Ionospheric effects on synthetic aperture radar at 100 MHz to 2 GHz

Recently, there has been increasing interest in the use of spaceborne synth etic aperture radar (SAR) for measuring forest biomass. However, it is note d that conventional SAR using C-band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavele ngths, typically P-band (500 MHz). It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses. The varian ce of the refractive index fluctuations due to turbulence is approximately proportional to f(-4). In addition, the Faraday rotation due to the geomagn etic field in the ionosphere becomes significant. This paper presents an an alysis with numerical examples of the following effects in the frequency ra nge 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 iono spheric turbulence can reduce the coherent length below the equivalent aper ture size, and the azimuthal resolution becomes greater than D/2, where D i s the antenna aperture size. Second, the ionospheric dispersion causes a sh ift of the imagery due to the group velocity. Third, the dispersion also cr eates broadening of the pulse. In addition, multiple scattering due to iono spheric 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 exam ples are shown using typical ionospheric parameters, turbulence spectrum, a nd TEC values.