Evaluation and Comparison of Different Methods of Ionospheric Delay Mitigation for Future Galileo Mass Market Receivers

A Somieski, C. Burgi and E. Favey

Abstract:	The Earth’s ionosphere causes strongly varying range and range - rate errors in the measurements of ground - based GNSS receivers and is, therefore, a limiting factor for single - frequencyGNSS applications. Single - frequency users have to rely on ionospheric models or Satellite Based Augmentation Systems (SBAS) in order to mitigate errors due to ionospheric refraction sufficiently. In the framework of the EU funded project Galileo Receivers for Mass Market (GREAT) a preliminary navigation study was carried out dealing with the performance of the future Galileo ionospheric model NeQuick proposed to calculate corrections for single - frequency Galileo users. Primarily, this study intends to clarify if the effort for a dual - frequency Galileo receiver for mass market pays in terms of improved performance due to advancedmitigation of ionospheric refraction. The additional work and expense of the dual - frequency approach has to be related to the benefit from enhanced ionospheric corrections. In this context, the quality of the future ionospheric model of Galileo plays an important role, since it affects directly the positioning accuracy of a single - frequency Galileo user. Therefore, the Galileo model NeQuick is investigated in detail and compared to common methods accounting for ionospheric effects. Secondly, the ”ionospheric” linear combination is applied to code measurements (C1 and P2) of the Global Positioning System (GPS) in order to evaluate the potential of a Galileo receiver performing code measurements on E1 and E5a. According to the Galileo Interface Control Document (ICD), the main input parameter of NeQuick is the effective ionization level Az, which is approximated with a second - degree polynomial function of the modified dip latitude MODIP. The three coefficients of this polynom are supposed to be determined on a daily basis utilizing a future global network of Galileo tracking stations. Since these Az coefficients are not yet available they have to be simulated accordingly. For that purpose, Global Ionosphere Maps (GIM) of the Center for Orbit Determination in Europe (CODE) are used to retrieve them from the Vertical Total Electron Content (VTEC) by a least - squares adjustment. The VTEC values needed are derived from GIMs by interpolating for 22 worldwide distributed monitoring stations. These stations represent a possible future network of Galileo ground stations used to monitor the ionosphere. For evaluation purpose, the results for the years from 2004 to 2006 are compared to the GPS ionospheric model (Klobuchar) and the ionospheric corrections of theWide Area Augmentation System (WAAS). Additionally, dual - frequency C1P2 code measurements of the fundamental stationWettzell (Germany) were processed with the GPS Toolkit. The first - order ionospheric group delay and the Total Electron Content (TEC) along the signal propagation path are accessed directly by the ”geometry - free” or ”ionospheric” linear combination. The unknown differential hardware delay of the receiver is determined daily using observations during the nighttime when the electron content in the ionosphere is almost negligible. Applying a least - squares fit to all night TEC data both a simple model of the (nighttime) TEC and the receiver bias are estimated. Finally, the Global Ionospheric Maps (GIM), WAAS and the ionospheric models of GPS and Galileo are compared to each other using GIM as reference. GIM was chosen because it provides a continuous time series, global coverage and high accuracy arising from the usage of dual - frequency phase measurements from about 200 GPS and GLONASS sites of the International GNSS Service (IGS) and other institutions. The rms, relative errors and residuals are computed on a global scale. But the dual - frequency retrievals of station Wettzell are referenced to the corresponding values interpolated. Following from the results obtained, it can be stated that the ionosphericmodel of Galileo (NeQuick) is expected to offer corrections of good quality exceeding those of GPS (Klobuchar). It was found out, that NeQuick corrects for approximately 70% of the ionospheric delay, whereasKlobuchar only accounts for 50%. Since some assumptions and approximations (e.g. vertical TEC instead of slant TEC observations) had to be made in this study concerning the simulation of the NeQuick model its performance is expected to be even better than evaluated. Additional improvements could be achieved, if the NeQuick coefficients are estimated and broadcast more frequently than every 24 hours and the network of groundmonitoring stations is dense and globally well distributed. The intercomparison also revealed, that the Satellite Based Augmentation System (SBAS) of North America WAAS provides very accurate ionospheric corrections comparable to dual - frequency retrievals utilizing (C1P2) code observations. Considering the promising results of the NeQuick simulation and the expectation of an increasing number of SBAS systems in the future, we come to the conclusion that the effort for a dual - frequency receiver for mass market does not pay. The presentation will focus on the Galileo ionospheric model NeQuick and its comparison with the Klobuchar model (GPS), WAAS and the dual - frequency approach. The results are analyzed with respect to the requirements of a future Galileo mass market receiver. The conclusions of the study presented provide valuable information for both users and developers of GNSS receivers.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2854 - 2860
Cite this article:	Somieski, A, Burgi, C., Favey, E., "Evaluation and Comparison of Different Methods of Ionospheric Delay Mitigation for Future Galileo Mass Market Receivers," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2854-2860.
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