Single-difference Ionosphere Map Generation Based on a Reference CORS Network

摘要

This paper demonstrates a new approach to single-difference (SD) ionospheric delay estimation, based on Global Positioning System (GPS) observations from the State of Ohio Continuously Operating Reference Station (CORS) network. Once SD ionospheric delays are obtained, interpolation methods, such as Kriging, multi-quadric biharmonic functions or linear functions, etc., can be applied to provide ionospheric correction maps to users located within or in the vicinity of the regional network. Ionospheric maps can be generated, in general, in various modes, such as undifferenced, SD or double-difference (DD), mode. However, the undifferenced mode is subject to possible biases due to incorrect ambiguities resulting from the carrier-phase pseudorange smoothing process commonly used in this procedure. The process of interpolation on each level of ionosphere representation (i.e., differencing) has pros and cons due to inherent errors in GPS observations and geometric configuration between the reference network and the user, as well as spatial and temporal variation of the ionospheric features. In this paper, a fast and accurate SD approach is proposed to estimate the SD ionospheric delays that are easy to interpolate and handle by the user. In the SD approach, interchannel bias between C1 (or P1) and P2 pseudorange observables and ambiguities in phase observables are major error sources which affect the quality of the estimated SD ionospheric delay. The use of SD geometric range information is introduced here to remove the SD interchannel bias in pseudorange observables, and a bias removal technique, similar to pseudorange smoothing, is applied to remove the ambiguities in SD phase observables. As a result, SD bias-free ionospheric delay estimates are obtained from the regional GPS network. The, results are compared with the reference "truth," i.e., the DD ionospheric delays obtained from the external source with a few millimeter accuracy (Kashani et al., 2004b). The accuracy of the DD ionospheric corrections derived from the SD model is better than 15 cm in most cases.