Nadir-Dependent GNSS Code Biases and Their Effect on 2D and 3D Ionosphere Modeling

Martin Håkansson

doi:10.3390/rs12060995

Abstract

Recent publications have shown that group delay variations are present in the code observables of the BeiDou system, as well as to a lesser degree in the code observables of the global positioning system (GPS). These variations could potentially affect precise point positioning, integer ambiguity resolution by the Hatch–Melbourne–Wübbena linear combination, and total electron content estimation for ionosphere modeling from global navigation satellite system (GNSS) observations. The latter is an important characteristic of the ionosphere and a prerequisite in some applications of precise positioning. By analyzing the residuals from total electron content estimation, the existence of group delay variations was confirmed by a method independent of the methods previously used. It also provides knowledge of the effects of group delay variations on ionosphere modeling. These biases were confirmed both for two-dimensional ionosphere modeling by the thin shell model, as well as for three-dimensional ionosphere modeling using tomographic inversion. BeiDou group delay variations were prominent and consistent in the residuals for both the two-dimensional and three-dimensional case of ionosphere modeling, while GPS group delay variations were smaller and could not be confirmed due to the accuracy limitations of the ionospheric models. Group delay variations were, to a larger extent, absorbed by the ionospheric model when three-dimensional ionospheric tomography was performed in comparison with two-dimensional modeling.

Highlights

Signals in the frequency bands employed by global navigation satellite systems (GNSSs) are affected by the ions and free electrons in the part of the atmosphere referred to as the ionosphere [1]
We investigated how nadir-dependent group delay variations (GDV) affect ionosphere modeling by analyzing the model residuals that they give rise to
Slant total electron content (STEC) were estimated from the geometry-free linear combination according to Equations (2) and (3) from the GNSS signal combinations C1C-C2W, C6X-C1X, and C1X-C5X for global positioning system (GPS), BeiDou, and Galileo, respectively with signals denoted in accordance with the receiver independent exchange format RINEX 3 [49]

Summary

Introduction

Signals in the frequency bands employed by global navigation satellite systems (GNSSs) are affected by the ions and free electrons in the part of the atmosphere referred to as the ionosphere [1]. As the ionosphere is dispersive, GNSSs have been designed to employ multiple carrier frequencies. This allows almost complete elimination of the effect from GNSS code and phase observations. Availability of observations associated with multiple carrier frequencies allow for determination of the number of free electrons and ions in the ionosphere by forming the geometry-free linear combination [1]. By utilization of the geometry-free linear combination of GNSS observations, the total electron content (TEC) along the ray paths can be estimated. Doing this for several satellites and several receivers allows for estimation of the spatial distribution of TEC. A more complete three-dimensional (3D) model might be derived by computerized ionospheric tomography (CIT) [2,3,4], techniques that were inspired by and have emerged from computerized tomography (CT) for medical applications [5]

Methods

Results

Conclusion