As the number of ground and space-based receivers tracking the Global Positioning System (GPS) steadily increases, it is becoming possible to monitor changes in the ionosphere continuously and on a global scale with unprecedented accuracy and reliability. At the time of writing this abstract (March 2006), there are more than 1200 globally-distributed dual-frequency GPS receivers available using publicly accessible networks including, for example, the International GPS Service (GPS) and Continuously Operating GPS Stations (CORS).

To take advantage of the vast amount of GPS data, researchers use a number of techniques to estimate satellite and receiver interfrequency biases and the total electron content (TEC) of the ionosphere. Most techniques utilize grid methods, spherical harmonic expansion or basis function coefficient sets to separate the hardware-related biases from the ionospheric contribution. These methods often have a limitation of using up to a couple of hundred GPS receivers, utilizing a sequential least squares or Kalman filter approach to estimate satellite and receiver interfrequency biases as nuisance parameters. The biases are then later removed from the measurements to obtain unbiased TEC.

In our approach to calibrating GPS receiver and transmitter interfrequency biases, we take advantage of all available GPS receivers using a new processing algorithm, based on the Global Ionospheric Mapping (GIM) software developed at the Jet Propulsion Laboratory. This new capability is designed to estimate receiver biases for all stations. In this new approach, we solve for the instrumental biases by modeling the ionospheric delay and removing it from the observation equation using pre-computed GIM maps. The pre-computed GPS maps use about 200 globally-distributed GPS receivers to establish the “background” used to model the ionosphere at the remaining 1000 GPS sites.

To demonstrate this new technique we show three particular applications to which we have applied the algorithm. 1) We estimate satellite and receiver interfrequency biases for all 1200 GPS receivers on a daily basis and compare them to the interfrequency biases delivered as the official IGS product. 2) We demonstrate this automated tool to investigate quiet and storm-time ionospheric behavior for e.g., the Wide Area Augmentation System (WAAS) and other scientific studies. 3) We apply the technique to extract ionospheric signatures at the 0.1 TECU level caused by seismic-ionospheric interaction during a seismic event such as the December 26, 2004 Sumatra Earthquake event.

ESA/ESOC contributes to the activities of the IGS Ionosphere Working Group since the working group’s establishment in May 1998. When these activities began, single layer approaches were widely used, and the time resolution of the TEC maps produced was one day. Also there were significant gaps in the IGS ground station network at that time. In the meantime especially the time resolution has been enhanced, currently to two hours, and the IGS station coverage could also considerably be densified. The realization of near-real-time processing and real-time processing is under discussion. Beyond these aspects, a clear trend to the establishment of 3-dimensional ionosphere models evaluating “classical” TEC data, derived from GNSS observables, in combination with electron density data obtained from LEO occultations and ground based ionosondes, can be recognized.

At ESA/ESOC the currently still used single layer modelling has been improved with the aid of surface spherical harmonics, and to enhance the time resolution a special estimation scheme has been designed and implemented. Activities to realize an operational TEC map service locally for the ESA tracking sites will commence soon. Beyond that, activities are ongoing to develop a 3-dimensional ionosphere model which will use “classical” TEC data and electron density data as input. Additionally, there are considerations to establish global and regional ionosphere models based on wavelets.

This paper will provide an overview of the improvements made at ESOC in ionosphere modelling so far, as well as an outlook over further improvements which are currently under development.

The purpose of this talk is to summarize the last results obtained at UPC in two fronts, which suppose a significant improvement of the corresponding ionospheric determinations:

1) The application of a Kriging interpolation technique adapted to the global Total Electron Content mapping, especially useful in the context of the IGS reprocessing campaign.

2) The improvement of the Wide Area Real Time Kinematics (WARTK) technique (originally based on an optimal combination of a real-time precise ionospheric and geodetic modelling) with a simple Medium Scale Travelling Ionospheric Disturbance (MSTID) modelling.

Indeed, the Kriging technique has shown its potential in order to significantly improve the performance of the ionospheric interpolation, being an important point in global TEC mapping, due to the lack of receivers in large areas over the Seas and South Hemisphere (see for example Orus et al. 2005). The reason behind such improvement is that Kriging takes optimally into account the spatial error decorrelation.

In this context we will show the first systematic results of UPC Ionospheric VTEC maps reprocessing, applied during the first three months of 2000, in which Kriging provides an additional 10% of improvement, regarding to the other improvements developped in the last 6 years.

An other ionospheric front in which significant improvements has been achieved recently corresponds to high accuracy ionospheric corrections, supporting cm-error-level GNSS navigation at continental scales (based for example on the Satellite Based Augmentation reference receivers, SBAS systems, such as the EGNOS RIMS).

Indeed, in order to achieve cm-error-level navigation, is important to correctly estimate the carrier phase ambiguities. In this context, the WARTK technique allows such fixing in baselines up to hundreds of km, due to an optimal combination of a high precision real-time tomographic ionospheric model, with the geometric model (see for example Hernandez-Pajares et al. 2000). We will show that a new and simple real-time modelling of MSTIDs doubles the WARTK service area over Europe. WARTK can be applied for both GPS and the new Galileo and GPS modernized systems, and it is being demonstrated in recent and ongoing European projects, which will be also briefly summarized.

References

Hernández-Pajares, M., J.M. Juan, J. Sanz, O.L. Colombo, Application of ionospheric tomography to real-time GPS carrier-phase ambiguities resolution, at scales of 400-1000 km and with high geomagnetic activity, Geophysical Research Letters, Vol.27, No.13, p. 2009-2012, 2000.

Orús, R., M. Hernández-Pajares, J.M. Juan, J. Sanz, Improvement of global ionospheric VTEC maps by using Kriging interpolation technique, Journal of Atmospheric and Solar-Terrestrial Physics, Vol.67, Pag.1598-1609, 2005.

The purpose of this talk is to summarize the recent activities and present status of the IGS Ionosphere WG. The first part of this talk will be devoted to show the recent performance of the final and rapid Global Ionospheric VTEC maps, and DCBs, which constitute the main products and activities of the WG, based on the contribution of the four involved agencies, CODE, ESA, JPL and UPC.

In the second part of the talk, additional activities performed in the context of the WG will be summarized as well, such as the study and conclusions about the second order ionospheric correction, solicited by IGS in the IGS WS held in Bern, in 2004.

Finally several points and potential actions that could deserve the interest of the next chairman and the WG as a whole will be presented.