Review of the Current Status of Four-Dimensional Ionospheric Imaging

Recent developments in tomographic imaging allow the use of GPS satellite data to image the Earth’s ionosphere. Ground-based GPS receivers monitor the Earth’s ionosphere continuously and a comprehensive database of ionospheric measurements suitable for tomographic processing now exists. The tomographic inversion of these GPS data in a three-dimensional time-dependent inversion algorithm can reveal the spatial and temporal distribution of ionospheric electron density. This new technique is unique for studying ionospheric physics because it gives a time-continuous near-global view of the ionosphere. The tomographic algorithms have been under continuous development for several years and are now yielding new geophysical results. 1.0 INTRODUCTION In 1986 a new technique for ionospheric study was proposed, namely radio tomography. Section 2 includes a review of ionospheric tomography, while Section 3 describes current developments in ionospheric imaging and practical applications of the work. Initial research established the validity of the technique by comparisons between co-located tomographic images and incoherent scatter radar observations. Tomographic images have now revealed a number of physical processes in the ionosphere, some of which are signatures of events originating in the magnetosphere. Recent work has shown the potential to extend the algorithms to make threeand four-dimensional images (movies) using multiple satellites and receivers. Radio tomography and inversion imaging offer powerful new methods for the routine monitoring of the near-Earth space-plasma. In addition to scientific investigations, the research has important applications in improving radio communication and navigation systems. Section 4 outlines current limitations and future directions for this particular research topic. These future directions fall into two areas. Firstly, a number of new satellites are being launched that will provide increased global coverage and opportunities for improved, higher resolution imaging. Secondly, the prospect of incorporating mathematical inversion theory into physical models of the solar-terrestrial regions to better understand the underlying physics that is producing the images. 2.0 HISTORY OF IONOSPHERIC TOMOGRAPHY AND IMAGING The tomographic technique Radon (1917),, so successful in the medical field, has been applied relatively recently to producing images of the electron concentration in the ionosphere. This has been done using a polar-orbiting satellite as the transmitter platform, together with arrays of receivers on the ground. Dual frequency trans-ionospheric signals are analysed and inverted using an algorithm to produce two-dimensional images of electron concentration. This application of tomography to the ionosphere falls into the category of Bust, G.S.; Mitchell, C.N. (2006) Review of the Current Status of Four-Dimensional Ionospheric Imaging. In Characterising the Ionosphere (pp. 31-1 – 31-18). Meeting Proceedings RTO-MP-IST-056, Paper 31. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 01 JUN 2006 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Review of the Current Status of Four-Dimensional Ionospheric Imaging 5a. CONTRACT NUMBER