Ionospheric effects on synthetic aperture radar at VHF

Synthetic aperture radars (SAR) operated from airplanes have been used at very high frequency (VHF) because of their enhanced foliage and ground penetration compared to radars operated at higher frequencies. A satellite-borne VHF SAR would have considerable utility but in order to operate with high resolution it would have to use both a large relative bandwidth and a large aperture. The presence of the ionosphere in the propagation path of the radar will cause a deterioration of the imaging because of dispersion over the bandwidth. In this paper we present calculations of the effects of a deterministic ionosphere on SAR imaging for a radar operated with a 100 MHz bandwidth centered at 250 MHz and over an angular aperture of 23/spl deg/. A daytime ionosphere induces a point spread function with an approximate width of 100 m in slant-range compared to the nominal resolution of 1.5 m.