When the satellite era commenced more than a quarter of a century ago, one could hardly foresee the world wide revolution it heralded in the development of aids to navigation for merchant shipping. However, early investigations into the possible application of satellites to maritime needs led to an understanding of the powerful potential of satellite techniques for navigation. It became clear that if the international maritime community was really interested in a global all-weather, high-precision and commercially viable navigation system; such a system could only be satellite-based. This is evident from the situation that has recently arisen in IMO, where after exhaustive discussion on the mandatory carriage of electronic position-fixing equipment on ships in designated areas, the organization could not express a preference for any particular aid, until it was decided that efforts should be made to develop a global satellite navigation system capable of meeting a new standard of navigational accuracy. Moreover, in preparing the navigational accuracy standard, account was taken of experience gained with existing satellite navigation systems.