On developing space weather services for the end users of GNSS

Abstract

Electromagnetic waves transmitted from navigation satellites to users at the Earth or to other satellites interact with the anisotropic plasma of the Earth’s ionosphere. Thus, depending on their frequency and the ionospheric propagation conditions, the signals suffer a delayed travel time, ray path bending, polarization changes and rapid signal strength fluctuations. At the L-band frequencies used commonly by GPS, GLONASS and the future GALILEO the ionosphere impact leads to ranging errors up to 100 meters. Due to the dispersive nature of the ionosphere, first order ionospheric propagation delays may be corrected quite well by dual frequency measurements separating the Total Electron Content (TEC) along the ray path through the plasma. More serious problems arise due to plasma density irregularities from small to large scales causing steep TEC gradients, rapid phase fluctuations and radio scintillations which may even result in loss of lock. Since the ionosphere is strongly controlled by the solar radiation, solar wind, coupling processes with the thermosphere and magnetosphere, signal degradation effects in GNSS applications are often related to severe space weather phenomena. The talk addresses these problems by discussing samples how GNSS users may be impacted by space weather effects. Consequently, specific space weather information can help GNSS users to improve accuracy and reliability of their positioning and navigation solutions or simply to get faster solutions by reducing the fixing time in carrier phase based solutions. Corresponding user needs are discussed in different fields of application which may focus either on high precision positioning or on extreme reliable solutions fulfilling safety of life standards. Future oriented space weather services have to meet these permanently growing challenges according to the state of art. Principally, this requires qualified space weather information, in particular based on a deep understanding of complex coupling processes in the space environment, on high resolution monitoring and modelling/reconstruction of the ionospheric electron density distribution. Fortunately, well established ground-based and innovative space-based GNSS measurement techniques itself offer the unique chance for a permanent high resolution monitoring of the plasma environment of the Earth thus providing the main data base for establishing space weather services for GNSS users. This data base may be completed by operational ground based measurements. To warn the GNSS users for space weather events having the potential to degrade GNSS signals, more comprehensive space weather information are needed which may be provided by the European Space Weather Network (SWENET).