Distributed Computing for Ionospheric Data Processing
With the advent of the GALILEO satellite system, one issue to take into account is the use of ionospheric models for single-frequency receivers, since the STEC (Slant Total Electron Content) correction must be provided to these users. In this framework, the aim of this study is to show, through the comparison of TEC (Total Electron Content) values, the difference in performance between the NeQuick model and the TEC maps provided by IGS (International GPS Service), and between the NeQuick model and the Klobuchar model, presently used in the Global Positioning System (GPS). These two comparisons will be used, with the aid of other more specific cases, to determine the most suitable implementation of the NeQuick algortihms for real time navigation purposes. These comparisons will be performed against an independent source of TEC data, such as altimetric satellites (e.g.TOPEX/Poseidon) that provide direct measurements of TEC.
The NeQuick model, a quick run model for ionospheric applications, provides electron density profiles using global ionosonde parameters in order to model anchor points for the E layer, F1 layer and F2 layer. The outputs provided by NeQuick make it a very suitable model for transionospheric propagation applications. Specifically, the output, which is going to be used for this study, is the electron density from the E region upwards as a function of time- hour and month-, height, geographic latitude, geographic longitude and solar activity. The driver of the model will be an effective ionization level parameter that will take into account not only the solar daily activity but also the geomagnetic information at user point.
The reference data used in this study are Global Ionospheric Maps (GIMs) of TEC with a 2h x 5 x 2.5 - time, longitude and latitude resolution - provided by IGS. These GIMs are computed by the IGS Associate Analysis Centres (IAACs) on a daily basis. The global accuracy of this combined TEC is about 2-8 TECU (1 TECU corresponds to an ionospheric delay of 16 cm in L1) depending on the epoch in the Solar Cycle, season, latitude and proximity of available GPS receivers.
Another way to determine its performance in correcting the ionospheric delay is to compare the model predictions with the TOPEX /Poseidon TEC derived form the dual-frequency altimeter. These data come from an altimetric satellite, at an averaged height of about 1330 km. Among other sensors, it has a dual transmitter -receiver in C-band (5.5 GHz) and a Ku-band (13.6 GHz) that provide TEC with accuracies, including systematic biases, of 2-3 TECU (1 TECU = 1016 e-/m2). Above oceans, few GPS data are available thus the comparative with TOPEX becomes a good test to analyse the accuracy of the TEC estimations far away from the ground stations. For the comparisons, the bias and rms of the models are computed regarding the TOPEX data.
First run
The first run was started on 17.06.2005 and completed on 22.06.2005.
Second run
The second run was started on 25.08.2005 and completed on 02.09.2005.