Development of experimental ionospheric tomography

This article describes the development of experimental work on ionospheric tomography carried out in Europe. It starts with early two-station observations and discusses the recent complex coordinated campaigns that have involved ionosondes to determine the bottomside profile and incoherent scatter radar to verify the image.©1994 John Wiley & Sons Inc     

Investigations of the ionosphere by satellite radiotomography

An overview of tomographic approach to remote sensing of ionosphere by radio waves is presented. Various methods, including diffraction, statistical, and ray radiotomography are discussed. The experimental procedures of satellite radiotomography are described. Results from satellite experiments in 1984–1993 are shown demonstrating the great possibilities of the radiotomographic method in ionospheric research.©1994 John Wiley & Sons Inc     

Three-dimensional tomographic reconstruction of mesospheric airglow structures using two-station ground-based image measurements

A new methodology is presented to create two-dimensional (2D) and three-dimensional (3D) tomographic reconstructions of mesospheric airglow layer structure using two-station all-sky image measurements. A fanning technique is presented that produces a series of cross-sectional 2D reconstructions, which are combined to create a 3D mapping of the airglow volume. The imaging configuration is discussed and the inherent challenges of using limited-angle data in tomographic reconstructions have been analyzed using artificially generated imaging objects. An iterative reconstruction method, the partially constrained algebraic reconstruction technique (PCART), was used in conjunction with a priori information of the airglow emission profile to constrain the height of the imaged region, thereby reducing the indeterminacy of the inverse problem. Synthetic projection data were acquired from the imaging objects and the forward problem to validate the tomographic method and to demonstrate the ability of this technique to accurately reconstruct information using only two ground-based sites. Reconstructions of the OH airglow layer were created using data recorded by all-sky CCD cameras located at Bear Lake Observatory, Utah, and at Star Valley, Wyoming, with an optimal site separation of ~100 km. The ability to extend powerful 2D and 3D tomographic methods to two-station ground-based measurements offers obvious practical advantages for new measurement programs. The importance and applications of mesospheric tomographic reconstructions in airglow studies, as well as the need for future measurements and continued development of techniques of this type, are discussed.     

Synthetic Incoherence via Scanned Gaussian Beams

Tomography, in most formulations, requires an incoherent signal. For a conventional transmission electron microscope, the coherence of the beam often results in diffraction effects that limit the ability to perform a 3D reconstruction from a tilt series with conventional tomographic reconstruction algorithms. In this paper, an analytic solution is given to a scanned Gaussian beam, which reduces the beam coherence to be effectively incoherent for medium-size (of order 100 voxels thick) tomographic applications. The scanned Gaussian beam leads to more incoherence than hollow-cone illumination.     

Truncation artifacts in tomographic reconstructions from projections

The artifacts in tomographic reconstructions from truncated sets of projections are analyzed. The shift-variant impulse response of the tomographic system for parallel-beam geometry is derived. A number of propositions are made describing the observed artifacts. A graphical scheme for the prediction of the location and shape of the truncation artifacts is presented and applied to reconstructions from simulated projections. The artifact analysis is applied to images obtained with the commonly used convolution backprojection reconstruction algorithms, and it is extended to reconstructions from fan-beam projections. The analysis is performed for the continuous imaging domain so as to separate the clipping artifacts clearly from those attributed to the digital implementation of the reconstruction algorithms.     

A novel iterative optimizing quantization technique and its application to X-ray tomographic microscopy for three-dimensional reconstruction from a limited number of views

The iterative optimizing quantization technique (IOQT) is a novel method in reconstructing three-dimensional (3D) images from a limited number of 2D projections. IOQT can reduce the artifacts and image distortion due to a limited number of projections and limited range of viewing angles. Equivalently, by reducing the number of projections required for reconstruction, the use of IOQT can reduce the dose delivered to the specimen, simplify the complexity of an experimental setup, and consequently support the development of techniques to nondestructively image microstructures of materials. In this article, we will demonstrate the capability of IOQT to reconstruct an accurate 3D image of an object from a limited number of views, using a computer simulation and an actual 3D test pattern experiment with submicrometer features. In addition, we will introduce a promising application of IOQT to X-ray tomographic microscopy to study microbiological specimens by presenting the 3D reconstructions of the two different-conditioned human sperm cells from six projections. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 204–213, 1997     

Estimate of the incoherent-scattering contribution to lidar backscatter from clouds

Lidar backscatter from clouds in the Delft University of Technology experiment is complicated by the fact that the transmitter has a narrow beam width, whereas the receiver has a much wider one. The issue here is whether reception of light scattered incoherently by cloud particles can contribute appreciably to the received power. The incoherent contribution can come from within as well as from outside the transmitter beam but in any case is due to at least two scattering processes in the cloud that are not included in the coherent forward scatter that leads to the usual exponentially attenuated contribution from single-particle backscatter. It is conceivable that a sizable fraction of the total received power within the receiver beam width is due to such incoherent-scattering processes. The ratio of this contribution to the direct (but attenuated) reflection from a single particle is estimated here by means of a distorted-Born approximation to the wave equation (with an incident cw monochromatic wave) and by comparison of the magnitude of the doubly scattered to that of the singly scattered flux. The same expressions are also obtained from a radiative-transfer formalism. The ratio underestimates incoherent multiple scattering when it is not small. Corrections that are due to changes in polarization are noted.     

Data from orbiting navigation satellites for tomographic reconstruction

So far, orbiting navigation satellites are the only source for primary data in ionospheric tomography. Phase difference measurements give the input for tomographic reconstruction. Except for a constant, the initial phase difference value, the data can be considered to be the line integral of electron density along the straight line from the satellite to a ground based receiver (“slant electron content”). In ionospheric physics a projection onto the vertical is used: (vertical) electron content (TEC). Many investigations have been based on this quantity alone. This work discusses some of the propeties of the measured data (instrumental and “ionospheric” limitations in section II), and the role of TEC evaluation assumptions (“mean ionospheric height” and initial phase constant, in section III). Some of the problems inherent in ionospheric tomography using orbiting navigation satellites are discussed by means of model calculations (section IV). The models are mathematically defined two-dimensional electron density distributions that are used to calculate the latitude dependence of both slant and vertical TEC. Among others, the model calculations are useful in showing that strongly different electron density distributions can lead to similar electron contents. Tomographic reconstruction would probably not be able to distinguish between the different distributions, which leads to the recommendation to incorporate additional data in the reconstruction process. It is well known, and reflected in the model electron contents, that without additional data tomographic reconstruction is not accurate in height determination. Layer height variations with latitude, however, can have a strong influence on slant and vertical electron content.©1994 John Wiley & Sons Inc     

Morphology,Mineralogy and Mixing of Individual Atmospheric Particles Over Kanpur (IGP): Relevance of Homogeneous Equivalent Sphere Approximation in Radiative Models

Estimation of the direct radiative forcing (DRF) by atmospheric particles is uncertain to a large extent owing to uncertainties in their morphology (shape and size), mixing states, and chemical composition. A region-specific database of the aforementioned physico-chemical properties (at individual particle level) is necessary to improve numerically-estimated optical and radiative properties. Till date, there is no detailed observation of the above mentioned properties over Kanpur in the Indo-Gangetic Plain (IGP). To fill this gap, an experiment was carried out at Kanpur (IITK; 26.52°N, 80.23°E, 142 m msl), India from April to July, 2011. Particle types broadly classified as (a) Cu-rich particles mixed with carbon and sulphur (b) dust and clays mixed with carbonaceous species (c) Fe-rich particles mixed with carbon and sulfur and (d) calcite (CaCO₃) particles aged with nitrate, were observed. The frequency distributions of aspect ratio (AR; indicator of extent of particle non-sphericity) of total 708 particles from April to June reveal that particles with aspect ratio range >1.2 to ≤1.4 were abundant throughout the experiment except during June when it was found to shift to high AR range, >1.4 to ≤1.6 (followed with another peak of AR i.e. >2 to ≤2.4) due to dust storm conditions enhancing the occurrence of more non-spherical particles over the sampling site. The spherical particles (and close to spherical shape; AR range, 1.0 to ≤1.2) were found to be <20% throughout the experiment with a minimum (11.5%) during June. Consideration of Homogeneous Equivalent Sphere Approximation (HESA) in the optical/radiative model over the study region is found to be irrelevant during the campaign.     

General approach for representing and propagating partially coherent terahertz fields with application to Gabor basis sets

We discuss a general theoretical framework for representing and propagating fully coherent, fully incoherent, and the intermediate regime of partially coherent submillimeter-wave fields by means of general sampled basis functions, which may have any degree of completeness. Partially coherent fields arise when finite-throughput systems induce coherence on incoherent fields. This powerful extension to traditional modal analysis methods by using undercomplete Gaussian-Hermite modes can be employed to analyze and optimize such Gaussian quasi-optical techniques. We focus on one particular basis set, the Gabor basis, which consists of overlapping translated and modulated Gaussian beams. We present high-accuracy numerical results from field reconstructions and propagations. In particular, we perform one-dimensional analyses illustrating the Van Cittert-Zernike theorem and then extend our simulations to two dimensions, including simple models of horn and bolometer arrays. Our methods and results are of practical importance as a method for analyzing terahertz fields, which are often partially coherent and diffraction limited so that ray tracing is inaccurate and physical optics computationally prohibitive.     

Test of exercise experiments proposed for the MIR '92 mission

During exercise, heart rate and blood pressure drives can be elicited by receptors situated in the interstitial space of the muscle. It was recently shown that these receptors are sensitive to the local state of hydration: Increased reflex responses during dehydration were shown in rat preparates as well as in humans during dynamic and static exercise. Weightlessness could affect these receptor mechanisms through the redistribution of body fluids and through secondary changes in the interstitial structure. To investigate such effects we suggested to determine heart rate and blood pressure responses to light isometric calf exercise at different calf volumes during the MIR '92 mission (experiment ISX). The First North Sea Parabolic Flight campaign provided an opportunity to test the setup and some operational aspects of this experiment. The experience of this campaign led to some modifications of the original setup.     

Tomographic reconstruction of multiple-layer specimens with the scanning tomographic acoustic microscope

In this article we report the recent results of tomographic image reconstructions of multiple-layer specimens at 100 MHz with the Scanning Tomographic Acoustic Microscope (STAM) system. The experiment utilizes 12 uniformly spaced projections and the results show significant improvement over the holographic images.     

Resolution analysis of tomographic reconstruction of electron density profiles in the Ionosphere

Traditionally, knowledge of the ionospheric electron density is obtained using Faraday rotation or differential Doppler techniques which measure total electron content in columns of the ionosphere. Conventional data processing can only image the electron density in the direction perpendicular to these columns, thereby forming one-dimensional images. Because this data is proportional to line integrals through the region of interest, tomographic techniques may be used to reconstruct two-dimensional electron density images. In this paper, the resolution limit of the image reconstruction process is analyzed in terms of limited-angle tomography.     

Modelling of low-latitude ionosphere using modified planar fit method for GAGAN

Several countries have been developing satellite-based augmentation systems (SBAS) for improving positional accuracy of global positioning system (GPS). India is also developing one such system popularly known as GPS aided geo augmented navigation (GAGAN) system. Modelling of ionospheric effects is one of the major challenges in developing precise and reliable GAGAN. The high values of total electron content (TEC), the large diurnal and seasonal variability and intense irregularities present in the low-latitude ionosphere, lead to unacceptable positional errors in GAGAN service region. Todd Walter et al. of Stanford University, USA have made significant contribution in the area of SBAS ionospheric grid modelling by developing the popularly known planar fit model. It is reported by Walter et al. (2000) that a constant decorrelation value of 35 cm was proposed over the wide area augmentation system (WAAS) service region. To provide accurate estimation of ionospheric delays at user ionospheric pierce points (IPP) during a storm or intense irregularities, an irregularity detector and a decorrelation adaptor are incorporated in the modelling. As planar fit model is not adequate to model intense irregularities of Indian ionosphere, a modified planar fit model is applied for GAGAN data on similar lines as was done for WAAS and the results are encouraging.     

A status report on applying discrete inverse theory to ionospheric tomography

Discrete inverse theory (DIT) provides an orderly framework in which to combine measurements of total electron content (TEC) with a priori information to image the ionosphere tomographically. We have developed a DIT-based tomographic processor for use with relative TEC data. The processor's a priori information comprises the global mean of over 17,000 profiles generated from an ionospheric model, for use as a “generic background”; empirical orthogonal functions (EOFs) spanning the same model profiles, for use as vertical basis functions; and a red power-law horizontal spectrum. Relative TEC data are used to evaluate coefficients multiplying the EOFs and harmonics, thus quantifying a perturbation electron-density field. The perturbation field, which need not be small, is added to the a priori background to produce the image. We present here several images produced by employing the processor with simulated TEC data based on in situ ionospheric measurements and incoherent-scatter radar observations.©1994 John Wiley & Sons Inc     

A block iterative algorithm for tomographic reconstruction of ionospheric electron density

Ionospheric tomography is a technique whereby a vertical cross section through ionospheric electron density can be imaged. The vertical resolution of ionospheric tomography systems is inherently poor, but can be improved by using a priori information in the tomographic reconstruction algorithm. Care must be exercised in using a priori information, since if too much of it is used, the reconstruction algorithm may discard some of the information contained in the tomographic data in favor of satisfying some of the a priori assumptions. Orthogonal decomposition (OD) is an existing technique that uses a priori information to constrain the reconstruction to lie in a space of reasonable images without weighting the reconstruction toward any particular a priori image. In this way a priori information can be used in a manner that does not overwhelm information contained in the data. Gauss-Seidel (GS) is an iterative algorithm that is used to calculate solutions for large systems of linear equations. In this article, a block version of the GS algorithm will be used to calculate the solution of the least-squares problem that is created using OD. The complete algorithm presented here will be called the residual correction method (RCM), since it involves calculation of successively better approximations based on the residual error. RCM is a fast and numerically stable algorithm that extracts as much information from the data as possible. A numerical example demonstrating the properties of RCM will also be presented. © 1996 John Wiley & Sons, Inc.     

Two different small size testing technique for the creep residual life evaluation of an equiaxed Ni-based superalloy for GT blade and vane application

Two miniaturised creep testing techniques have been applied to a polycrystalline Ni-based superalloy employed for gas turbine blades and vanes. The test campaign was conducted on as-delivered material to assess both testing techniques and evaluate their effectiveness in determining creep residual life and creep residual properties of operated material. The first technique involved small size uniaxial creep testing, conducted on samples with a diameter lower than 3 mm. The diameter effect was deeply studied and a calibration was done to correlate results from standard creep test pieces with those from small diameter samples. The second technique involved the small punch creep test, and the assessment on virgin material enabled determination of the correlation factor, k_sp, to compare small punch test results with uniaxial creep tests. All the collected results were related to the coarse grain macrostructure that characterises this class of alloys and that increases the scatter of the collected experimental results.     

Complete wavefront reconstruction using sequential intensity measurements of a volume speckle field

The recording of the volume speckle field from an object at different planes combined with the wave propagation equation allows the reconstruction of the wavefront phase and amplitude without requiring a reference wave. The main advantage of this single-beam multiple-intensity reconstruction (SBMIR) technique is the simple experimental setup because no reference wave is required as in the case of holography. The phase retrieval technique is applied to the investigation of diffusely transmitting and reflecting objects. The effects of different parameters on the quality of reconstructions are investigated by simulation and experiment. Significant enhancements of the reconstructions are observed when the number of intensity measurements is 15 or more and the sequential measurement distance is 0.5 mm or larger. Performing two iterations during the reconstruction process using the calculated phase also leads to better reconstruction. The results from computer simulations confirm the experiments. Analysis of transverse and longitudinal intensity distributions of a volume speckle field for the SBMIR technique is presented. Enhancing the resolution method by shifting the camera a distance of a half-pixel in the lateral direction improves the sampling of speckle patterns and leads to better quality reconstructions. This allows the possibility of recording wave fields from larger test objects.     

Magnetospheric-ionospheric changes caused by flights of space vehicles

The process of formation and disappearance of an ionospheric “hole” in the F2 layer caused by the ejection of water molecules during the flight of a space vehicle has been considered using the method of computational experiment based on the mathematical ionospheric-magnetospheric model in a coordinate system fixed on the magnetic force line of the Earth. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 6, pp. 11–15, November–December, 2006.