The impact of multi-satellite altimetry on the representation of eddies across the spectrum of meso-scales from 500 km to 30 km is assessed. A multi-scale three-dimensional variational data assimilation (MS-3DVAR) system has been employed to deal with multiple spatial scales in fine resolution regional models and observations, and thus to more effectively assimilate multi-satellite altimetry data alongside sparse in-stiu observations, shore-based high-frequency radar velocities, and satellite sea surface temperatures (SSTs). The model that is used has a resolution of about 1 km. Leveraging the capability developed to assimilate a variety of observations, a set of observing system experiments (OSEs) have been conducted to quantify the impact of the existing multi-satellite altimetry observations on mesoscale eddy predictions on top of other available observations within the framework of an advanced observing system for the California coastal ocean and during the Salinity Processes in the Upper Ocean Regional Study (SPURS) field campaign in the North Atlantic Ocean. The result demonstrates positive impact on the representation of mesoscale eddies whose scales can be down to a few ten km.