UC San Diego

Refractivity is an important atmospheric parameter which determines the propagation speed of electromagnetic (EM) waves traveling through the lower troposphere. The vertical refractivity profile of the atmosphere dictates how much the wave front of a radio wave will bend away from the straight-line path from its transmitter. The standard atmospheric refractivity profile over a terrestrial path is well known, experiences little fluctuation, and causes radio waves to curve towards the Earth. This effectively increases the `radio horizon' for non-optical frequencies. Over a marine channel however, the refractivity profile of the atmosphere is highly variable and can form spurious natural waveguides known as `ducts,' which allow for abnormally long range propagation and other anomalous effects. As a result, it is desirable to be able to track the refractivity profile of the atmosphere over marine channels to better predict and exploit the propagation paths of EM waves.

Currently, the state of the art in atmospheric refractivity research involves inference of the atmospheric refractivity profile from observations of the propagation loss of narrowband signals transmitted from a known location. Though this method has found some success, it is limited by the fact that there are often many refractivity profiles resulting in the same observed propagation loss. Thus, it is often impossible to know for certain the atmospheric refractivity profile using propagation loss alone.

In this dissertation, it is suggested that additional information about atmospheric refractivity can be found by measuring the direction of arrival (DOA) of a wave front in addition to its propagation loss. The topic of DOA estimation is discussed in depth, from classical techniques to the development of new super-resolution algorithms. The construction of a passive receiver array and first long term measurements of the DOA of an over the horizon signal propagating through a marine channel are detailed. These measurements show that the time series of DOA information is not well correlated with the time series of propagation loss information, implying that wave front DOA could be used to infer additional information about the atmospheric refractivity profile.