The Impact of the External Ionospheric Models on the Accuracy of RTK Position Estimation

Dorota A. Grejner-Brzezinska, Israel Kashani, Pawel Wielgosz, Dru A. Smith, Paul S. J. Spencer, Douglas S. Robertson and Gerald L. Mader

Abstract:	Real-time long-range RTK GPS is one of the most challenging modes of GPS positioning, as the spatiallycorrelated effects, such as atmospheric errors, increase with the growing distance between the base and the rover stations, ultimately preventing the integer ambiguity resolution (AR). The distance, over which carrier-phase ambiguity can be resolved, may be significantly increased by employing, for example, a multi-reference station (i.e., network) approach. This can be achieved either by introducing geometric conditions based on the fixed reference locations, and/or through the use of reference network data to estimate the atmospheric corrections to GPS observations that can be broadcast to the users, or by creating a virtual reference station (VRS), close to the user location. These methods generally allow the AR and increase the accuracy and reliability of the user position. In this paper, three methods of the ionospheric correction modeling and estimation, with varying spatial and temporal resolutions, based on reference GPS permanently tracking network, are presented, and the impact of these models on the positioning accuracy at the user location is discussed. The three ionospheric models that will be tested are: (1) absolute (biased) carrier phasebased model, decomposed from double-differenced (DD) ionospheric delays, (2) absolute model based solely upon undifferenced dual frequency ambiguous carrier phase data, and (3) tomographic model using pseudorangeleveled phase data. Methods 1-2 assume that the ionosphere is an infinitesimal single layer, while method (3) considers the ionosphere as a 3D medium. A 24-hour data set collected by the Ohio Continuously Operating Reference Station (CORS) network on August 31, 2003 is presented and analyzed, with a special emphasis on varying ionospheric conditions during the day and the night times. One CORS station was selected as rover, and the data reduction was performed in the post-processing mode. However, it should be mentioned that the algorithms implemented in the Multi Purpose GPS Processing Software (MPGPS™), developed at The Ohio State University, are designed for real-time applications. The ionospheric reference “truth” was generated in the form of DD ionospheric corrections, estimated directly from the dual frequency carrier phase data using fixed DD ambiguities. Then, the three ionospheric models were used to derive the DD ionospheric delay corrections for the rover. Subsequently, the rover position estimation was performed in each case, and the analysis of the applicability of each model to high-accuracy RTK GPS was performed. In particular, we studied the time required to fix the ambiguities (i.e., time-to-fix) applying various ionospheric models, and the quality of the resulting rover position coordinates. The positioning results for the rover receiver were obtained with MPGPSÆË using L1 and L2 fixed model with data accumulation to resolve the unknown integers in the on-the-fly (OTF) mode. In this analysis, the ionospheric corrections from the three selected models were used to assist the AR procedure. This method may continue in the instantaneous mode, using the ionospheric correction prediction, based on the previous epoch of data. The advantages of various approaches to ionosphere modeling and rover positioning are discussed.
Published in:	Proceedings of the 2005 National Technical Meeting of The Institute of Navigation January 24 - 26, 2005 The Catamaran Resort Hotel San Diego, CA
Pages:	462 - 470
Cite this article:	Grejner-Brzezinska, Dorota A., Kashani, Israel, Wielgosz, Pawel, Smith, Dru A., Spencer, Paul S. J., Robertson, Douglas S., Mader, Gerald L., "The Impact of the External Ionospheric Models on the Accuracy of RTK Position Estimation," Proceedings of the 2005 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2005, pp. 462-470.
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