The low-frequency radio telescope at the Clark Lake Radio Observatory (CLRO) near Borrego Springs, California, provides a unique and powerful method for studying ionospheric structure at mid-latitudes. By observing the refractive offsets that ionospheric gradients impose upon the signals from natural radio sources, we are able to measure these gradients simultaneously at many points in the ionosphere. The telescope is a highly sophisticated system which operates in the 10- to 125-MHz frequency range—a range in which ionospheric refraction completely dominates tropospheric refraction. It is sensitive enough to detect many thousands of natural radio sources and, because it is electronically steered, it can be pointed anywhere in the sky in less than a millisecond. The telescope can be programmed to rapidly cycle over a grid of sources whose astronomical coordinates are well known and can be used to determine the refractive displacements of the apparent positions of these sources. Thus the ionosphere can be “punctured” at separations of a few kilometers to a few hundreds of kilometers by the lines of sight to these sources and ionospheric gradients can be measured at each of these puncture points. Ionospheric waves are observed; their amplitude, wavelength, direction of propagation, and speed of propagation can be measured. The two-dimensional shapes and sizes of ionospheric irregularities are determined as well as large-scale ionospheric gradients.
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