Multiple frequency atmospheric radar techniques
This thesis is concerned with the use of multiple frequency coding to improve the vertical resolution of pulsed-Doppler very high frequency (VHF) atmospheric radars, especially with regards to the two-frequency technique known as frequency domain interferometry (FDI). This technique consists of transmitting alternate pulses on two distinct carrier frequencies. The two resulting time series are used to evaluate the normalized cross-correlation function, whose magnitude and phase are related to the thickness and position of a scattering layer. These same time series are also used to evaluate cross-spectra, which yield magnitude and phase values for each Doppler frequency component of the return signal. Examples of FDI cross-correlation and cross-spectral data are presented. These data show that considerable differences exist between scattering layers located in the lower and upper mesosphere. Lower mesospheric layers are relatively long-lived and undergo slow oscillatory motions with periods up to several hours. Although their mean thickness is about 500 m, they may sometimes consist of thin (less than 150 m) pancake-like structures that are randomly tilted within a few degrees of the horizontal. Cross-spectra from this region are often adequately explained by a combination of beam- and turbulence-broadening. Layers located in the upper mesosphere, on the other hand, are comparatively short-lived and occasionally possess mean thickness values less than 200 m. They often demonstrate short-period vertical oscillations and sometimes appear to jump from one height to another. Upper mesospheric cross-spectra may sometimes be explained by a combination of the layer-displacement and turbulence-broadening effects.
Geophysics, Physics, Atmospheric Science, Remote Sensing
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign