The increasing congestion in some portions of the radio spectrum, especially in the AM band, makes it virtually impossible to obtain information about the ionosphere in those bands by conventional sounding. This raises the question, if ionospheric information can be obtained in a passive mode by measuring the angle of arrival of signals transmitted by stations of known location over a short or medium length path involving ionospheric reflection. No difficulties are expected if only one wave is observed at the receiving site. If the signal can propagate over two different paths, the two superimposed waves can be separated numerically in amplitude and direction of arrival, provided that amplitude and phase are recorded at four spaced receiving antennas. It appeared that the ionospheric sounding facility at Brighton, Colorado, had the capability to test this principle. Several tests showed that the high data quality makes this approach very promising. The two-wave separation, however, could not be tested, since the two receiving antenna pairs are sampled in sequence, and ionospheric changes cause already significant changes in the phase relation of the two waves over periods much shorter than the sampling interval of 20 ms. The relevance of these tests to direction finding will also be discussed.
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