This paper was first presented at GNSS 2002 – European Navigation Conference – held in Copenhagen, 28th to 30th May, 2002.

At the 5th GNSS Symposium, held in Seville in 2001, the authors presented a paper entitled ‘The Portuguese DGPS network’, which began by addressing the current need for a DGPS service, given the removal of Selective Availability (Moore et al., 2001). In that paper, the benefits of DGPS for mariners, namely in terms of accuracy and integrity, were discussed and presented, in order to show that the option to embrace DGPS was, and is, still valid because it can provide the best accuracy and integrity at sea. However, the accuracy advantage afforded and the integrity check performed by DGPS are only useful if the DGPS service reaches very high standards of availability and continuity. It would be almost useless to have a DGPS service with low availability and continuity, because it could mean a ship losing the differential corrections in restricted waters, where the service is most needed. In summary, the level of benefit gained from DGPS is directly proportional to the user availability of the service and to its continuity. Having discussed accuracy and integrity last year, this paper covers issues related to availability and continuity (continuity replaced the previously used concept of reliability). The availability and continuity requirements recently adopted by the International Maritime Organization (IMO) and the International Association of Lighthouse Authorities (IALA) are presented, as well as the ways by which DGPS services can meet the very high requirements for the harbour entrance and approach phase of navigation. The concepts used in the Portuguese DGPS network to minimize downtime and ensure the highest reliability and availability, are presented as an example.