Abstract
With the emergence of BeiDou and Galileo as well as the modernization of GPS and GLONASS, more available satellites and signals enhance the capability of Global Navigation Satellite Systems (GNSS) to monitor the ionosphere. However, currently the International GNSS Service (IGS) Ionosphere Associate Analysis Centers (IAACs) just use GPS and GLONASS dual-frequency observations in ionosphere estimation. To better determine the global ionosphere, we used multi-frequency, multi-constellation GNSS observations and a priori International Reference Ionosphere (IRI) to model the ionosphere. The newly estimated ionosphere was represented by a spherical harmonic expansion function with degree and order of 15 in a solar-geomagnetic frame. By collecting more than 300 stations with a global distribution, we processed and analysed two years of data. The estimated ionospheric results were compared with those of IAACs, and the averaged Root Mean Squares (RMS) of Total Electron Content (TEC) differences for different solutions did not exceed 3 TEC Unit (TECU). Through validation by satellite altimetry, it was suggested that the newly established ionosphere had a higher precision than the IGS products. Moreover, compared with IGS ionospheric products, the newly established ionosphere showed a more accurate response to the ionosphere disturbances during the geomagnetic storms.
Highlights
The ionosphere constitutes the upper part of the atmosphere, extending from approximately 60 to 1500 km above the Earth’s surface, enriched with free electrons and ions
The International Global Navigation Satellite Systems (GNSS) Service (IGS) Ionosphere Working Group was initialized in 1998 to exploit the global ionosphere estimation based on the IGS network of stations, and routinely provides IGS Global Ionosphere Maps (GIMs) based on a weighted combination of ionosphere maps regularly produced by IGS Ionosphere Associate Analysis Centers (IAACs) [4], such as the Center for Orbit Determination in Europe (CODE), European Space Agency (ESA), Jet Propulsion Laboratory (JPL) and Polytechnic University of Catalonia (UPC)
The GIMs are determined in a solar-geomagnetic reference frame using spherical harmonic (SH) expansion in combination with a daily Differential Code Bias (DCB) estimation [5,6,7,8]
Summary
The ionosphere constitutes the upper part of the atmosphere, extending from approximately 60 to 1500 km above the Earth’s surface, enriched with free electrons and ions. With the modernization of GPS and GLONASS as well as the rapid deployments of BeiDou System (BDS) and Galileo, more and more satellites are available to transmit multi-frequency signals, which brings both opportunities and challenges for ionosphere monitoring and modelling. JPL and UPC just use GPS data to estimate the GIMs. The Total Electron Content (TEC) is modelled in a solar-geomagnetic reference frame using bi-cubic splines on a spherical grid in JPL. The advantage of combined ionosphere determination with dual-frequency measurements from GPS, GLONASS, BDS and Galileo has been assessed through a 60-day performance analysis by Ren et al [13]. With a priori IRI model, the ionospheric improvements brought by multi-frequency and multi-constellation GNSS were compared with the solutions from other IAACs and independently validated by satellite altimetry [4,17].
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