The International Maritime Organisation (IMO), in its Sub-Committee for the Safety of Navigation, has recommended the acceptance of GPS as a component of the World-Wide Radionavigation System. However, the Sub-Committee highlighted that GPS accuracy is not sufficient for harbour entrances and approaches and that it does not provide instantaneous integrity to alert users of any malfunction. The Sub-Committee considered that DGPS could improve both the accuracy and the integrity of GPS. In practice, the use of differential corrections to GPS signals allows elimination of most of the errors of GPS and improves the integrity of the service significantly. While GPS does not provide instantaneous integrity, the use of DGPS reference stations, which continuously monitor the signals of the visible satellites, enables the timely warning (under 15 seconds) of any malfunction or failure. However, the recent discontinuation of Selective Availability (SA), which led to an improvement in stand-alone GPS accuracy, has changed some of the premises that justified differential services. This paper presents the needs of mariners in terms of radio positioning, with the aim of evaluating the ability of unaugmented GPS and DGPS to comply with marine navigation requirements. The impact of the removal of SA on DGPS and its benefits for mariners will also be discussed in order to show that DGPS is still adequate and useful for mariners.