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Abstract. This paper demonstrates the concept and practical examples of instantaneous mapping of regional ionosphere, based on GPS observations from the State of Ohio continuously operating reference stations (CORS) network. Interpolation/prediction techniques, such as kriging (KR) and the Multiquadric Model (MQ), which are suitable for handling multi-scale phenomena and unevenly distributed data, were used to create total electron content (TEC) maps. Their computational efficiency (especially the MQ technique) and the ability to handle undersampled data (especially kriging) are particularly attractive. Presented here are the preliminary results based on GPS observations collected at five Ohio CORS stations (~100 km station separation and 1-second sampling rate). Dual frequency carrier phase and code GPS observations were used. A zero-difference approach was used for absolute TEC recovery. The quality of the ionosphere representation was tested by comparison to the International GPS Service (IGS) Global Ionosphere Maps (GIMs), which were used as a reference.

Instantaneous (single-epoch) ambiguity resolution may not always be possible because of the increasing noise and lack of sufficient redundancy to verify the integer selection in long-range real-time kinematic (RTK) GPS positioning. The algorithm proposed here is based on a single-baseline solution aided by doubledifference (DD) ionospheric delay from the previous epoch (up to 60 s latency in DD ionospheric delay can be accepted), which supports the ambiguity resolution and provides a quality instantaneous kinematic position over long distances. However, obtaining the initial DD ionospheric delay requires atmospheric corrections provided by the reference network and some accumulation at the beginning of the session-i.e., on-the-fly (OTF) initialization is needed. Afterwards, instantaneous RTK positioning is assured. In this paper, a feasibility test of instantaneous real-time rover positioning is presented, and the effect of the latency of the DD ionospheric corrections on the instantaneous ambiguity resolution in long-range RTK is investigated. The positioning accuracy achieved in the tests, expressed as the differences between the resultant coordinates and the known "true" coordinates, is at millimeter-level accuracy for the horizontal components and centimeter-level for the vertical one for baselines over 100 km long.

The primary objective of this paper is to test several methods of modeling the ionospheric corrections derived from a reference GPS network, and to study the impact of the models' accuracy on the user positioning results. The five ionospheric models that are discussed here are: (1) network RTK (NR) carrier phase-based model-MPGPS-NR, (2) absolute, smoothed pseudorange-based model-MPGPS-P4, (3) IGS Global Ionosphere Model-GIM, (4) absolute model based on undifferenced dual-frequency ambiguous carrier phase data-ICON, and (5) carrier phase-based data assimilation method-MAGIC. Methods 1-4 assume that the ionosphere is an infinitesimal single layer, while method (5) considers the ionosphere as a 3D medium.The test data set was collected at the Ohio Continuously Operating Reference Stations (CORS) network on August 31, 2003. A 24-hour data set, representing moderate ionospheric conditions (maximum Kp = 2o), was processed. The ionospheric reference "truth" in doubledifference (DD) form was generated from the dualfrequency carrier phase data for two selected baselines, ~60 and ~100 km long, where one station was considered as a user receiver at an unknown location (simulated rover). The five ionospheric models were used to generate the DD ionospheric corrections for the rover, and were compared to the reference "truth." The quality statistics were generated and discussed. Examples of instantaneous ambiguity resolution and RTK positioning are presented, together with the accuracy requirements for the ionospheric corrections, to assure integer ambiguity fixing.

The network-based approach to kinematic GPS positioning significantly increases the distance, over which carrier-phase ambiguity resolution can be performed. 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 corrections to GPS observations that can be broadcast to the users. The Multi Purpose GPS Processing Software (MPGPS) developed at The Ohio State University uses the multiple reference station approach for wide area and regional differential kinematic GPS positioning. The primary processing algorithm uses the weighted free-net (WFN) approach with the distance-dependent weighting scheme to derive optimal estimates of the user coordinates and realistic accuracy measures. The WFN approach, combined with the single epoch (instantaneous) ambiguity resolution algorithm is presented here as one approach to realtime kinematic (RTK) GPS. Since for baselines exceeding $100 km, the instantaneous ambiguity resolution may not always be possible due to the increasing observation noise and insufficient number of observations to verify the integer selection, an alternative approach, based on a single-(or multiple-) baseline solution, supported by a double-difference (DD) ionospheric delay propagated from the previous epoch is also presented. In this approach, some data accumulation, supported by the network-derived atmospheric corrections, is required at the beginning of the rover data processing to obtain the integer ambiguities; after this initialization period, the processing switches to the instantaneous RTK positioning mode. This paper presents a discussion on the effects of the network geometry, station separation and the data reduction technique on the final quality and reliability of the rover positioning solution. A 24-h data set of August 31, 2003, collected by the Ohio Continuously Operating Reference Station (CORS) network was processed by both techniques under different network geometry and reference station separation. Various solutions, such as (1) single-baseline solution for varying base-rover separation, (2) multi-baseline solution with medium-range base separation (over 100 km), and (3) multi-baseline solution with long-range base separation (up to 377 km), were obtained and compared for accuracy and consistency. The horizontal positioning accuracy achieved in these tests, expressed as the difference between the estimated coordinates and the known rover coordinates, is at the sub-decimeter level for the first approach, and at the centimeter-level for the second method, for baselines over 100 km. GPS Solut (2005) 9: 212-225