Ionospheric total electron content (TEC) measurements, obtained simultaneously at several locations, can be processed using computerized tomography (CT) algorithms to obtain two-dimensional images of ionospheric electron density. Using TEC data, computerized ionospheric tomography (CIT) reconstructs an image of the electron density structures in a vertical slice above the receiving stations. We successfully applied this technique to realistic simulations of ionospheric density variations over 16 ø of latitude and a height range of 50 to 1000 km. A method for approximating the peak height and scale height of the electron density profile will be discussed as well as a reconstruction technique based on the multiplicative algebraic reconstruction technique algorithm and a back projection based initial guess. The quality of reconstructions is considered for two geometries and image resolutions. In particular, the image of a mid-latitude trough with background horizontal density gradient and large-scale irregular structures has been reconstructed from TEC data generated from a model based on an incoherent scatter radar observation. The CT reconstructed image was compared with the original contour map obtained by the incoherent scatter radar. Good agreement has been achieved. The CIT technique has also been applied to a modeled ionosphere to calculate the range rate corrections for a Doppler-tracking radar. INTRODUCTIONRecently, Austen et al. [1988] have demonstrated the feasibility of using a computer tomography (CT) technique for imaging ionospheric electron density. The CT technique involves using one-dimensional information to reconstruct a two-dimensional image. The data consist of measurements of the line integral of electron density in the ionosphere for many different paths. The line integral paths can be considered as many unique rays traversing the plane of the image. The image region can be gridded into small areas or pixels, and within each pixel the parameter will assume some constant value. What is desired is the distribution of the parameter over the image. In the case of the ionosphere the integrated parameter is electron density. The line integral of electron density over some path is the total electron content (TEC). Using a chain of stations receiving the beacon signal from an orbiting satellite, a geometry can be created such that the TEC measurements can be used to reconstruct a vertical slice image, or vertical cross section, of ionospheric electron density in a vertical plane between the satellite and the ground stations.The feasibility of using CT for imaging ionospheric electron density has been shown by Austen et al. [1988] by applying the technique to successfully reconstruct iononspheric structures from synthetic TEC data generated from computer models. In this paper we will discuss the results from a further test of the technique. From a mapping of the mid-latitude ionospheric trough obtained by the Chatanika radar [Weber et al., 1985] the Air Force Geophysics Laboratory (AFGL) group (J. A...

The potential of imaging large ionospheric structures by applying tomographic techniques has only been proposed recently and has already received interest and increasing attention by workers in ionospheric research. In the combined case of an idealized plane geometry and complete and continuous data extending to +_oo the inversion is exact and unique. This is no longer the case if any of the idealized conditions are removed. Specifically, we investigate the limitations arising from limited angles, sampled data of finite receiving aperture and circular geometry. Under these various nonidealized but more realistic conditions, fidelity of the reconstructed image is investigated by computing the response to an impulse source function. The deterioration of the reconstructed image, the sensitivity to the position in the image plane, and the possible existence of ghost images are all numerically investigated and illustrated. Possible methods of improvement are suggested.

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, owing to limited angle measurements or nonoptimal receiver location. Improvements in both reconstruction algorithms and CIT measurement systems are being implemented to overcome these difficulties. New imaging systems being developed employ CIT for large area mapping of the plasma densities in the ionosphere.

The relatively recent development of inexpensive, high-speed computational resources has made computerized tomography a readily accessible tool for remote sensing. A variety of geophysical remote applications have made use of the technique; in particular, tomography has been used to image the electron density in the Earth's ionosphere. As in most cases there are problems unique to the application. A review of the algorithms used to date is presented with a sampling of experimental results.

A campaign was conducted in late September 1991 to obtain experimental measurements for use in ionospheric tomography. Four stations receiving signals from the Navy Navigation Satellite System satellites were set up in a meridional chain in Scandinavia covering some 10° of latitude. Measurements of total electron content were made for six satellite passes over a 2‐day period coinciding with an extended run of the CP3f latitudinal scan common program of the European incoherent scatter radar. A new reconstruction technique involving the use of two‐dimensional basis vectors has been used to convert the total electron content measurements to images of electron density in a height versus latitude plane. Comparisons of the tomographic images and the independent measurements of electron density from the radar show good general agreement. Broadly similar troughs and enhancements are observed by the two techniques, and a latitudinal gradient in the height of the peak density is reproduced in both data sets.