J. M. Juan

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.

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.

Neural network modeling of the ionospheric electron content at global scale using GPS data

The adaptative classification of the rays received from a constellation of geodetic satellites (the Global Positioning System (GPS)) by a set of ground receivers is performed using neural networks. This strategy allows us to improve the reliability of reconstructing the ionospheric electron distribution from GPS data. As an example, we present the evolution at global scale of the radially integrated electron density (total electron content (TEC)) for October 18, 1995, coinciding with an important geomagnetic storm.

Enhanced Precise Point Positioning for GNSS Users

This paper summarizes the main results obtained during the development of an Enhanced Precise Point Positioning (EPPP) Global Navigation Satellite Systems multifrequency user algorithm. The main innovations include the application of precise ionospheric corrections to facilitate the resolution of undifferenced carrier phase ambiguities, ambiguity validation, and integrity monitoring. The performance of the EPPP algorithm in terms of accuracy, convergence time, and integrity is demonstrated with actual GPS and simulated Galileo data. This can be achieved with very limited bandwidth requirements for EPPP users (less than 300 b/s for dual-frequency GPS data).

Combining ionosonde with ground GPS data for electron density estimation

Abstract Dual frequency Global Positioning System (GPS) receivers provide integrated total electron content (TEC) along the ray path (slant TEC, affected by a bias). By inverting this observable, it is possible to obtain the vertical total electron content with some assumptions about the horizontal structure of the ionosphere. The large number of permanent receivers distributed around the world provide enough information to obtain such TEC observables with high spatial and temporal resolutions. Nevertheless, the geometry (mainly vertical) of the ground GPS observations does not allow to solve the vertical structure of electron density of the ionosphere. Mixing different kinds of complementary data in a tomographic context helps to overcome this problem. Several works have obtained successful results achieved by combining occultation and ground GPS data to estimate the local three-dimensional structure of ionospheric electron density. This paper proposes the use of just ground data to obtain similar or better results. To do this, the ground GPS data are mixed with vertical profiles of electron density derived from ionosonde data instead of GPS occultation observations. In this paper, the complementarity between vertical profiles of electron density (estimated using the NeQuick model) and ground GPS data (from GPS IGS permanent network) are shown as well as the performance of the resulting combination.