COMBION

Aim of the project is to use different space geodetic techniques for ionosphere monitoring and to develop a sound combination procedure for computation of integrated global models. The combined models of the ionosphere should make best use of the advantages of each particular space geodetic technique, having a more homogeneous global coverage, and being more accurate and reliable than the results of each single technique. In a first step, the models will be generated in terms of Vertical Total Electron Content (VTEC) as a function of latitude, longitude, and time in the form of Global Ionosphere Maps (GIM). Further, the single shell GIM will be expanded by additional layers in order to assess the vertical variations in the ionosphere. In the final stage of the project the combined GIM will be used as basis for developing an integrated 4D global model, representing the ionosphere in latitude, longitude, time, and height. The combined global ionosphere maps will be useful for correcting single- frequency measurements done by many observation techniques using radio frequencies and for validation and improvement of ionosphere parameters derived by other individual techniques as well as of theoretical models. They can also be utilised as information source for the technique-specific instrumental biases, such as the GNSS DCB, which will be estimated as a by-product. Generally, the combined models will contribute to various studies of the physics of the upper Earth atmosphere and the solar terrestrial environment. Space geodetic techniques to be investigated within the research project are the Global Navigation Satellite System (GNSS), satellite altimetry missions, the Very Long Baseline Interferometry (VLBI), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and Low Earth Orbit Satellites (LEOs). Following main steps will be carried out within COMBION: Investigation of the ionosphere parameters derived by each individual technique as well as consideration and modelling of technique-specific biases. Definition of the 3D model and computation of GIM from GNSS data with temporal resolution of two hours. Development of a procedure for rigorously combining the GNSS GIM with data from each other techniques on normal equation level, determination of the optimal relative weighting of the individual data sets within the combined model, and thorough error analysis of the results. Extension of the combined 3D model through additional ionosphere layers. Investigation of different approaches for 4D ionosphere modelling and of data allowing vertical profiling and subsequent development of a 4D combined model. Final validation of the integrated ionosphere models by comparison with measurements from external techniques and data from theoretical models.

The rapid development of new-technological systems for navigation, telecommunication, and space missions which transmit signals through the Earth`s upper atmosphere - the ionosphere - makes the prediction of the meteorological conditions of space around the Earth more essential. In the last decades space geodetic techniques have turned into a capable tool for measuring ionospheric parameters such as the electron density or the Total Electron Content (TEC). For space geodetic techniques, operating in microwave band, ionosphere is a dispersive medium; thus signals traveling through this medium are in the first approximation affected proportional to inverse of the square of their frequencies. This effect allows gaining information about the parameters of the ionosphere. These parameters can then be expressed by means of spherical harmonic base functions in order to provide a Global Ionosphere Map (GIM). The classical input data for development of GIMs are obtained from dual-frequency observations carried out at Global Navigation Satellite Systems (GNSS) ground stations. However, GNSS stations are in-homogeneously distributed around the world, with large gaps particularly over the oceans; this fact reduces the precision of the GIM over these areas. On the other hand, dual-frequency satellite altimetry missions such as Topex/Poseidon (T/P) or Jason-1 provide information about the ionosphere precisely above the oceans; and furthermore Low Earth Orbiting (LEO) satellites, such as Formosat-3/COSMIC (F/C) provide well-distributed information of ionosphere globally. This project investigates global modeling of TEC through combining GNSS observations with satellite altimetry measurements and also with global TEC data derived from the occultation measurements of the F/C mission. The investigations mainly focused on the duration between the last solar minimum (i.e. 2006) and a few years after that (i.e. till 2009). Within several experimental days for which the combination process was accomplished, the combined GIMs of vertical TEC (VTEC) showed a maximum difference of about 1.5 TEC units (TECU) with respect to the GNSS-only GIMs. The root mean square error (RMS) maps of combined solution showed a reduction in the whole day. This decrease of RMS could reach up to 0.5 TECU in areas where no or only few GNSS observations were available, but high numbers of F/C measurement were carried out. This proved the assumption that the combined GIMs provide a more homogeneous global coverage and higher reliability than results of each single method. So it can be inferred that combining altimetry and also F/C measurements with GNSS observations for global modeling of the ionosphere will significantly improve the accuracy and reliability of the GIMs, especially when a high number of occultation measurements is available. Within this project, all comparisons and validations done, provide vital information regarding combination and integration of various observation techniques in the Global Geodetic Observing System of the International Association of Geodesy (IAG).