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Ionospheric effects on synthetic aperture radar at 100 MHz to 2 GHz
Author(s) -
Ishimaru Akira,
Kuga Yasuo,
Liu Jun,
Kim Yunjin,
Freeman Tony
Publication year - 1999
Publication title -
radio science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.371
H-Index - 84
eISSN - 1944-799X
pISSN - 0048-6604
DOI - 10.1029/1998rs900021
Subject(s) - ionosphere , physics , faraday effect , optics , computational physics , ionospheric heater , radar , geophysics , magnetic field , telecommunications , quantum mechanics , computer science
Recently, there has been increasing interest in the use of spaceborne synthetic aperture radar (SAR) for measuring forest biomass. However, it is noted that conventional SAR using C‐band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavelengths, typically P‐band (500 MHz). It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses. The variance of the refractive index fluctuations due to turbulence is approximately proportional toƒ −4 . In addition, the Faraday rotation due to the geomagnetic field in the ionosphere becomes significant. This paper presents an analysis with numerical examples of the following effects in the frequency range from 100 MHz to 2 GHz in order to show the frequency dependence and the effects of total electron content (TEC) of the ionosphere. First, the ionospheric turbulence can reduce the coherent length below the equivalent aperture size, and the azimuthal resolution becomes greater than D /2 where D is the antenna aperture size. Second, the ionospheric dispersion causes a shift of the imagery due to the group velocity. Third, the dispersion also creates broadening of the pulse. In addition, multiple scattering due to ionospheric turbulence gives rise to pulse broadening. Fourth, we consider the Faraday rotation effect and show that the ellipticity change is negligible, but the orientation angle changes significantly at P‐band. Numerical examples are shown using typical ionospheric parameters, turbulence spectrum, and TEC values.