This document presents the conceptual design and analysis of a Synthetic Aperture Radar (SAR) system based on geosynchronous satellites (GEOSAR) working at Ka-band. These satellites, apparently orbiting in a fixed position relative to the Earth, are affected by slight perturbations of the perfectly geostationary orbits (small inclinations and non zero eccentricities) that may be used to generate a synthetic aperture radar for Earth Observation. The main advantage of GEOSAR is to obtain short revisit pass (daily or less) with local coverage, thus complementary to typical Low Earth Orbit SAR that achieve global coverage but with weekly or monthly revisit times. A further advantage is the look direction, mainly South-North, that is truly complementary to the LEO satellites looking at East-West. The LEO long revisit times prevent the observation of fast dynamic scenarios like natural hazards, disasters, atmosphere, etc. The GEOSAR main characteristics are presented and different configurations are proposed. Some geosynchronous orbit considerations are shown to understand the particularities in the GEOSAR geometry. The compromises between spatial resolution and sensitivity are obtained together with basic operation parameters like Doppler Analysis, power link budget and ambiguities.