Computerized tomography (CT) techniques can be used to produce a two‐dimensional image of the electron density in the ionosphere. The CT problem requires that the measured data be the line integral through the medium of the unknown parameter; transionospheric satellite beacon total electron content data recorded simultaneously at multiple ground stations fulfill this requirement. In this paper the CT problem is formulated as it applies to ionospheric imaging and limitations of the technique are investigated. Simulations are performed assuming a 1000‐km‐altitude polar‐orbiting satellite and both five and three ground stations; the results demonstrate the feasibility of this technique.
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...
In a program designed to compare measurements of scintillation activity in the equatorial anomaly region, observations were made of satellite beacon signals transmitting at frequencies ranging from 137 MHz to 7 GHz. Recordings were made at Ascension Island in January–February 1981, during a month of very high solar flux and a high occurrence of scintillations. Saturation was noted in the VHF‐UHF range with levels of 8 dB peak to peak at 4 GHz and 3 dB peak to peak at 7 GHz. Statistics of occurrence of various levels for 1.5 and 4 GHz are given in the paper. The hypotheses of vertical or horizontal irregularity sheets or rods within the patches of irregularities were examined with data from the Global Positioning System satellites. Vertical sheets were eliminated as a possibility. A comparison of scintillations with 6300‐Å airglow images, which map regions of depleted electron density over the entire sky, showed that in the anomaly region, maximum scintillation activity occurs within the patch and not at the walls of the patch.
Equatorial ionospheric scintillation data at 1541.5 Mhz (L‐band) and 3945.5 Mhz (C‐band) showing time shifts of up to one second between similar fades of the two signals are presented. Simple model computations show that systematic refractive effects due to equatorial plasma bubbles in the particular propagation geometry may explain the observed data. Implications on equatorial ionospheric irregularities and scintillation theory are discussed.
Abstract. Computerized ionospheric tomography (CIT) is an imaging technique thatproduces a two-dimensional image of the electron density in the ionosphere. This technique uses total electron content (TEC) measurements from multiple ground stations as input data to computerized tomography (CT) algorithms. Unfortunately, the TEC data suffer from ambiguities that are introduced during the measurement process. Ionosphere researchers model these ambiguities as an unlaiown constant which is added to the true TEC data. This is known as the constant of integration problem. Traditional methods for resolving this problem, such as the single-and two-station methods, are based on ionospheric models and may perform poorly when the true ionospheric structure differs greatly from the model. This paper
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