A. Flores

4D tropospheric tomography using GPS slant wet delays

Tomographic techniques are successfully applied to obtain 4D images of the tropospheric refractivity in a local dense network of global positioning system (GPS) receivers. We show here how GPS data are processed to obtain the tropospheric slant wet delays and discuss the validity of the processing. These slant wet delays are the observables in the tomographic processing. We then discuss the inverse problem in 4D tropospheric tomography making extensive use of simulations to test the system and define the resolution and the impact of noise. Finally, we use data from the Kilauea network in Hawaii for February 1, 1997, and a local 4 × 4 × 40 voxel grid on a region of 400 km2 and 15 km in height to produce the corresponding 4D wet refractivity fields, which are then validated using forecast analysis from the European Center for Medium Range Weather Forecast (ECMWF). We conclude that tomographic techniques can be used to monitor the troposphere in time and space.

Tomography of the lower troposphere using a small dense network of GPS receivers

The application of tomographic techniques to the troposphere with GPS signals was demonstrated in previous work using data from the Kilauea permanent network, Hawaii. Local orography of the network considered there, however, played a key role in the resolution capabilities of the technique. The authors explore the possibilities of tomographic reconstruction of the four-dimensional (4D) structure of water vapor using a very small network of global positioning satellite (GPS) receivers with virtually no height differences between the stations. The analyzed campaign consisted of seven GPS receivers located at the Onsala Space Observatory, Onsala, Sweden, and was carried out in August 1998. Traditional meteorological data sources and tools such as the numerical weather model NCAR Mesoscale Model (MM5), satellite data from the National Oceanic and Atmospheric Administration (NOAA), Washington, DC, and data and analysis from the European Center for Medium-Range Weather Forecasting (ECMWF), Reading, UK, have been used to evaluate the results. A good agreement is found between GPS tomography and classical methods, even in meteorological situations with complex vertical structure of water vapor.

Estimation of tropospheric zenith delay and gradients over the Madrid area using GPS and WVR data

We have analyzed data from an experiment over the Madrid (Spain) area obtained from 5 GPS receivers and 3 Water Vapor Radiometers (WVR), in order to compare their retrievals of Tropospheric Slant Delays. For this purpose we have fitted a simple gradient model to both types of data, using a Kalman filter to account for the temporal variability of the zenith and gradient parameters. We show that the retrieved gradients with the two instruments are compatible, thus suggesting that the derived slant delays can be useful for tomographic analysis. We compare the estimates of ZWD obtained with the GIPSY and the Bernese software packages. Finally, we compare the estimated gradients with those obtained with HIRLAM, a Numerical Weather Prediction model.

The contributions of the MAGIC project to the COST 716 objectives of assessing the operational potential of ground-based GPS meteorology on an international scale

Abstract MAGIC (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean) is a 3 year project financed in part by the European Commission for research on deriving and validating robust GPS integrated water vapor (IWV) and zenith tropospheric delay (ZTD) data sets and developing methods to assimilate the data into numerical weather prediction models (NWP) and test their impact. It was conceived independently from the COST 716 action, which seeks to coordinate research in the domain at an international scale, but addresses some of the same objectives. This has led to a productive cooperation between the two initiatives and their participants, and motivated the decision of MAGIC participants to provide research results as part of the COST demonstration system. Currently a database of 1.5 years of ZTD data are available on the MAGIC web site which has been validated through comparisons with radiosondes which gives differences with a standard deviation of 10 mm ZTD or the equivalent error in IWV of 1.6 kg/m2. NWP assimilation tests will be carried out in the final year of the project.

Ionospheric calibration of radar altimeters using GPS tomography

We compare TEC measurements from the NASA Radar Altimeter and DORIS instrument on board TOPEX/POSEIDON with GPS TEC estimates, and evaluate different GPS data analysis strategies. We verify that global tomographic GPS analysis using a voxel grid is well suited for ionospheric calibration of altimeters. We show that a 1-day fit of 20-second-averaged NRA ionospheric correction data versus GPS tomographic TEC data has a bias of 3.4 TECU and a root mean square deviation of 3.2 TECU. Tomographic inversion using simulated data from the Parametrized Ionospheric Model highlights the strong correlation between GPS bias constants, electronic densities at the highest layer, and unmodeled protonospheric TEC. This suggests that GPS TEC estimates at the TOPEX/POSEIDON altitude are more accurate if the bias constants are estimated and if a layer above TOPEX/POSEIDON is added to the grid.

Sensing atmospheric structure: Tropospheric tomographic results of the small-scale GPS campaign at the Onsala Space Observatory

Tropospheric tomography using data from local networks of Global Positioning System (GPS) receivers is producing encouraging spatio-temporal representations of the wet refractivity field. In this work we present the results from a small-scale geodetic experiment that we carried out at the Onsala Space Observatory. Seven GPS receivers distributed within a radius of 3 km from the center, were deployed during 21 days in the summer 1998. The limited number of sites and their spatial configuration present a challenge for tropospheric tomography. Using novel GPS techniques to determine the vertical structure of the atmosphere, we observed, for one session, a strong horizontal water-vapor gradient with a leading edge at higher altitude than the trailing edge, entering from the north. The vertical structure obtained independently using tomographic techniques matched such situation. These results suggest tomography is a promising technique for the determination of the spatio-temporal structure of the atmosphere. We will present preliminary results of the tropospheric tomography, using simulations and experimental data, together with some comparisons with radiosonde data.

Spatio-temporal tomography of the lower troposphere using GPS signals

The obtaining of the spatio-temporal representation of the wet refractivity distribution in the lower troposphere using GPS has been a line of research that has recently achieved very promising results. We here present a review of the work done and discuss some aspects as well as trace some future lines of development to increase the impact of GPS data in meteorological studies. Starting from the refinement of the tomographic technique, we assessed its capabilities using simulations based on the ground network of GPS receivers at mount Kilauea, Hawaii, and finally applied the whole procedure to the GPS data campaign conducted at the Onsala Space Observatory, verifying the results there obtained using traditional meteorological tools and analysis.

Ionospheric tomography using Ørsted GPS measurements - preliminary results

Abstract Data from the Turbo-Rogue GPS receiver onboard the Danish Orsted Satellite are used to derive satellite-to-satellite Total Electron Content (TEC). We present preliminary results obtaining Electronic Density fields from ionospheric tomography using TEC Orsted data. TEC is based on single frequency measurements, since only the L1 signal is of good quality. The C/A pseudo range (C1) and the L1 phase are used to obtain the TEC measurement. Electron density profiles from individual occultations are derived, based on the Abel transform, and compared with the tomographical solution. The assumption of spherical symmetry in the Abel transform limits the accuracy of the profiles and often results in a bias. We select occultations observed along the orbital plane to minimize the spherical assymetry effects. The tomographic grid is also confined to a narrow torus close to the orbital plane. The TEC calculations are based on data from 6 hour (3.6 orbits) periods. Two days have been selected for this study, December 5, 1999 and February 12, 1999.

A PIM-aided Kalman Filter for GPS Tomography of the Ionospheric Electron Content

Abstract We develop the formalism to perform PIM-based stochastic tomography of the ionospheric electron content with a Kalman filter, in which the inversion problem associated with four-dimensional ionospheric stochastic tomography is regularized. For consistency, GPS data is used to select dynamically the best PIM parameters, in a 3DVAR fashion. We demonstrate the ingestion of (10S and GPS/MEI) GPS data into a parameterized ionospheric model, in order to select the set of parameters that minimize a suitable cost functional. The resulting PIM-fitted model is compared to direct 3D voxel tomography. We demonstrate the value of this method analyzing IGS and GPS/M ET GPS data, and present our results in terms of a 4D model of the ionospheric electronic density.

A near real time system for tropospheric monitoring using IGS hourly data

The availability of real-time tropospheric data is an important question for atmospheric parameters to be ingested in weather prediction models or in monitoring studies. In the frame of the EU funded project CLIMAP we have developed an on-line system that acquires GPS data from a number of IGS stations distributed around Europe providing hourly batches of GPS observables. An off-line system processing data with a one-day delay was run during one year to tune the characterisitics of the present on-line system. The hourly data are processed using GIPSY/OASIS II software, to obtain the Zenith Total Delay in a near-real-time basis. Every hour, the new batch is merged with the observations of the previous 23 hours to allow the system to process enough data to yield robust results. The question of what prediction for orbits to use has also been addressed, designing the system to always use the best orbits available from the Center for Orbit Determination in Europe, using the quality codes for the prediction of the orbits to remove “bad quality” satellites from the solution. To validate the results, the ZTD time series for each station have been compared against the solutions obtained through a Precise Point Positioning using final precise orbits and clock corrections. The estimation processing has been tuned to account for the particularities of a near-real-time scenario such as the satellite clock correction and the day boundary issue.