Enhanced TEC Maps Based on Different SpaceGeodeti

Authors: Jiantong Zhang Michael Schmidt Denise Dettmering Liqiu Meng Yueqin Zhu Yanbin Wang

Publish Date: 2013

Volume: , Issue: , Pages: 21-34

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Abstract

The ionosphere is defined as part of the upper earth’s atmosphere where the density of free electrons and ions is high enough to influence the propagation of electromagnetic radio frequency waves The ionisation process is primarily depending on the Sun’s activity and varies strongly with time as well as with geographical location The knowledge of the electron density is the critical point for many applications in positioning and navigation During the last decade dualfrequency Global Navigation Satellite Systems GNSS in particular the Global Positioning System GPS have become a promising tool for monitoring the Total Electron Content TEC ie the integral of the electron density along the raypath between the transmitting satellite and the receiver Hence geometryfree GNSS measurements provide information on the electron density which depends on spatial position and time ie fourdimensional 4D At present the International GNSS Service IGS provides timedependent vertical TEC VTEC maps based on more than 100 permanent ground stations however these stations are mainly located on the continents and provide less accurate results over the oceans New spacebased observation techniques especially various LowEarthOrbiting LEO satellite missions such as FORMOSAT3/COSMIC and CHAMP as well as dualfrequency radar altimetry missions such as Jason1 Jason2 and Envisat can also contribute ionospheric evaluation on a global scale The former get the TEC values from GPSLEO occultation observations whereas the latter provide VTEC observations from the onboard doublefrequency radar altimeter In order to enhance the IGS VTEC maps ie balancing the insufficient GNSS coverage over the sea efficient and inexpensive occultation observations and altimetry measurements can be collected and utilized In this way the IGS VTEC products can benefit from additional data sources In this paper we combine both occultation and altimetry measurements to enhance the IGS VTEC maps Our model consists of a given reference part background model computed from the IGS VTEC products and also of an unknown correction term In contrary to the traditional spherical harmonic approach we use a global multidimensional Bspline approach for modelling the unknown correction term We rely on normalized endpointinterpolating Bsplines for modelling the latitude and the timedependency and trigonometric Bsplines for the dependency on the longitude Several constraints eg for the poles for meridian have to be considered carefully Since Bsplines are localizing functions ie they are characterized by a compact support data gaps can be handled appropriately The unknown series coefficients of our multidimensional Bspline expansion are calculable from the LEO and the altimetry measurements applying parameter estimation The relative weighting between the different data sources the prior information and the constraints will be performed by Variance Component Estimation VCE We compare the enhanced VTEC maps between the combined approach which is based on VCE and a second approach using FORMOSAT3/COSMIC and Jason1 data only within selected periods It will be shown that an improvement from the additional data sources is visible in the areas with good data coverage In regions with limited amount of observations the background model values from IGS will be conserved

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