Radar Imaging and Holography
Focusing on holography and tomography for quasimonochromatic and broadband signals, this book provides a detailed coverage of the basic physical methods, inverse problems and mathematical principles.
TABLE OF CONTENTS Radar Imaging and Holography
Chapter 8: Phase Errors and Improvement of Image Quality
Possible sources of phase fluctuations of an echo signal, which negatively affect the aperture synthesis, are turbulent flows in the troposphere and ionosphere. Fluctuations of the refractive index due to tropospheric turbulence impose restrictions on the aperture centimetre wavelengths. Ionospheric turbulence affects far-decimetre wavelengths. Phase fluctuations decrease the resolving power of a synthetic aperture, leading to a lower image quality.
8.1 Phase errors due to tropospheric and ionospheric turbulence
8.1.1 The refractive index distribution in the troposphere
Fluctuations in the troposphere may arise from changes in the meteorological conditions and air whirls. As a result, there are non-uniform local distributions of temperature and humidity, leading to a non-uniform distribution of refractivity N:
where n is the refractive index.
At the centimetre wavelengths, a static air volume has refractivity N defined by the Smith Wentraub formula:
where P is the total atmospheric pressure measured in millibars, T is temperature in Kelvin degrees and e is the specific water vapour pressure in millibars.
It follows from Eq. (8.2) that the value of N at centimetre and longer wavelengths strongly depends on the water vapour concentration, while its variation with the wavelength ? is insignificant. The latter fact is quite important because it makes it possible to obtain phase fluctuation spectra for various wavelengths in the microwave range, using an experimental spectrum measured at any wavelength. The major type of non-uniformity responsible for amplitude and phase fluctuations of an electromagnetic wave are so-called globules. These represent...
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