Performance Assessment of the New Rapid-Static Module of the Online Positioning User Service „ OPUS-RS

Dorota A. Grejner-Brzezinska, Pawel Wielgosz, Israel Kashani, Gerald L. Mader, Dru Smith, Doug Robertson and Attila Komjathy

Abstract:	The National Geodetic Survey (NGS) operates the Online Positioning User Service (OPUS) as means to provide the GPS users’ with easier access to the National Spatial Reference System (NSRS). OPUS allows the users to submit their GPS data files to NGS, where the data are processed to determine position coordinates using the NGS computers and software. OPUS is completely automatic, and the results are e-mailed back to the user, usually within a few minutes. OPUS is intended for the use in the conterminous U.S. and most territories. Currently, OPUS accepts a minimum of two hours of GPS data, with a recommendation of submitting at least four hours in order to obtain the best possible solution (http://www.ngs.noaa.gov/OPUS/). To extend the number of OPUS users to those, who normally collect shorter data spans, the effort, sponsored by NGS, and performed in collaboration with the Satellite Positioning and Inertial Navigation (SPIN) group at The Ohio State University (OSU), resulted in the development of a rapidstatic module (OPUS-RS), as an extension of the currently used OPUS model. In this paper, the performance analysis of the OPUS-RS module, based on the stations of the CORS (Continuously Operating Reference Station) network is presented. Selected sub-network with ~250-300 km station separation is used to produce the ionospheric and tropospheric corrections that are tested in the rover positioning solution in multi-baseline mode, simulating the expected reference network and user geometry. One of the CORS stations is considered a user receiver with known coordinates, providing the ground truth to our tests. The data processing is performed in the postmission mode. Even though the OPUS-RS module is designed to estimate the ionospheric correction internally, additional tests of the positioning module accuracy and reliability were performed using the external ionospheric models. Four external ionospheric models were tested for their accuracy and applicability to support fast ambiguity resolution (AR) for short data sets (10 minutes) over long baselines: (1) absolute (biased) carrier phase-based model, decomposed from double-differenced (DD) ionospheric delays (OSU MPGPS-NR), (2) tomographic method based on pseudorange-leveled carrier phase data (NGS MAGIC), (3) MAGIC enhanced by the L1-L2 data fit, and (4) code and carrier phase-based JPL global ionospheric maps (JPL GIM). Model 1 assumes that the ionosphere is an infinitesimal single layer, while method 2 and 3 consider the ionosphere as a 3D medium; model 4 is based on a set of local horizontal basis function representation of the global ionosphere. An experimental data set of February 18, 2005 was used; with the Kp index reaching up to 6+, this data set represents a variable ionospheric condition during the course of the day: minor geomagnetic storm around 00-04 UTC, and more quiet ionosphere for the rest of the day (Kp index between 2+ and 5o). The work presented in this paper is a continuation of the investigation presented by Kashani et al. (2005a), where the primary implementation modules of OPUS-RS, as well as the preliminary performance evaluation were presented.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2595 - 2605
Cite this article:	Grejner-Brzezinska, Dorota A., Wielgosz, Pawel, Kashani, Israel, Mader, Gerald L., Smith, Dru, Robertson, Doug, Komjathy, Attila, "Performance Assessment of the New Rapid-Static Module of the Online Positioning User Service „ OPUS-RS," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2595-2605.
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