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Vector antenna and maximum likelihood imaging for radio astronomy

机译：矢量天线和射电天文学的最大似然成像

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Radio astronomy using frequencies less than ¿¿¿100 MHz provides a window into non-thermal processes in objects ranging from planets to galaxies. Observations in this frequency range are also used to map the very early history of star and galaxy formation in the universe. Much effort in recent years has been devoted to highly capable low frequency ground-based interferometric arrays such as LOFAR, LWA, and MWA. Ground-based arrays, however, cannot observe astronomical sources below the ionospheric cut-off frequency of ¿¿¿10 MHz, so the sky has not been mapped with high angular resolution below that frequency. The only space mission to observe the sky below the ionospheric cut-off was RAE-2, which achieved an angular resolution of ¿¿¿60 degrees in 1973. This work presents alternative sensor and algorithm designs for mapping the sky both above and below the ionospheric cutoff. The use of a vector sensor, which measures the full electric and magnetic field vectors of incoming radiation, enables reasonable angular resolution (¿¿¿5 degrees) from a compact sensor (¿¿¿4 m) with a single phase center. A deployable version of the vector sensor has been developed to be compatible with the CubeSat form factor. Results from simulation as well as ground testing of the vector sensor are presented. A variety of imaging algorithms, including expectation-maximization (EM), space-alternating generalized expectation-maximization (SAGE), projected gradient ascent maximum likelihood (PGAML), and non-negative least squares (NNLS), have been applied to the data. The results indicate that the vector sensor can map the astronomical sky even in the presence of strong interfering signals. A conceptual design for a spacecraft to map the sky at frequencies below the ionospheric cut-off is presented. Finally, the possibility of using multiple vector sensors to form an interferometer is discussed.
机译：使用小于100 MHz的频率的射电天文学为从行星到星系的物体的非热过程提供了一个窗口。在此频率范围内的观测值还用于绘制宇宙中恒星和星系形成的早期历史。近年来，人们已经投入大量精力来开发高性能的低频地面干涉式阵列，例如LOFAR，LWA和MWA。但是，基于地面的阵列无法观测到电离层截止频率?? 10 MHz以下的天文源，因此尚未绘制出低于该频率的高角度分辨率的天空。观测电离层边界以下天空的唯一太空任务是RAE-2，它在1973年实现了60度的角分辨率。这项工作提出了用于绘制卫星上方和下方天空的替代传感器和算法设计。电离层截止。矢量传感器的使用可测量入射辐射的全部电场和磁场矢量，可实现具有单相中心的紧凑型传感器（？4 m）的合理角分辨率（？5度）。矢量传感器的可部署版本已开发为与CubeSat尺寸规格兼容。介绍了矢量传感器的仿真和地面测试结果。各种成像算法，包括期望最大化（EM），空间交替广义期望最大（SAGE），投影梯度上升最大似然（PGAML）和非负最小二乘（NNLS），都已应用于数据。结果表明，即使存在强干扰信号，矢量传感器也可以绘制天文天空图。提出了一种航天器的概念设计，用于在电离层截止以下的频率处绘制天空图。最后，讨论了使用多个矢量传感器形成干涉仪的可能性。

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