Helen Na

Two-dimensional mapping of the plasma density in the upper atmosphere with computerized ionospheric tomography (CIT)

Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine electron densities. The integral of electron density along vertical or oblique paths is obtained with radio transmissions from low-earth-orbiting (LEO) satellite transmitters to a chain of receivers on the earth’s surface. Similar measurements along horizontal paths can be made using transmissions from Global Position System (GPS) navigation satellites to GPS receivers on LEO spacecraft. Also, the intensities of extreme ultraviolet (EUV) emissions can be measured with orbiting spectrometers. These intensities are directly related to the integral of the oxygen ion and electron densities along the instrument line of sight. Two-dimensional maps of the ionospheric plasma are produced by analyzing the combined radio and EUV data using computerized ionospheric tomography (CIT). Difficulties associated with CIT arise from the nonuniqueness of the reconstructions, owi...

Resolution degradation parameters of ionospheric tomography

Tomographie reconstruction of the electron density structure in the ionosphere has generated much interest due to the possibility of high-resolution two-dimensional image reconstruction using total electron content (TEC) data. An algorithm based upon the filtered backprojection algorithm of X ray tomography that provides such two-dimensional reconstructions has been developed; however, the resolving capability is limited by the nature of the ionospheric system. In ionospheric tomography, each piece of data contains information similar to a sample on a projection in X ray tomography. Collectively, however, the information content of ionospheric data sets is significantly different from a set of tomographic projections. These differences are the result of characteristics of the data acquisition system and are directly related to the resolution limits of the reconstructed images. In this paper the data acquisition system is analyzed to determine the sources of this degradation. Two key parameters of nonuniform sample spacing and angular reindexing error are analyzed to determine their effect upon the reconstruction process. A method of evaluating sampling schemes to measure the information content of the resulting projections is presented. Finally, a relationship between the angular reindexing error and the point spread function of the imaging system is derived.

Ionospheric tomography using the residual correction method

Ionospheric tomography systems provide data that can be used to reconstruct images of ionospheric electron density. Since ionospheric tomography systems have fundamentally poor vertical resolution, a priori information on the vertical distribution of ionospheric electron density must be used in the reconstruction algorithm. This can be accomplished using an existing technique called orthogonal decomposition. However, the poor vertical resolution of the imaging system makes the reconstruction problem numerically ill conditioned. This paper presents a new ionospheric reconstruction algorithm called the residual correction method (RCM). The RCM is a fast, efficient, and numerically stable ionospheric tomography algorithm. This paper will present results demonstrating the performance of the RCM algorithm using a realistic example.

Three-dimensional computerized ionospheric tomography using volumetric constraints

A new algorithm for the reconstruction of three-dimensional images of ionospheric electron density using volumetric a priori information is presented in this paper. This method relies upon the total electron content (TEC) data measured by an ionospheric tomography system to derive necessary distribution-related information from a priori ionospheric images for projection domain and image domain volumetric correction processes. It involves extraction of shape information from a priori ionospheric images on an extremely localized basis to simultaneously satisfy measured TEC data and the need for coherent, convergent reconstruction guidance, necessary for the limited angle tomography system. The algorithm uses three-dimensional polar geometry for computerized ionospheric tomography (CIT) which permits direct use of slant TEC data without the interpolations that are necessary for conventional two-dimensional imaging of the ionosphere. Reconstructions based on data recorded in two recent CIT campaigns are also presented.

Resolution and coverage analysis for ionospheric tomography

This paper presents results of investigations of the relationship between optimal image resolution and associated sky-coverage for ionospheric tomography. Several measures of the variation of image information content as a function of resolution are presented for understanding the trade-offs between information content and computational resources. A new quantitative description of imaging geometry that enables sky-coverage analysis for this limited information tomographic system is introduced here. This paper also presents examples of commonly encountered nonideal system geometries, diagnosis of associated image artifacts, and improvements achieved by minor logistical modifications to the imaging system.

A localized space-frequency algorithm for computerized ionospheric tomography

Ionospheric tomography is a rapidly developing technique for imaging ionization distributions. Owing to the system configuration, the resolving capability of these systems is still limited. A number of reconstruction algorithms have been developed which strive to form high resolution images by enhancing the available data with information provided by model ionospheres. This paper presents a new Fourier domain algorithm for ionospheric tomography which uses unique characteristics of ionospheric spectra to enhance the reconstructions. Unlike most model based algorithms, this solution is not limited to a linear combination of model ionospheres and data. This algorithm also uses a localized approach which enables reconstructions of varying resolution in different regions of the ionosphere.

Time-varying reconstruction of the ionosphere. 1. The algorithm

Total electron content data can be used to reconstruct images of ionospheric electron density using computed ionospheric tomography (CIT). All existing CIT algorithms are formulated with the assumption that the ionosphere does not move during data collection. Since existing algorithms are static reconstruction algorithms, the motion of the ionosphere becomes a source of image degradation. This article presents a time‐varying CIT algorithm that reconstructs several time slices of the ionosphere instead of a single static image. Thus, the new algorithm is not adversely affected by the motion of the ionosphere. The new algorithm uses a priori information on the vertical distribution of ionospheric electron density, but no a priori information on ionospheric motion, so the motion is reconstructed solely on the basis of information contained in the data.

Multisource volumetric tomography for over‐the‐horizon radar

This paper presents a new technique for the mapping of ionospheric electron density distributions for determining ambient HF radio propagation characteristics necessary for applications such as over-the-horizon-radar (OTHR). The multisource volumetric inversion technique (MSVIT) combines information from conventional ionospheric probes used for OTHR, such as quasi-vertical incidence and oblique backscatter ionosondes, with total electron content data obtained by computerized ionospheric tomography (CIT), to form three-dimensional maps of ionospheric electron density distribution. MSVIT is an iterative reconstruction technique that draws on the wide-area coverage of VHF-UHF data from a CIT imaging system colocated with an OTHR system to enhance the reliability of sector-wise localized predictions of HF propagation conditions. A detailed analysis of the MSVIT algorithm together with simulations using the international reference ionosphere model of 1990 and experimental data from recent CIT campaigns shows that MSVIT has considerable potential for improving the quality of OTHR operations.

TIME-VARYING RECONSTRUCTION OF THE IONOSPHERE. 2. DATA SOURCE ANALYSIS

A time‐varying computed ionospheric tomography (TVCIT) algorithm for reconstructing moving images of the ionosphere is proposed. Since the TVCIT algorithm provides more information in the reconstructions than is provided in reconstructions using conventional static computed ionospheric tomography, the TVCIT algorithm requires information from additional data sources for accurate reconstruction. There are several data sources that may be available for TVCIT reconstruction, including ionosonde data and total electron content data from additional satellites. This article evaluates the efficacy of various combinations of data sources for TVCIT reconstruction. In addition, a quantitative measure is used to compare the amount of information derived from several different combinations of data sources and to evaluate the effect of varying the number of ground stations.

Limited angle tomography using volumetric constraints

This paper introduces a method for incorporation of volumetric a priori information into a limited angle tomographic image reconstruction process, such as that encountered in computerized ionospheric tomography (CIT). Data-dependent localized volumetric correction in projection and image domains is provided by the iterative inversion algorithm for enhanced reconstruction of two and three dimensional images of ionospheric electron density.