The General Lighthouse Authorities' (GLAs) marine Differential Global Positioning System (DGPS) is a satellite-based navigation system. It is an element of the mix of visual, audible and electronic aids to navigation provided by the three GLAs of the UK and the Republic of Ireland under their Marine Navigation Plan (MNP).
The MNP resulted from widespread consultation with the maritime community on the requirement for Marine Aids to Navigation into the 21st century. The plan has been devised to ensure the ongoing provision of a satisfactory, economical and reliable aids to navigation service to meet the changing requirements of all classes of mariner.
DGPS, which became operational on 1 July 2002, is a network of 14 ground-based reference stations providing transmissions with coverage of at least 50 nautical miles around the coasts of the United Kingdom and Republic of Ireland. It is an open system—available to all mariners—and is financed from light dues charged on commercial shipping and other income paid into the General Lighthouse Fund.
Details of the exact locations, ranges and transmission characteristics of all of the GLAs' DGPS reference stations at the lighthouse sites are given in the accompanying tables.
DGPS can assist the safe passage of all classes of vessel from cargo ships, cruise liners and fishing vessels to small yachts, by:
- Monitoring the integrity of the US NAVSTAR Global Positioning System (GPS).
- Improving accuracy for safe navigation in waters where the freedom to manoeuvre is restricted.
Users of the GLAs DGPS need to be aware, however, that:
- DGPS relies inherently on GPS, the operation and characteristics of which are outside the control of the GLAs and their respective governments;
- All radio navigation systems are susceptible to interference (including jamming) and environmental effects, which can adversely affect their availability;
- Various DGPS receiver types are available, some of which may not:
- have the facility to provide transformations from WGS84 Datum to the regional datum of the chart;
- provide appropriate or timely integrity warnings in respect of the system.
- Signal reception may become unreliable in certain extreme environmental conditions towards the limits of the geographical coverage.
The GLAs strongly advise that no single aid to navigation system should be used in isolation and that GPS/DGPS users should use all alternative means available to cross-check the information received. Users should also ensure that they have a receiver which gives sufficient warning of the complete loss of the DGPS signal and reversion to GPS.