Regional to global satellite based navigation systems

The author claims that implementation of a satellite based navigation system with a highly precise service can be conceived only with a significant cost reduction with respect to GPS (Global Positioning System). The lessons learned from various investigations indicate that the satellite constellation plays a predominant role in the cost-reduction initiative. In this perspective, a combination of geostationary satellite and satellites in inclined elliptic orbit (molniya or tundra) is expected to provide a promising solution, offering among other advantages, the possibility of starting with a regional service using a few space elements which could be upgraded to extend the service to a global scale