In this feasibility study, prospective satellite altimetry missions are studied for their usefulness in oceanography. Sea surface height (SSH) anomalies obtained from Global Navigation Satellite System Reflectometry (GNSS-R) are assimilated with a 4D-VAR method. A space-borne GNSS-R detector on low Earth orbit is assumed and simulated. The proposed GNSS-R measurements will surpass the radar-based satellite altimetry missions in temporal and spatial resolution but are less precise. The characteristics of the SSH observations are estimated and respective observations are sampled from a high resolution numerical ocean model. The Agulhas region is chosen as test region. The Agulhas current system transports warm and salty water masses from the Indian ocean into the Southern Ocean and into the Atlantic. These transports impact present and future climate on local and global scales. However, size and variability of the respective transports are still much debated. With a twin-model adjoint data assimilation approach the question is answered how the regional internal water mass properties and transports in the Agulhas region are determined by the high-density but low-accuracy GNSS-R based SSH observations.