Low frequency radio navigation systems continue to play an important role in the provision of precise navigation for vessels sailing in coastal waters, and in other zones requiring high accuracy. Modernization of the existing Loran C chains, and deployment of new ones, shows there is strong interest in these systems despite the appearance of global navigation satellite systems (GNSS) such as ‘Navstar’ and ‘Glonass’. This continuation of interest is connected to the relatively low cost of operation of the systems, the low cost of receiver-indicators and the need to provide users with very precise but reliable positioning information, which at present can only be obtained by joint use of GNSS and Loran C. To make the most of such an approach, Loran C should provide accuracy and reliability similar to GNSS.