G41A-0595
New Atmospheric Observations from the Airborne GNSS Instrument System for Multistatic and Occultation Sensing (GISMOS)
The Airborne GNSS Instrument System for Multistatic and Occultation Sensing (GISMOS) was deployed on the NCAR HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) aircraft to make atmospheric observations over the Gulf of Mexico coastal region in February 2008. The objective of the measurements was to test the performance of the system in comparisons with radiosonde profiles and dropsonde profiles that were also collected during the field campaign. The airborne GNSS radio occultation measures of GNSS signals from satellites that are setting or rising behind the Earth's limb relative to the receiver on board an aircraft. High-gain side-looking antennas and a 10MHz GPS Recording System that records the raw RF signal make this set of instrumentation unique, and especially adapted for open-loop tracking observations in the lower atmosphere. Measurements of the amount of refraction in the signal ray paths are inverted using an Abel transform procedure to retrieve a profile of refractivity, which depends on atmospheric pressure, temperature and relative humidity. The airborne geometry, in contrast to the space- borne satellite occultation geometry, is affected by a large drift in the tangent point location, that is the location of the closest point to the Earth surface, as the ray path descends in the atmosphere. Therefore plans for the validation campaign included releasing dropsondes in the plane of the line of sight of the satellite-receiver occultation geometry in order to study this effect. Careful timing and location of the flight path was used to coordinate occultation times with operational and supplementary radiosonde launches. A total of 6 days of balloon sounding data were collected with 20 dropsondes and 28 supplementary radiosonde profiles. A discussion of the technical performance of the system will be presented, which describes the signal characteristics and antenna performance. Preliminary results on the quality of retrieved refractivity profiles will also be shown.
G41A-0596
El Nino-Southern Oscillation : A New Atmospheric Perspective via GPS Radio Occultation
We present initial results of a study on El-Nino-Southern Oscillation (ENSO) focusing on free tropospheric water vapor derived from Global Positioning System Radio Occultation (GPSRO) observations. The globally- distributed, high vertical-resolution, cloud penetrating profiles used in this study are derived from two sources: the CHAllening Minisatellite Payload (CHAMP) and the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC). The CHAMP data for the 2002-2008 period, allowed us to discover the signature of El Nino episodes, as indicated by the wettest portions of the free troposphere over the central Pacific at the peak of the El Nino. The location of the wettest profiles shift with the ENSO cycle, suggestive of a new atmospheric ENSO index. Additionally, the observed dryness of the free troposphere during January followed by very wet profiles in the Southern Pacific convergence zone in March/April preceding La Nina episodes, suggests a predictive skill a year in advance of the La Nina cold phase. To emphasize the importance of the results from GPSRO we contrast them to the results from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, which contain a much weaker signal in the wettest profiles. A more in-depth study is possible with data from the six-satellite COSMIC constellation, which provides far denser coverage than previous GPSRO datasets. While far too short for a climatology (mid-2006 to the present), the tropical system has experienced both warm (fall 2006- spring 2007) and cold (fall 2007-spring 2008) ENSO phases over this short interval. In order to establish correlations between the 4D water fields and other key ENSO variables such as SST and OLR, we have created a new vertically-resolved, gridded water vapor dataset via a cluster analysis. We will summarize our approach and some of key spatio-temporal patterns that we have found in this study.
G41A-0597
Tropospheric water vapour over Antarctia from 12 years of globally reprocessed GPS data.
Atmospheric water vapour — a natural greenhouse gas of importance in the climate system — remains poorly monitored for some regions of the Earth. This paper presents an analysis of Global Positioning System (GPS) data that results in a new precipitable water (PW) dataset for Antarctica. We have undertaken a global reprocessing of a 60 station GPS network for the 1995-2006 period. In order to obtain optimal and homogeneous estimates of PW, emphasis has been placed on using the latest and most up-to-date GPS observation models in a consistent manner. These models include the VMF1 tropospheric mapping function and absolute antenna phase centre variations. Zenith delays, estimated at two-hourly intervals, were converted to PW for twelve, mainly coastal, Antarctic locations. Comparison with radiosonde derived PW time series shows that the reprocessed GPS measurement technique exhibits good temporal stability. GPS / radiosonde biases are small, at the sub-millimetre level; correlations are of the order of 0.95. Comparison with PW datasets obtained from MODIS, AIRS and AMSR-E satellite instruments shows the AIRS instrument to give the best agreement with the GPS derived time series, again with sub-millimetre biases. The PW time series capture the extremely dry climate of Antarctica. Periodic signals in the PW time series were also observed, including a strong annual signal at all locations, and a semiannual signal at coastal East Antarctic locations. A 12-hourly (S1) periodic variation is observed in summer PW measurements at many sites. There is also a possible increase in summertime PW over the 1995- 2006 period in coastal East Antarctica and at the South Pole. We conclude that globally reprocessed GPS solutions can provide accurate measurements of PW that will become increasingly useful for meteorological and climatological applications, provided that the GPS data are (re)processed consistently and homogeneously.
G41A-0598
Ground-based GPS Climatology: Ten-year Comparisons of GPS and Radiodonde Retrieved Precipitable Water Vapors over the Japanese Islands
We performed 10 year comparisons of precipitable water vapor (PWV) retrieved from ground-based GPS
stations of GEONET, Japan, with PWV from 18 radiosonde stations. RTNet software developed by GPS
Solutions Inc. was used for GPS data processing. Although there were some events of changes both in GPS
antenna and in radiosonde sensor, both PWV agreed with 0.6 mm of mean bias (GPS minus radiosonde) and
2.7 mm of standard deviation. There is a small decrease of GPS PWV in GPS antenna replacement events,
and it was corrected by using the PWV difference between GPS and radiosonde by assuming that newer
GPS antenna (choke-ring antenna) is the reference because phase center variation model is well
established. The correction helped to see good agreement of increase of PWV in the 10 years. We found
scale dry biases of PWV of about 5% in radiosonde observation in daytime operation of 09 LST, and it is
consistent with reports in some researches focusing on different radiosonde type, where the scale bias is
less than 1% in nighttime. The results suggest that GPS PWV can be considered as one of the climatological
benchmark (reference) for PWV observation, and thus GPS is used for climatological researches such as re-
analysis experiments with historical archived dataset, monitoring long-term water vapor variations. Also, a
correction of scale biases of humidity profile of radiosonde is important to reduce biases in the analysis field
of numerical weather model because the primal moisture data for the weather model has been provided from
radiosonde observation. We review history of GEONET, show statistics of 10 year PWV products, and
introduce current status and future plan of GEONET for meteorological and climatological application.
http://www.cosmic.ucar.edu/~iwabuchi
G41A-0599
Ground-based GNSS/GPS measurements of water vapor for climate monitoring: Strength, Limitation and Reprocessing
Atmospheric precipitable water (PW) from ground-based GPS measurements has become more and more valuable for climate monitoring because of increased accuracy in raw GPS measurements, significant improvements in techniques to map zenith wet delays onto PW, the dramatic growth of global and regional GPS networks, and more importantly longer GPS data record (> 10 years). We have produced a global, 2- hourly atmospheric precipitable water (PW) dataset from ground-based GPS measurements of zenith tropospheric delay (ZTD) using the International GNSS (Global Navigation Satellite Systems) Service (IGS) tropospheric products (~80-370 stations, 1997-2007), U.S. SuomiNet product (169 stations, 2003-2006) and Japanese GEONET data (1223 stations, 1997-2007). The dataset has been used for various climate studies. Based on our experience, we found both the strength and the limitation of ground-based GPS data for climate monitoring and call for reprocessing of global ZTD data. The strength is its availability under all weather conditions, the high accuracy of raw GPS measurements (clock and positions), and its high temporal resolution. If careful procedures are taken, the high accuracy of raw GPS data would imply high accuracy in derived PW. Unfortunately this is not always the case because various models, assumptions, and supplemental data are involved in the derivation of PW. The high temporal resolution enables us understanding the diurnal cycle and examining the impacts of under-sampling of diurnal cycle in other climate datasets on climate trend analysis. To illustrate this point, the diurnal cycle of the 11-year PW trends will be shown at some stations. The main limitation is inhomogeneity of GPS data associated with changes in instruments and processing software. As the GPS data record is becoming longer and longer, such problem becomes more and more serious. We will show some examples of inhomogeneity based on comparisons with radiosonde data and neighboring stations and among different ZTD products. To ensure the homogeneity of GPS data and enhance its value in climate monitoring, we strongly call for consistently reprocessing ZTD data from ground-based GPS measurements at global and regional GPS networks using the state-of-art GPS processing models and creating and maintaining a comprehensive metadata database to document any changes in both hardware and software.
G41A-0600
Reanalysis and In Situ Real-time Weather Data in GPS Tropospheric Delay Studies
The Scripps Orbit and Permanent Array Center (SOPAC) maintains an archive of continuous GPS network data dating back to 1990. The tropospheric delay data contained in the archive would be a tremendous resource for climate studies if there were concomitant meteorological data available for the GPS sites, since it is a simple matter to convert that data into values for precipitable water (integrated water vapor). Unfortunately, since most of the GPS networks were intended for geodetic and geophysical studies, there are very few co-located meteorological stations. A way around this limitation has been found by using pressure- level geopotential height data from the North American Regional Reanalysis (NARR) to compute station pressures at the GPS sites. This technique has been applied to over 500 GPS sites in California and Nevada, which are particularly dense spatially due to numerous studies of the active tectonics in the region. The interpolated pressures that were obtained have been tested by using the same technique to compute pressures at Metar sites, where hourly station pressure data is available. It has been found that station pressure can be calculated from the NARR data with a mean accuracy of approximately 0.5 mb (hPA). This yields precipitable water with inferred errors of 5% or better. Precipitable water values from the GPS sites were then interpolated onto a grid. As a test of the technique, time series of precipitable water were compared to NOAA GPS sites that had co-located meteorological stations and also to values from interpolated balloon sounding locations; both were found to be in excellent agreement. This technique is useful in climate science in a number of different ways: it allows spatially dense maps of precipitable water to be constructed which can then be correlated with specific meteorological events; it is possible to examine diurnal cycles of atmospheric water vapor across the region; and long term trends in mean precipitable water can be examined as a diagnostic of climate change. Additionally, this technique may have applications to solid earth geophysics, for example by using the delay data deconstructed into hydrostatic and wet delays to improve synthetic aperture radar interferometry. In parallel, with funding from the California Department of Water Resources we are installing meteorological sensors at continuous GPS stations in California operated by PBO and SCIGN. This effort will contribute to the short- to long-term applications mentioned above, but also provide essential short-term data for flood control in California basins and coastal regions. Therefore, we are focusing our efforts at continuous GPS stations that have been integrated into the California Real Time Network (CRTN), which provides high-rate (1 Hz) data with low latency (about 1 s). The short-term meteorological applications currently require a latency of 15-30 minutes, which is most efficiently met by CRTN.
G41A-0601
An Evaluation of Recent Improvements in Observational Modeling and the Associated Impact in GPS Precipitable Water Vapor Accuracy
Over the last few years a number of significant improvements in the modeling of GPS observations have
been identified and implemented within standard high precision analysis packages. These improvements
include the transition to absolute antenna phase center models for both satellites and antennas, a transition
to the IGS05 reference frame, and the increasing use of mapping functions that are directly related to the
current atmospheric state at a station. The COSMIC/UCAR program processes both global and regional GPS
networks for atmospheric applications (www.suominet.ucar.edu). We evaluate what impact these processing
changes have had on the estimation of precipitable water vapor (PWV) estimates since January of 2007.
Switching to the new absolute phase center models creates roughly a 1 mm drop in PWV, with the magnitude
of the change dependent on the type of antenna at the station. This switch reduces biases seen between
GPS techniques and other measurement methods, including microwave water vapor radiometers and
radiosonde comparisons. Effects for both global and regional networks will be presented, including a
summary of each major antenna type. We also evaluate the use of ray tracing based on Global Forecast
System models to generate time and location specific tropospheric mapping functions. The greatest impact
of this direct mapping is seen at polar sites. Finally we will show that using a numerical weather model to
estimate the weighted mean temperature of the atmosphere for computing the pi factor to convert zenith wet
bending delay to precipitable water vapor has become the standard for climate studies of PWV. The effects
of using NWM grids versus surface temperature and Bevis regression on both diurnal and seasonal
frequencies will be presented.
http://www.suominet.ucar.edu
G41A-0602
Continuous Estimates of Precipitable Water Vapor Within and Around Hurricane Systems
This study investigates how estimates of precipitable water vapor (PW) from Global Positioning System (GPS) stations can be used to quantify how atmospheric moisture influences the intensity of tropical storms and hurricanes. The motivation for this study is based on the fact that hurricanes derive their strength through water vapor that is both evaporated from warm ocean surfaces and the existing moisture in the surrounding atmospheric environment. Observationally, there are relatively few instruments that can accurately measure water vapor in the presence of clouds and rain. Retrievals of PW using GPS stations may be the most reliable way to continuously monitor column integrated water vapor. Using storm information from the National Hurricane Center (www.nhc.noaa.gov), we have compared storm intensity to PW estimates for all tropical storms and hurricanes making landfall within 100-km of a GPS station between 2003 and 2008. We find that PW is inversely correlated (r**2 < -0.7) to the drop in surface pressure observed at that station. We have also begun to relate atmospheric PW at a station to the local sea surface temperature (SST). This comparison can be used to measure how strongly atmospheric water vapor and SST are coupled. It can also be used to measure how quickly the atmosphere responds to changes in SST. Finally we have compared the estimated PW to the Global Forecast System (GFS) analysis fields that are used to initialize numerical weather prediction models. This comparison indicates that the GFS analysis fields have significantly larger errors in atmospheric moisture in the Caribbean and Gulf of Mexico when compared to differences over the continental United States. These results illustrate that estimates of PW are an important data set for atmospheric scientists and forecasters attempting to improve the prediction of hurricane intensity.
G41A-0603
Near Real time GPS tropospheric delay estimation and validation: a study case in Brazil
Data from GNSS continuous stations in Brazil have been used mainly for geodetic applications. Recently, with the implementation of the NTRIP Caster, real time data are now available from several GNSS stations in Brazil. Therefore, product like tropospheric delay can be made available near real time. In this presentation we are going to estimate and validate the quality of near real time tropospheric delay using data of this network. Near real time PPP with a sliding window of seven hours will be used in order to obtain tropospheric delays for the last hour. The Gipsy-Oasis II (GOA-II) software has been used in this approach. The impact of using absolute antenna calibration and different mapping functions in the approach will be analyzed. The results will compared with those obtained from a post-processing procedure using IGS precise ephemeris. The methodology, results and analyses, together with perspectives of using such product will be presented.
G41A-0604
GPS Data Processing for Water Cycle Studies Over West Africa : Statistical Analysis of GPS Wet Tropospheric Delay Estimates
Six permanent GPS stations have been deployed in West Africa within the AMMA project framework since
2005 and 2006 in order to obtain precipitable water vapour (PWV) estimations during the complete
experiment. We therefore aim to retrieve the PWV evolution with a time resolution of 1 hour over the AMMA
area. PWV series are used to study climate at different time scales (especially important for diurnal cycle
studies). As our main objectives are to ensure long term stability and high accuracy in the GPS PWV, we
must take account of the latest improvements of models used in GPS scientific software.
This recent progress had consequences on PWV estimations so we carried out some methodological and
sensitivity tests, especially concerning the atmospheric modelling parametrization : a priori temperature and
pressure models, mapping functions (GMF, VMF), number of tropospheric gradient. On the GAMIT GPS
software, the model for zenithal tropospheric delay (ZTD) takes the form of a piecewise linear function over
the session. The tabular points of the function can be constrained using a first-order Gauss-Markov process.
However, the default parametrization could not be optimal and should change according to the dry or wet
season. Analysing ZTD GPS estimates computed with loose constraints, first-order Gauss-Markov
parameters could be estimated and introduced in a new GPS data processing. After each data processing,
new ZTD estimates are available but although phase residuals, estimated positions ... All geodetic estimated
parameters can be used to evaluate the quality of the processing and the ZTD estimated parameters. The
use of radiometer is envisaged to have an external validation.
G41A-0605
Assimilation of ground-based GPS-PW and its Impact on Rainfall Forecast over the Korean Peninsula
Accurate information of the spatial distribution and temporal variations of water vapor is essential for the study of short-term severe weather phenomena, such as localized heavy rainfalls that often cause serious damages during the summer season on the Korean Peninsula. In particular, a good description of the initial conditions for the three-dimensional water vapor field is crucial for the simulation of convective systems and situations leading to heavy rainfall events in numerical model. Since the emergence of a new technique to retrieve precipitable water (PW) from ground-based GPS observation, numerous meteorological studies using PW derived from GPS measurements have been conducted and showing encouraging results in various fields of meteorology, especially for improving moisture fields of numerical weather prediction from development (Kuo et al. 1993) to operational use (Gutman et al. 2003). In this study, an impact of ground-based GPS-PW assimilation on a rainfall forecast is examined. A series of GPS-PW assimilation experiments were performed using the Weather Research and Forecasting (WRF) model and its three-dimensional data assimilation (WRF 3DVAR) system. PW data derived from 39 GPS sites over the Korean Peninsula is assimilated with cold-start and cycling techniques. Both negative bias and the rms error of the PW contained in original analysis (background) are significantly reduced after the assimilation of GPS-PW. As a results, the improved moisture field in initial condition lead to positive effect on the forecast of rainfall initiation at the right time and intensity. Also, the importance of sequential update of background in assimilation process has been proved by cycling 3DVAR experiment. GPS-PW assimilation with cycling mode shows better results on the forecast of rainfall intensity and position than cold-start experiment. References [1] Gutman, S.I., K.L. Holub, S.R. Sahm, J.Q. Stewart, T.L. Smith, S.G. Benjamin, and B.E. Swartz, 2003: Rapid retrieval and assimilation of ground based GPS-met observations at the NOAA Forecast Systems Laboratory: Impact on weather forecasts. Proceeding of International Workshop on GPS Meteorology, Tsukuba, Japan, Japan Meteorological Agency, 1-12-1-1-12-10. [2] Kuo, Y.-H., Y.-R. Guo, and E.R. Westwater, 1993: Assimilation of precipitable water measurements into a mesoscale numerical model. Mon. Wea. Rev., 121, 1215-1238.
G41A-0606
GPS tomography validation using radiosoundings, ground-based and airborne lidar
The Convective and Orographically-induced Precipitation Study (COPS) is an international field campaign with the overarching goal of enhancing the quality of forecasts of orographically-induced convective precipitation by 4D observations and modeling of its life cycle. The region in France and Germany (Vosges mountains and Black Forest) has been selected, where, on the one hand, severe thunderstorm activity is frequent in summer with significant amounts of precipitation and risk of flash flood events. On the other hand, the skill of numerical weather forecasts in this region is particularly low. A dedicated network of more than 80 GPS stations has been operating during the 2007 summer. The main goal of the GPS measurement is to provide a 4D field of water vapor both at local and regional scale trough GPS tomography based technique. GPS measurements are supported by numerous radiosoundings, radiometers, ground-based and airborne lidar. The GPS tomography has been modified to assimilate all water vapor measurements both integrated, dense and continuous (GPS) and vertically resolved, sparse in time and space (radiosoundings) measurements. Extensive validation of the GPS tomography products has been conducted both at the temporal scale of the entire field campaign period and for different specific events. The influence of the grid size and GPS network density has been evaluated. Moreover the number of radiosoundings assimilated (both spatial and temporal density) necessary to achieve an accurate 4D water vapor field with a vertical resolution of less than 500 meters is under the scope of this experiment. GPS with radiosoundings assimilated tomography is both a proxy for future GPS slant delay assimilation in numerical weather model and an unique way to combine the strength of the different water vapor measurements currently available.
G41A-0607
Local tomography troposphere model over mountains area
The term GNSS meteorology refers to the utilization of Global Navigation Satellite System's (GNSS) radio signals to derive information about the state of the troposphere. GNSS tomography allows to resolve the spatial structure and temporal behavior of the tropospheric water vapor. The paper presents the verification of GNSS tomography over dense local GNSS network. Paper address problem of obtaining stable tomographic solution from ill-conditioned system of linear equations. The main interests are in suitable horizontal and vertical resolution in given conditions. Here the Moore-Penrose pseudo inverse of variance- covariance matrix is used. The results are validate with the help of simulated weather conditions. Three various scenarios are tested. As general output of this paper the optimal model construction scheme is presented with possible further improvements. The verification of the tomography model based on the local GPS KARKONOSZE, situated in the mountains area.
G41A-0608
The calibration of mesoscale numerical weather prediction model NWP with the local meteorological observation for the local GNSS tomography
The mesoscale NWP models produce very fine meteorological forecast over large areas, but it may fail to reproduce the weather on the smaller scale. On the other hand, local meteorological parameters even measured at various elevations, are not sufficient for deriving the vertical troposphere structure. The paper addresses the problem of calibrating the mesoscale NWP model to suite local atmospheric measurements. The key issue is to obtain the spatial and temporal distribution of pressure, temperature and humidity. Methodology to interpolate, compare and subtract bias for both data sets is presented, as well as final atmospheric structure. Local data sets constitutes of 13 meteorological stations measuring three basic parameters with one hour time resolution. The average distance between sites is 20km. The Coupled Ocean Atmospheric Mesoscale Prediction System (COAMPS) model (Hodur, 1997) is the source of same three variables on the 3D grid, which has horizontal resolution of 1.44 km, with the 30 horizontal levels, and spans over 40 thousands square kilometers. Data are delivered with one hour time step. Study area is located in the south west of Poland in the Karkonosze mountains. The obtained data will be used for validating the GNSS tomography results from dense local network. Hodur, R.M., The Naval Research Laboratory's Coupled Ocean/Atmospheric Mesoscale Prediction System, Monthly Weather Review, 135, 1414-1430, 1997
G41A-0609
Water Vapor Tomography Using CGPS Network in Taiwan and FORMOSAT-3/COSMIC Radio Occultation Observations
GPS water vapor tomography presents time-dependent spatial distribution of wet refractivity in the troposphere which is useful for the improvement of numerical weather system in the region with densely- spaced ground GPS receivers. The GPS water vapor tomography is composed of numerous slant wet delays which estimated from double difference (DD) processes, precise point positioning (PPP), or directly achieved by using phase observations. It determines refractivity on each grid by using stochastic modeling and Kalman filtering technique is also applied. GPS water vapor tomography had been constructed from simulation and field experiments, and the results indicated that the resolution of the tomography was always depended on the number of satellites, the accuracy of slant wet delays at low elevations, and the number of ground GPS stations. Taiwan, located in the subtropical and mountainous region where heavy rainfalls and typhoons occurred frequently in summer, has been occupied by a dense continuous GPS network for the earthquake- related studies. Some continuous GPS stations also equipped with meteorological instruments. In this study we investigate the feasibility and capacity of the GPS water vapor tomography in Taiwan and study in particular at the time period of severe weather. The vertical profiles of atmospheric information retrieved from observations of GPS radio occultation from FORMOSAT-3/COSMIC spacecraft constellation are also used for deriving the accuracy and sensitivity of the tomography.
G41A-0610
The use of Troposphere Slant Delays in Regional Double Difference GPS Computation
The demand of accurate and stable time series of geodetic parameters is increasing. One factor limiting the accuracy is the troposphere, since it is hard to model and compute with sufficient resolution, both in time and space. We have studied the effect of numerical weather model (NWM) derived troposphere slant delays to most commonly used mapping functions, Niell and Vienna mapping functions, in GPS processing. We corrected the effect of troposphere on the observation level and computed a regional network using both modified and un-modified data. We used the data of thirteen Finnish permanent GPS stations for 184 days, i.e. 6 months. We computed the network using Bernese v. 5.0 in double difference mode, where the other end of the vector is kept fixed and the coordinates of the other end are estimated. The vector lengths vary between 110 and 1100 km. The results show that when no site-specific troposphere parameters or gradients are estimated, the use of NWM- based troposphere delays improves the results in all three coordinate components (north, east and up) statistically significantly, up to 60 percent. Using the more realistic troposphere model reduces also the baseline length dependence of the solution. This approach could be used e.g. for real-time navigation applications or for regular land surveying. If the site-specific troposphere parameters and the horizontal gradients are estimated, there is no statistically significant improvement between the different solutions. Our method of manipulating the observations has the advantage that it is processing software independent and can be used for other error sources, e.g. ionosphere delay or loading, as well.
G41A-0611
Impact of mapping functions based on spherical, ellipsoidal, gradient, and 3d atmospheric structures on GPS Precise Point Positioning
We evaluate the impact of mapping functions developed from different atmospheric structures on precise point positioning. In each case the atmospheric structure is derived from the same Numerical Weather Model (NWM). We compared five different structures -- from simpler to more realistic: spherical concentric, spherical osculating, ellipsoidal, gradient, and 3D -- and a state-of-art mapping function, Vienna Mapping Functions Site (VMF1). We used data from IGS station ALGO. Results correspond to comparisons with the IGS (non- cumulative) weekly solution. The spherical concentric model shows a large (cm-level) bias in the north component. The spherical osculating (and ellipsoidal) model shows an improvement in the up component, by almost one order of magnitude, over that of VMF1. The 3D atmosphere model reduces the horizontal bias to less than 1 mm, but there is no apparent improvement in the vertical position, which we attribute to unaccounted non-tidal atmospheric pressure loading. Finally, the gradient atmosphere shows biases with magnitude in between those of the spherical osculating and 3d models.
G41A-0612
An Evaluation of the Practicability of Current Mapping Functions using Ray-traced Atmosphere Slant Delays from JMA Mesoscale Numerical Weather Data
The Japan Meteorological Agency (JMA) meso-scale analysis data (MANAL data) which we used in our study provides temperature, humidity, and pressure values at the surface and at 21 height levels (which vary between several tens of meters and about 31 km), for each node in a 10km by 10 km grid that covers Japan islands, the surrounding ocean and eastern Eurasia. The 3-hourly operational products are available by JMA since March, 2006. We have simultaneously evaluated atmospheric parameters (equivalent zenith total delay and linear horizontal delay gradients) and position errors derived from slant path delays obtained by the KAshima RAytracing Tools (KARAT) through the MANAL data. Most of the early mapping functions developed for VLBI and GPS were based on the assumption of azimuthal isotropy. On the other hand, the recent geodetic analyses are carried out by applying the modern mapping functions based on the numerical weather analysis fields. The Global Mapping Function (GMF) by Boehm et al. (2006), and Vienna Mapping Function (VMF) by Boehm and Schuh (2004) have been successfully applied to remove the zenith hydrostatic delay in the recent years. In addition, the lateral spatial variation of wet delay is reduced by linear gradient estimation. Comparisons between KARAT-based slant delay and empirical mapping functions indicate large biases ranging from 18 to 90 mm, which is considered to be caused by significant variability of water vapor. Position error simulation reveal that the highly variability of the errors is clearly associated with severe atmospheric phenomena. Such simulation are very useful to investigate the characteristics of positioning errors generated by local atmospheric disturbances. Finally, we compared PPP processed position solutions using KARAT with those using the latest mapping functions covering a period of two week GEONET data. The KARAT solution is almost identical to the solution using GMF with linear gradient model, but some cases tends to be slightly worse under the extreme atmospheric condition. Though we need further investigations to evaluate the capability of KARAT to reduce atmospheric path delay under the various topographic and meteorological regimes, KARAT will promise an efficient reduction of atmospheric path delays considering that the numerical weather model will be improved concerning spatial and temporal resolution.
G41A-0613
A General Atmospheric Propagation Delay Formula for Geodetic Space Techniques
In recent years, some fundamental geodetic stations have been established to carry out multi-satellite
observations with the highest accuracy (~ 1mm) using GNSS (Global Navigation Satellite System), Very
Long Baseline Interferometry (VLBI) and SLR (Satellite Laser Ranging). However, the demand to increase
the accuracy of such geodetic space techniques has shown that their ultimate accuracy is limited by the
correction of the atmospheric propagation delays. We have developed a unified theory of atmospheric
corrections based on the geometric optics principle, and the perturbation technique to solve the propagation
problem. Using this theory, we have further developed a new atmospheric correction formula for linear
combinations of space geodetic measurements such as the two-colour SLR measurements and the co-
located SLR and GNSS measurements to a GNSS satellite equipped with a laser reflector array. Moreover,
this new formula can also be used for the dual frequency ionospheric correction of GPS/VLBI observations.
The proposed formula eliminates the total atmospheric density effect and takes into account the water
vapour density and the arc-to-chord correction. Unlike other correction formulae, this new formula does not
require any hydrostatic mapping function nor a hydrostatic horizontal gradient mapping function as the total
atmospheric density effect can be eliminated. Numerical simulations show that this new formula produces the
atmospheric range correction with an accuracy better than 1 mm at any elevation angle. The required
information about the water vapour distribution along the propagation path can be calculated using GPS or
Water Vapour Radiometer data. The accuracy demand on this data is moderate, thus we propose to use a
co-located GPS receiver. The arc-to-chord correction requires an atmospheric model which will be discussed
in detail. However, the required measurement precision for the difference of the two-colour SLR
measurements and the co-located GNSS and SLR measurements, i.e. better than 30 μm, exceeds the
capability of the current state-of-the art GNSS and SLR systems.
http://www.igms.tugraz.at
G41A-0614
Stochastic Modeling Considering Ionospheric Scintillation Effects on GNSS Relative and Point Positioning
Nowadays, Global Navigation Satellite Systems (GNSS), especially the Global Positioning System (GPS),
represent one of the most used techniques for geodetic positioning. The functional models related with the
GNSS observables are better understood than the stochastic models, considering that the development of
the latter is more complex. Usually, the stochastic models are used in a simplified form, as the standard
models, which assume that all the GNSS observables are statistically independent and have the same
variance. However, the stochastic models may be investigated in more detail, considering for example, the
effects of ionospheric scintillation. The high latitudes regions experiment strong influence of the ionospheric
effects, in particular ionospheric scintillation. Considering the availability of specially designed GNSS
receivers that provide ionospheric scintillation parameters, these effects can be mitigated through improved
stochastic models. This paper presents the methodology and results from GPS relative and point positioning
considering ionospheric scintillation in the stochastic modeling. Two programs have been developed to
obtain the results from relative and point positioning: "GPSeq" (currently under development at the
FCT/UNESP Sao Paulo State University – Brazil) and "pp_sc" (developed in a collaborative project between
FCT/UNESP and Nottingham University - UK). The point positioning approach can be realized considering an
epoch by epoch solution and the relative positioning using a Kalman Filter and the LAMBDA method to solve
the Double Differences ambiguities. Both programs have the option to estimate the ionospheric residuals as
one stochastic process using the white noise or random walk correlation models. In both cases it is also
possible to use the L1/L2 ion-free linear combination. The stochastic modeling considering ionospheric
scintillation has been implemented based in the models of Conker et al. (2003), following the approach
described in Aquino et al. (2008). Data from a network of GPS Ionospheric Scintillation and TEC Monitor
(GISTM) receivers set up in Northern Europe was used in the experiments as can be seen in De Franceschi
et al. (2006) and Romano et al. (2008). The point positioning results have shown improvements of the order
of 5 to 20 percent when considering the proposed stochastic modeling. In relative positioning, improvements
of the order of 20 percent have been achieved. These and further results will be discussed in this paper.
http://gege.prudente.unesp.br/
G41A-0615
Benefits of three frequency ionospheric corrections in Radio Occultation soundings
The Finnish Meteorological Institute (FMI) has assessed the potential benefits from using the third transmission frequency of the next generation GNSS systems in the sounding of the atmosphere with Radio Occultation (RO). This research has been performed in the framework of the Ionospheric Effects in GNSS Radio Occultation Data study funded by EUMETSAT. The objective of this study was to analyze the advantages of three frequency soundings as part of the planning of the future EUMETSAT satellite missions. The research has been performed by simulating the three frequency transmissions of the next generation GPS and GALILEO systems with the EGOPS (End-to-end GNSS Occultation Performance Simulator) software package developed by the international EGOPS consortium. EGOPS allows simulations of RO missions by propagating the orbits of the transmitting and receiving satellites, determining the geodetic locations and geometries of the soundings and ray tracings of the signals propagation paths. In the study we have ensured that all specified conditions are met by simulating over 1700 occultation soundings. The simulations included both the GPS and the future GALILEO constellations and signals. A LEO satellite at the orbit of the EUMETSAT Metop-A has been used to simulate an RO receiver. The global distribution of the occultations ensured that all occultation times and geometries of interest have been covered. Three solar activity levels have been used to simulated solar minimum, normal and solar maximum conditions. Two ionospheric correction techniques taking benefit of the third GNSS frequency have been tested in the study. The first tested method was a three-frequency linear combination technique. This method is an expansion of the two frequency linear combination that is currently widely used in GNSS navigation and in RO data processing. The disadvantage of this approach is that the noise level in the retrieved bending angle profile is significantly increased. The second tested methods is an ionospheric ambiguity removal with a combination of widelane (WL) and extra-widelane (EWL) signals. This method does not increase the noise level as much as the linear combination method, but is computationally slightly more complex and requires combining code phase observations with the carrier phase observations. The results of the simulations and the retrievals have been rigorously analyzed both statistically and by investigating selected individual observations. The results indicate that ionospheric correction with three frequencies can significantly reduce the ionospheric error in the neutral atmosphere sounding in the heights of 35 - 60 km. This can potentially increase the useful height range of RO soundings in operational NWP (Numerical Weather Prediction) and climate monitoring. This result is very important because very few atmospheric sounding techniques can provide global information from the upper stratosphere and lower mesosphere region. This presentation will show the results of the performed study including descriptions of the simulations and the tested ionospheric correction techniques. The bending angle retrieval accuracy benefits will be quantified both statistically and by analysis of selected complex signal propagation cases. Finally, the presentation will also address the potential benefits of three frequency soundings in space weather observations by RO.
G41A-0616
Ionospheric corrections estimation in a local GNSS permanent stations network: improvement of Code Point Positioning at sub-metric accuracy level
It is well know that GNSS permanent networks for real-time positioning were mainly designed to generate and transmit products for RTK (or Network-RTK) positioning. In this context, RTK products are restricted to users equipped with geodetic-class receivers. This work is a first step toward using a local network of permanent GNSS stations to generate and transmit real time products that could remarkably improve positioning accuracy for C/A receiver users. A simple experiment was carried out based on 3 consecutive days of data from 3 permanent stations that belong to the RESNAP-GPS network (w3.uniroma1.it/resnap-gps), located at the Lazio Region (Central Italy) and managed by DITS-Area di Geodesia e Geomatica, Sapienza University of Rome. In the first step the RINEX files were corrected for the differential code biases according to IGS recommendations and then processed with Bernese 5.0 CODSPP module (single point positioning using code measurements), using IGS precise ephemeris and clocks. One position per epoch (every 30 seconds) was estimated for P1 and for the ionosphere free combination (P3). The accuracy obtained with the P3 combination for the vertical component, which ranged from -1 to +1 m, was taken as the reference for the following discussion. For P1 observations, the vertical coordinate errors showed a typical signature due to the ionospheric activity: higher errors for day-time (up to 5 m) and smaller ones for night-time (around 1.5 m). In order to improve the accuracy of the P1 solution, ionospheric corrections were estimated using the La Plata Ionospheric Model, based on the dual-frequency observations from the RESNAP-GPS network. Those corrections were applied to the RINEX files of a probing station located within the reference network. With this procedure, the vertical coordinate errors were reduced to the range from -0.8 to 0.8 m. This methodological approach shows the possibility to remarkably improve the real time positioning based on Code measurements only using ionospheric corrections estimations and CODE DCB products.
G41A-0617
A Linkage Between the L-Band Amplitude Scintillations and the Steepest TEC Gradients at the Boundaries of the Equatorial Ionization Anomaly Crests
GPS derived total electron content (TEC) mapping with coupled amplitude scintillation data at the GPS L1 frequency (1.575 GHz) are used to study their specific relationships. We used data collected from the Brazilian GPS receiver network and a time-dependent inversion algorithm to create the 2-D images with the coupled TEC and scintillation measurements. We have found that the scintillation events are strongly correlated with the regions in the ionosphere where the steepest TEC gradients are observed (at the boundaries of the equatorial ionization anomaly crests). In order to study such relations, we carried out 3-D numerical simulations of equatorial plasma bubbles that include the dynamics parallel and perpendicular to the magnetic field. The simulations show that the large ionospheric density gradients at the boundaries of the anomaly are probably responsible for the observed large TEC fluctuations and the L-band amplitude scintillations.
G41A-0618
The diurnal/semi-diurnal and monthly variations of electron peak density and its height of the F2 layer in the mid-latitude using COSMIC/FORMOSAT-3 GPS radio occultation measurements
The ionospheric electron densities which are generally varied with the solar EUV/UV fluxes can affect on the GPS signals propagation from the GPS satellite through the ionosphere to the ground-based GPS receiver. The electron densities are mostly concentrated in the F2 layer, and then its climatology is important to monitor and model the ionosphere because the F2 peak electron density (NmF2) and its height (hmF2) are changed with the local time and month, and the solar and geomagnetic activities. In this paper, it is examined that the diurnal/semi-diurnal and monthly variations of the NmF2 and hmF2 in the mid-latitude between 30° and 60° N using the measurements from the COSMIC/FORMOSAT-3 GPS radio occultation system and ionosonde measurements. The variations of NmF2 and hmF2 are compared with the predictions of the International Reference Ionosphere (IRI- 2001) model.
G41A-0619
Detection of ionospheric scintillations and impact on GPS kinematic positioning
Monitoring the ionospheric state is essential for many scientific applications, e.g. accurate GPS-based positioning, radio-waves transmission, space weather. Additionally, the Earth's atmosphere is one of the major error sources degrading the potential of GPS kinematic positioning. To quantify the impact of high ionospheric activity on GPS kinematic positioning, a subset of 40 stations from the European Permanent Network (EPN) was processed using the Bernese v5.0 software. During the Halloween geomagnetic (super-) storm of 29-31 October 2003, we observed a decrease by a factor ten or greater of the hourly repeatability for the horizontal and vertical components compared to positions obtained during normal ionospheric activity. We also produced Total Electron Content (TEC) maps from continuous operating GPS stations of the EPN to characterize the ionospheric activity during this period. Those maps, computed each hour with a one degree step grid, have a higher spatial and temporal resolution than the CODE or IGS TEC maps, and allow detecting rapid and isolate abnormal ionospheric activity above Europe. We investigate the potential benefit of using residuals from the geometry-free carrier phases as well as slant TEC estimate to detect small scale ionospheric disturbances.
G41A-0620
Monitoring and Imaging Ionospheric Total Electron Content Without the Thin-Shell Approximation
The thin-shell model of the ionosphere relies on the coarse approximation that ionospheric electron density is non-negligible only in the vicinity of a specified reference height (typically the peak of the F-layer). The utility of this approximation resides primarily in the ease with which measurements of slant total electron content (TEC) may be converted into estimates of vertical TEC: if we identify the ionospheric pierce point (IPP) where a signal raypath intersects the shell height, then the vertical TEC at this IPP is estimated by scaling the TEC measured along the raypath by a simple geometric factor that depends upon the elevation angle of the signal. Developed to ensure the accuracy and integrity of user position estimates based upon global navigation satellite system (GNSS) measurements, all satellite-based augmentation systems (SBAS) to date, such as the United States' Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), and the Multi-Functional Satellite Augmentation System (MSAS) in Japan, use the thin-shell model as the basis for estimating vertical ionospheric delay at a specified set of regularly-spaced intervals in latitude and longitude, i.e., ionospheric grid points (IGPs). The vertical delay estimate at each IGP is calculated from a planar fit of neighboring slant delay measurements projected to vertical using the standard thin-shell obliquity factor. For an estimate of vertical TEC based upon the thin-shell approximation to be valid, two conditions must generally be satisfied: (1) the ionospheric electron density must be azimuthally symmetric with respect to the IPP; and (2) the choice of shell height must be appropriate. The successful operation of WAAS over the past fives years is a testament to the fact that these conditions are roughly satisfied under nominal ionospheric conditions at mid-latitudes. In the presence of significant horizontal electron density gradients, however, distinct measurements, which share a common IPP, can result in inconsistent estimates of the vertical TEC at the IPP. This is particularly true under storm conditions and at low latitudes, in regions characterized by complex ionospheric structure, high TEC values, and steep electron density gradients. The resulting error, often designated obliquity error, can seriously degrade the accuracy of the estimate. We present a two-stage approach for estimating TEC that does not use the thin-shell approximation and thereby eliminates obliquity error from the estimate. In the first stage, pseudo-measurements are calculated for a set of raypaths connecting earth grid points (EGPs) and satellites, where each fit of GNSS measurements used to define a pseudo-measurement is restricted to a spatial domain encompassing signals from only one satellite. In the second stage, we perform a fit of pseudo-measurements at each EGP. The resulting fit parameters may then be interpolated to provide an estimate of slant TEC along any arbitrary raypath. This method is designated the multi-cone model, since the spatial domain of each fit is a cone with a satellite (stage one) or an EGP (stage two) at the vertex of the cone. We assess the improvement in fit accuracy by comparing results achieved with the multi-cone model to those of the thin-shell model using data sets from networks of dual-frequency GPS receivers in Mexico and the United States, under both quiet conditions and disturbed conditions representative of those likely to be encountered at the next solar maximum.
G41A-0621
GNSS Active Network of West of Sao Paulo State Applied to Ionosphere Monitoring
In Brazil, a research project of atmospheric studies from reference stations equipped with dual frequency GNSS receivers is in initial phase. These stations have composed the GNSS Active Network of West Sao Paulo State (Network-GNSS-SP) and have been broadcasting GNSS data in real time. Network-GNSS-SP is in tests phase and it's the first Brazilian network to provide GNSS measurements in real time. In Spatial Geodesy Study Brazilian Group (GEGE) has been researched the ionosphere effects on L band signal, as well as the GPS potential on ionosphere dynamic monitoring and, consequently, the application of this one to spatial geophysics study, besides dynamic ionosphere modeling. An algorithm based on Kalman filter has been developed for ionosphere modeling at low latitude regions and estimation of ionospheric parameters as absolute vertical TEC (VTEC) for the monitoring of ionosphere behavior. The approach used in this study is to apply a model for the ionospheric vertical delay. In the algorithm, the ionospheric vertical delay is modeled and expanded by Fourier series. In this paper has been realized on-line processing of the Network-GNSS-SP data and the initial results reached with the algorithm can already be analyzed. The results show the ionospheric maps created from real time TEC estimates.