPS context. The dynamic modeling used is from Center for Weather Forecasting and Climate Studies of the National Institute for Space Research (CPTEC/INPE), which has operationally available ZTD prediction for South American region (available in: http:satelite.cptec.inpe.br/htmldocs/ztd/zenital.htm). Some experiments were carried out using an in-house software developed at FCT/UNESP and data of different GPS baselines lengths: 75, 165, 237 and 443 km. The stations used are from RBMC (Brazilian Continuous Network of Monitoring GPS Satellites) (PPTE, PARA stations) and from GPS Active Network of West of São Paulo State (SEM2, OURI, ROSA stations). The station SEM2 was considered base station. The stations PPTE, OURI, ROSA and PARA were considered rovers. It was processed 2 days of data, December 29 and 30, 2007. The improvement obtained in DGPS using dynamic modeling for the 75, 165, 237 and 443 km baseline was on average 0.35%, 3%, 2.8% and 12.1%, respectively, for the two days in relation to the Hopfield model. These results show that in all evaluated baseline the dynamic modeling has been improved the results if compared with Hopfield empirical model. It is important to verify that with the baseline growth the improvement was very significant.
A53C-07
Atmospheric Effects Mitigation in GNSS Relative Positioning using Wavelet Multiresolution Analysis
In GNSS relative processing involving long baselines, the atmospheric effect, mainly, that due to the ionosphere is one of the major errors. Although linear combination (called ion-free) of observations at the two GPS frequencies or ionosphere models can be used to mitigate ionospheric effects, they do not remove all the errors. Furthermore, depending of the region and of the ionospheric behavior, these remaining errors are very significant. This happens because the ionosphere has several irregularities and variations, such as, for instance diurnal, seasonal and solar cycle variations, as well as traveling ionospheric disturbances (TIDs), scintillation, and other geomagnetic disturbances. The short and medium-scale ionosphere irregularities (TIDs and scintillation, for example) are difficult to be taking into account or modeled. Thus, in atmospheric applications, especially for ionospheric studies, wavelet multiresolution analysis (MRA) is a powerful method to be used because it allows a time-frequency decomposition to analyze the signal in several resolutions in order to detect different scale phenomena and disturbances. In this sense, it will be present the wavelet method based on applying the MRA to the double-difference (DD) observables to detect and remove short and medium-scale ionosphere variations and disturbances, as well as short-term troposphere variations. Experiments were carried out in Brazil involving data of different baseline lengths. It was showed that even using only single-frequency data, but with MRA, one can obtain very good results, even better than those using ion-free linear combination.
A53C-08
Potential Contribution of GNSS Data Based Tropospheric Zenith Delay to Weather Forecasts in Alpine Areas
The importance of high resolution meteorological analysis of the alpine atmosphere due to local and regional extreme precipitation events increased over the past years. A detailed analysis of the humidity field is an important precondition for a better monitoring of these 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. 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. 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. 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).
A53C-09
Impact of the Combination of GNSS and Altimetry Data on the Derived Global Ionosphere Maps
The classical input data for development of Global Ionosphere Maps (GIM) of the Total Electron Content (TEC) is the so called "geometry free linear combination", obtained from the dual-frequency Global Navigation Satellite System (GNSS) observations. Such maps in general achieve good quality of the ionosphere representation. However, the GNSS stations are inhomogeneously distributed, with large gaps particularly over the sea surface, which lowers the precision of the GIM over these areas. On the other hand, the dual-frequency satellite altimetry missions such as Jason-1 and TOPEX/Poseidon provide information about the parameter of the ionosphere precisely above the sea surface, where the altimetry observations are preformed. Due to the limited spread of the measurements and some open issues related to systematic errors, the ionospheric data from satellite altimetry is used only for cross-validation of the GNSS GIM. It can be anticipated however, that some specifics of the ionosphere parameter derived by satellite altimetry will partly balance the inhomogeneity of the GNSS data. Such important features are complementing in the global resolution, different biasing and the absence of additional mapping, as it is the case in GNSS. In this study we create two-hourly GIM from GNSS data and additionally introduce satellite altimetry observations, which help to compensate the insufficient GNSS coverage of the oceans. The combination of the data from around 180 GNSS stations and the satellite altimetry mission Jason-1 is performed on the normal equation level. The comparison between the integrated ionosphere models and the GNSS-only maps shows a higher accuracy of the combined GIM over the seas. A further effect of the combination is that the method allows the independent estimation of daily values of the Differential Code Biases (DCB) for all GNSS satellites and receivers, and of the systematic errors affecting the altimetry measurements. Such errors should include a hardware delay similar to the GNSS DCB as well as the impact of the topside ionosphere, which is not sampled by Jason-1. At this stage, for testing purposes we estimate a constant daily value, which will be further investigated. The final aim of the study is the development of improved combined global TEC maps, which make best use of the advantages of each particular type of data and have higher accuracy and reliability than the results derived by the two methods if treated individually.