M. Hernández-Pajares

New approaches in global ionospheric determination using ground GPS data

Abstract Since 1 June 1998, the group of Astronomy and Geomatics of the Polytechnic University of Catalonia (gAGE/UPC) is contributing to the international project of defining an ionospheric product (Total Electron Content, TEC) from the data gathered by the permanent ground GPS receivers of the International GPS Service (IGS) network. The strategy and algorithms related to such a preliminary product, its calibration with synthetic observations generated from the International Reference Ionosphere (IRI), and the comparison with TOPEX TEC data are presented. Finally, these methods are applied combining ionosonde with ground GPS data, in order to obtain the vertical structure of the free electron distribution.

Medium-scale traveling ionospheric disturbances affecting GPS measurements: Spatial and temporal analysis

[1] In this work we present a simple technique to estimate the medium-scale traveling ionospheric disturbances (MSTIDs) characteristics (such as occurrence, velocity, vertical propagation) with periods lower than 20 min and its application to a set of GPS data both temporally and spatially representative (near one solar cycle and four local networks in the Northern and Southern Hemispheres, respectively). Some of the main results presented in this paper are the MSTIDs which occur at daytime in local winter and nighttime in local summer, related to the solar terminator and modulated by the solar cycle. They present equatorward (from � 100 to 400 m/s) and westward (� 50 to 200 m/s) horizontal propagation velocities, respectively. The corresponding periods are compatible (higher) with the theoretical prediction, which is given by the neutral atmosphere

Improving the Abel inversion by adding ground GPS data to LEO radio occultations in ionospheric sounding

GPS radio occultations allow the sounding of the Earths atmosphere (i.e. troposphere and ionosphere). The basic observable of this technique is the additional delay, due to the refractivity index, of a radio signal when passing through the atmosphere. This additional delay is proportional to the integrated refractivity, in such a way that we can obtain an estimation of the vertical refractivity profiles using observations at different elevation angles by solving an inverse problem. Traditionally, the solution of this inverse problem is obtained by using the Abel inversion algorithm assuming a refractivity index that only depends on the altitude. In this paper we present a modified Abel inversion algorithm for ionospheric sounding that overcomes the spherical symmetry assumption of the traditional Abel inversion algorithm. Processing a set of simulated data and 1 day of real data with this algorithm, a clear improvement over the traditional one can be obtained when comparing the derived critical frequencies with the ionosonde measurements. It is also shown that this improvement is sufficient to measure critical frequencies associated with the ionospheric E layer.

Application of ionospheric tomography to real‐time GPS carrier‐phase ambiguities Resolution, at scales of 400–1000 km and with high geomagnetic activity

Theinfluenceoftheionospherecanbeoneofthe mainobstacles toGPScarrierphaseambiguityresolution in real-time,particularlyoverlongbaselines. Thisisimportant toallusersofGPSrequiringsub-decimeterpositioning, per- haps in real time, especially with high geomagnetic activity or close to the Solar Maximum. Therefore, it is desirable to have a precise estimation of the ionospheric delay in real- time, to correct the data. In this paper we asses a real-time tomographic model of the ionosphere created using dual- frequency phase data simultaneously collected with the re- ceiversofanetworkofstationsintheUSAandCanada,with separationsof400-1000km,duringaperiodofhighgeomag- netic activity (Kp=6). When the tomographic ionospheric correctionisincluded,theresolutionon-the-fly(OTF)ofthe widelanedouble-dierencedambiguitiesatthereferencesta- tions is nearly 100% successful for satellite elevations above 20 degrees, while theresolution of theL1,L2ambiguities at the rover is typically more than 80% successful.

Global observation of the ionospheric electronic response to solar events using ground and LEO GPS data

We present in this work the temporal evolution of the three-dimensional electron density at global scale during two ionospheric storms (October 18–19, 1995, and January 10, 1997) computed using only actual Global Positioning System data. The tomographic model is solved by means of a Kaiman filtering with a filter updating time of 1 hour in a Sun-fixed reference frame, and with a resolution of 10 × 10 deg in latitude/local time and 100 km in height including also a protonospheric component (eight layers). The data set contains the data from the International GPS Service IGS (with more than 100 ground GPS stations worldwide distributed) and the GPS/MET low orbiting GPS receiver (both positive and negative elevation observations are used). This means for each storm 1,000,000 of delays, 400 occultations and 3000 unknowns per batch. The International Reference Ionosphere and data coming from the ionosonde of the Ebre observatory are used to show the reliability of the results.

Performance of different TEC models to provide GPS ionospheric corrections

Abstract The existence of a worldwide international GPS service (IGS) permanent network of dual-frequency receivers makes the computation of global ionospheric maps (GIMs) of total electron content (TEC) feasible. The GIMs computed by the IGS Associate Analysis Centers on a daily basis and by other kinds of forecast GIMs, which can be computed from, for instance, the international reference ionosphere (IRI) model, and the GPS broadcast models in the navigation message, can be applied to a broad diversity of fields, for instance as, navigation and time transfer. In this context, the performance of different kinds of models are presented in order to determine the accuracy of the different GIM. This is carried out by comparison with the TOPEX data that provides an independent and precise (at the level of few TECU) vertical TEC determination over the oceans and seas. Thus, the obtained accuracies, in terms of global relative error, ranging from 54% corresponding to the GPS broadcast model, to about 41% corresponding to IRI climatological model, and to less than 30% corresponding to GPS data driven models.

The Kohonen self-organizing map method: An assessment

The “self-organizing map” method, due to Kohonen, is a well-known neural network method. It is closely related to cluster analysis (partitioning) and other methods of data analysis. In this article, we explore some of these close relationships. A number of properties of the technique are discussed. Comparisons with various methods of data analysis (principal components analysis, k-means clustering, and others) are presented.

A two‐layer model of the ionosphere using Global Positioning System data

We present a new approach to model the Ionosphere based on GPS data. Previous authors have used models with an unique shell. In this case we have included a second shell to account for the distribution of the electrons in the outer part of the Ionosphere. We have analyzed the ionospheric electron content of a region above 30 degrees in declination in different conditions of ionospheric activity using the Kalman filter. The data used has been obtained from the International GPS Service for Geodynamics (IGS) network. Simultaneously we have studied the receiver and transmitter differential biases showing the effects of neglecting the outer part of the Ionosphere in the model. It appears a systematic variations for the receivers—depending on its latitude—not for the satellites.

Combining GPS measurements and IRI model values for space weather specification

Abstract We will discuss various ways in which the International Reference Ionosphere (IRI) model and ionospheric data deduced from GPS measurements can be combined to improve ionospheric determinations. A number of research groups are analyzing GPS data products and providing global maps of vertical Total Electron Content (TEC) on a regular basis. IRI predictions can guide the interpolation of regional TEC estimations, computed from GPS data, to obtain global TEC maps. GPS measurements, on the other hand, can be used to update the IRI monthly averages to actual conditions. This can be done by using the GPS-derived TEC maps or by using the actual GPS measurements of the electron content along the signal path from satellite to ground receiver. We will discuss the updating results using the actual GPS measurements.

Comparative testing of four ionospheric models driven with GPS measurements

In the context of the European Space Agency/European Space Operations Centre funded Study A¢Â�Â�GNSS Contribution to Next Generation Global Ionospheric Monitoring,A¢Â�Â� four ionospheric models based on GNSS data (the Electron Density Assimilative Model, EDAM; the Ionosphere Monitoring Facility, IONMON v2; the Tomographic Ionosphere model, TOMION; and the Neustrelitz TEC Models, NTCM) have been run using a controlled set of input data. Each model output has been tested against differential slant TEC (dSTEC) truth data for high (May 2002) and low (December 2006) sunspot periods. Three of the models (EDAM, TOMION, and NTCM) produce dSTEC standard deviation results that are broadly consistent with each other and with standard deviation spreads of ~1 TECu for December 2006 and ~1.5 TECu for May 2002. The lowest reported standard deviation across all models and all stations was 0.99 TECu (EDAM, TLSE station for December 2006 night). However, the model with the best overall dSTEC performance was TOMION which has the lowest standard deviation in 28 out of 52 test cases (13 stations, two test periods, day and night). This is probably related to the interpolation techniques used in TOMION exploiting the spatial stationarity of vertical TEC error decorrelation.