Ionosphere-gradient based filtering approach for precise relative navigation in LEO

Formation flying missions require the knowledge of the relative positions of the satellites for formation, maintenance and scientific purposes. In Low Earth Orbit, this task is typically performed by GPS-based navigation filters. However, the final accuracy, especially over long baselines, is strongly affected by the capability of correctly estimating the ionospheric delays. In this paper the performance of a real-time relative positioning filter are improved with the introduction of an ionospheric model capable of representing horizontal ionospheric gradients, referred to as Linear Thin Shell model. This model can be used as an alternative to the most common isotropic ionospheric model. Filter performance is investigated in high and low ionospheric intensity conditions using real flight data. Results show that, in intense ionospheric conditions, positioning accuracy is improved with respect to the case of using an isotropic model.