The General Lighthouse Authorities' (GLAs) public marine Differential Global Positioning System (DGPS) is part of the mix of visual, audible and electronic aids to navigation provided by the three General Lighthouse Authorities of the UK and the Republic of Ireland under their Marine Navigation Plan. 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. DGPS can assist the safe passage of all classes of vessels from cargo ships, cruise liners and fishing vessels to small yachts, by:-
The DGPS 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 the Republic of Ireland, of which 4 are in the Northern Lighthouse Board area of Scotland and the Isle of Man.
Details of the locations, ranges and transmission characteristics of all of the GLAs DGPS reference stations at the lighthouse sites are given in the accompanying tables.
GPS was developed by the US Department of Defense as a worldwide all weather navigation and positioning and timing resource, mainly for military use. It is based on a constellation of 24 satellites orbiting the earth which act as reference points.By measuring the travel time of signals transmitted from four satellites, a receiver can measure its distance from each satellite and combine these measurements to calculate its latitude, longitude, altitude, course and speed.GPS positions are accurate 95% of the time to ± 9 metres and updates can be received every second. A higher accuracy level is required for some specialised navigation and positioning purposes and the integrity of the system needs to be monitored for safety-related applications.
DGPS can provide higher accuracies of 5 metres (95%) or better in moving applications and even better in stationary situations, by cancelling out most of the natural and man-made errors arising from normal GPS measurements.DGPS works by having a fixed receiver at a known ground-based reference station which continuously monitors the GPS errors and transmits corrections to the measurements taken by mobile receivers operated by users of the system. The GLAs' DGPS service is intended to offer accuracies for general navigation of 5 metres for vessel position-fixing in areas where the freedom to manoeuvre is restricted. DGPS is provided from radiobeacons by modulating the carrier with the correction data and information regarding the system: DGPS receivers should comply with the International Electrotechnical Commission (IEC) Standard 1108-4 (BS EN 61108-4) and may be combined with the GPS receiver; the messages provided include information about the health of the reference station and the transmitter. The radiobeacon system uses the LF/MF band 283.5 - 315 kHz in Europe. A full description of the technical characteristics of the system can be found in ITU-R Recommendation M.823-2. This incorporates the RTCM SC-104 protocol and the International Association of Lighthouse Authorities has adopted these standards for use with maritime radiobeacons to transmit DGPS corrections.
DGPS is referenced to the World Geodetic System 1984 (WGS84) Datum. Charts covering the waters within the GLAs' areas of responsibility produced by the Hydrographic Office of the United Kingdom are referred to WGS84. Most DGPS receivers will have a facility to transform positions from WGS84 Datum to the regional datum of the chart, but the resulting accuracy will depend on the transformation parameters contained within the software of the user's receiver. The GLAs strongly advise mariners to keep the DGPS receiver referenced to WGS84 Datum and to apply the position shift values denoted on the relevant nautical chart before plotting a position. Reference should also be made to the current Volume 2 of the Admiralty List of Radio Signals.
DGPS is no longer simply a navigation technique - it is a means to measure positions and movement of any kind to a high degree of accuracy.In the marine environment, the output from a suitable DGPS receiver could form the position sensor input to an Integrated Navigation System (INS), an Electronic Chart System (ECS) or an Automatic Identification System (AIS). In these applications the inherent integrity checking, high accuracy and stability of the differential system are particularly important. In positioning mode, DGPS is used for buoy-laying and hydrographic surveying. DGPS may also be used, often in combination with other systems, for a variety of other marine applications such as dynamic positioning, rig moves, pipe-laying, cable-laying and dredging.
There are marine radiobeacon DGPS services in over 45 countries throughout the world including the majority of North West European States. With advances in technology, the signals from radionavigation systems have progressively transcended national boundaries to provide long-range transmissions from a network of satellite and terrestrial stations. This has significant benefits for the safety of navigation since it enables a consistent means of navigation throughout all phases of the voyage, during the day or night and in conditions of reduced visibility. Such systems will also help with improved voyage planning to reduce ship operators' costs. A high level of availability of the right mix of aids to navigation systems also plays some role in determining which ships will visit our ports and in demonstrating that effective measures have been taken to minimise the risk of pollution from ships.
The GLAs DGPS system is provided for maritime use. Implementation was completed in August 1998 and the system achieved Operational Capability on 1 July 2002. Problems with DGPS receiver equipment should be taken up with the supplier, whereas users who experience problems with the system are invited to complete and return an Incident Report Form, which can be obtained as follows: Northern Lighthouse Board (Scotland and the Isle of Man) Northern Lighthouse Board 84 George Street Edinburgh EH2 3DA Tel: 0131 473 3100 Fax: 0131 220 2093 E-mail: firstname.lastname@example.orgTrinity House (England, Wales and the Channel Islands) The Navigation Manager Trinity House Tower Hill LONDON EC3N 4DH Tel: 020 7481 6900 Fax: 020 7480 7662 The Commissioners of Irish Lights (all Ireland) Harbour Road Dun Laoghaire Co. Dublin Ireland Eire Tel: +353 -1- 2715400 Fax: +353 -1- 2715566. E-mail: email@example.com
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