A New Method to Model the Ionosphere Across Local Area Networks

In addition to an overall quasi-periodic 24 hour cycle, the ionosphere features many irregular disturbances. The detailed structure and dynamical evolution of the ionosphere continues to be one of the largest unpredictable factors in GNSS measurements. Across local area networks, however, the total electron content on the line of sight between the stations of a network and a given satellite is highly correlated between the stations due to the proximity of the signal rays as they propagate through the ionosphere. In this paper we consider local area networks of 4 to 50 stations with baselines of the order of 10 to 100 kilometers. Across these networks, we characterize the ionosphere around the piercing points in terms of the average total electron content and spatial corrections to first and higher order. To estimate the state of the ionosphere and integer ambiguities of the individual stations we process the phase measurements of the stations by means of a Kalman filter. Together with the code information and a tropospheric model for a full network solution (such as GPSNet, see Vollath et al. 2000, Vollath 2004) for reference station networks for RTK positioning, our novel approach leads to improved fixing results even during geomagnetic disturbances and for elevation angles as low as 10°. The validity of this technique is demonstrated for various networks around the globe. We find that the model correctly reproduces the expected ionospheric characteristics for all regions. While providing improved ambiguity fixing performance and reliability, the model also gives a physical description of the ionosphere throughout the day.