Near-optimal inversion of incoherent scatter radar measurements: coding schemes, processing techniques, and experiments

Incoherent scatter radar;Inversion;Parameter estimation;Deconvolution methods;Regularization;Amplitude modulation;Model order selection;subset selection

Nikoukar, Romina

关键词: Incoherent scatter radar;    Inversion;    Parameter estimation;    Deconvolution methods;    Regularization;    Amplitude modulation;    Model order selection;    subset selection;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/16990/Nikoukar_Romina.pdf?sequence=11&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Accurate and efficient estimation of the key ionospheric state parameters such as electron density, ion composition, and electron and ion temperatures is required to understand fundamental issues of terrestrial plasma physics such as redistribution of energy and momentum, and coupling within terrestrial upper atmosphere regions. This work focuses on developing a modern coding scheme and inversion methodology using Arecibo incoherent scatter radar (ISR) to achieve efficient, and near-optimal estimates of the key ionospheric parameters. In particular, this work considers two aspects of the ISR inversion problem: (i) ISR lag estimates at individual altitudes, and(ii) modulation techniques that can provide more accurate estimates with a specific range resolution. These two aspects suggest a unifying framework for ISR inversion in which modern computational technology and ISR methodology are utilized in a robust estimation procedure.The first contribution of this work is the development of a discrete forward model for F-region incoherent scatter measurements, where long pulses are utilized in transmission. The range smearing imposed on measurements by long pulses is modeled as a one-dimensional convolution along the range in the simplified case where the receiver sampling is instantaneous. The next major phase of this research is to develop an efficient hybrid technique that allows for estimation of plasma parameters by removing range smearing from measurements. The inversion technique incorporates both quadratic and edge-preserving regularization approaches in order to provide smooth plasma auto-correlation function (ACF) lag profiles in the presence of noise while still resolving the sharp gradients.Another contribution of this thesis is to develop a technique for optimal modulation design in ISR experiments. The optimal resolution supported by ISR measurements is used as one criterion for the optimal design. The model order selection framework is applied to the problem at hand to find the optimal resolution. The results indicate that, compared with a long pulse, amplitude-modulated codes yield finer range resolutions with nearly similar parameter estimation errors or smaller estimation errors with the same range resolution. In medium to high SNR scenarios, a smaller on-off ratio of the transmitted waveform is recognized as a determining factor for allowing more freedom in removing range ambiguities as well as resulting in improved statistical accuracy for integrated data in range and lag directions. In order to find the optimal amplitude modulation for Arecibo ISR measurements in medium to high SNR scenarios, a modified form of the sequential backward selection algorithm is applied to the space of all amplitude modulated pulses with a certain on-off ratio. Due to the vast search space, there is no possibility for an exhaustive search. Therefore, the problem of finding the optimal amplitude modulation is viewed as an optimization problem. Three optimality criteria, namely, sum of squared errors, uniformity of estimation errors, and condition number of convolution matrices, are considered.The final contribution of this work was implementing and conducting several experiments in April 2004, August 2005, and July 2006 using the incoherent scatter radar at Arecibo Observatory, and applying the inversion technique to estimate the plasma parameters. In these experiments, the original mode of MRACF was modified to utilize amplitude modulation. The results of all these experiments verify that when the SNR is sufficiently high, compared with an unmodulated long pulse, improved range resolution with nearly the same statistical accuracy is obtained when an amplitude modulationis utilized.The results of the developed methodology and experimental design of this work can be extended to other incoherent scatter radars (such as Jicamarca radar in Peru, and advanced modular incoherent scatter radar in Alaska) to improve the estimation task in other altitude and latitude regions, and to extract many further ionospheric parameters such as electric field strength, conductivity, current, and neutral wind speed.