DGPS: what is the position?
Colin Day
The General Lighthouse Authorities of the United Kingdom and the Republic of Ireland--The Commissioners of Irish Lights, Trinity House and the Northern Lighthouse Board will be providing an unencrypted Differential Global Positioning System (dgps) service.
The General Lighthouse Authorities of the United Kingdom and the Republic of Ireland--The Commissioners of Irish Lights, Trinity House and the Northern Lighthouse Board will be providing an unencrypted Differential Global Positioning System (dgps) service.
Notice to Mariners no. 4 (1998) issued by the Commissioners of
Irish Lights on 15th May 1998
DGPS antennas above the dome at Loophead Lighthouse.
Notices to Mariners are usually about alterations to light-houses or buoys--but this one...? Differential Global Positioning System...? Unencrypted...? What does it all mean?
The Global Positioning System, or gps, is a system developed by the US Department of Defense, primarily 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 at least four satellites, a gps receiver can calculate its latitude, longitude and altitude. A standard gps receiver has an accuracy of +/- 100 metres 95% of the time. The system is controlled by the US Military which has purposely introduced an error on the signals, referred to as Selective Availability, for non-military users.
A Differential gps Reference Station, which is at a known surveyed position, receives the signals and computes a calculated position. The calculated position and the known position are compared and the difference is computed for each satellite (hence differential). These difference corrections are broadcast by radio transmissions and can be received by a dgps receiver. The dgps receiver applies the corrections to achieve greater accuracy, in the order of +/- 5 metres or better.
The DGPS installation team at Tory Island Lighthouse. (LR) Colin Day, Electrical Engineer; Tony Reynolds, Coast General Operative; Benny Goodman, Trinity House Research and Development; Paul Barron, Electronic Technician; Mark Cazzini, Lighthouse Technician Electrical; Brian Kelly, Lighthouse Technician Mechanical; Seamus Barry, Coast Tradesman; Duncan Hawksbee, Trimble Navigation Europe Ltd; and Sean Doherty, Attendant of Tory Island Lighthouse.
The General Lighthouse Authorities' (GLAs') system transmits the corrections from Marine Radiobeacon stations. The transmitted corrections are unencrypted - not coded in any way - and are therefore freely available to anybody with an appropriate receiver. The other important feature of dgps is the integrity monitoring of the satellite signals. A Reference Station can detect whether any satellite is broadcasting unusable information and can alert dgps users not to use that particular satellite. The format of differential correction trans-missions has been internationally agreed and dgps corrections are transmitted on Marine Radio-beacons worldwide, especially in Europe.
Three of the twelve Reference Stations in the GLAs' system have been installed by the Commissioners of Irish Lights, at Tory Island, Loop Head, and Mizen Head. These three sites already had Marine Radiobeacons in operation.
The gps reference station and integrity monitoring equipment was supplied and installed by Trimble Navigation Europe Ltd. The equipment at each Reference Station comprises duplicated gps receivers, duplicated Integrity Monitors, computer, communications equip-ment, and the Radiobeacon. Monitoring, control, and data from all the GLA sites can be downloaded by the Commissioners of Irish Lights at Dun Laoghaire, Trinity House Lighthouse Service at Harwich, and the Northern Lighthouse Board at Edinburgh.
The back up equipment and facilities were installed by Irish Lights Technicians before the arrival of the team of Trimble personnel who travelled to each of the twelve GLA sites in turn to install the main equipment. Together with the Lighthouse Technicians they worked long into the evenings to ensure that the installation programme remained on schedule.
The Trimble team included two Englishmen, a New Zealander and an American. International co-operation continued with Irish Lights' own Norwegian Electronic Technician. The Trimble team travelled and transported the equipment in a large hired van. The van was brought by barge to Tory Island where it caused quite a stir.
It was not all work, however. The best of West Cork hospitality was enjoyed one evening in Skibbereen, while Duncan, one of the Trimble team, wants to spend his next summer holidays on Tory Island.
The installation of the equipment at the three sites presented new challenges for Irish Lights' engineering personnel. The gps antennas had to be installed at each site so that they had a clear view of the sky in every direction to receive all the available satellite signals. At Tory Island and Loop Head this meant building a platform above the dome of the lighthouse. The design, fabrication, transport, and installation of these platforms was completed in a few months and it is a credit to the mechanical design and installation teams that the appearance of the platforms is so unobtrusive.
The dgps tower at Mizen Head with Radiobeacon aerial masts in the background (photos: Colin Day)
At Mizen Head a different solution for the siting of the gps antennas had to be found as there is no conventional lighthouse tower at this station. A new concrete structure, variously referred to as the tower, the turret, or the folly, was designed and built by the civil engineering section, to a very tight time schedule, on the side of the hill overlooking the Radiobeacon antenna. Visitors to Mizen Vision wanted to know if it was to be a new observation point for the heritage centre.
Once the siting of the antennas had been decided the next technical problem to be overcome was the route of the antenna cables. These very high specification cables are similar in rigidity to hydrodare water pipes--not the ideal cable to pull through the existing ducts at each station. The cables had to be installed in protective sleeving and laid in trenches that were then backfilled. The four antenna cables at each site were installed in record time due to the terrific co-operation of all personnel on site, including Benny from Trinity House Research and Development who became intimately acquainted with Irish mud!
Before the Trimble personnel arrived new batteries, chargers, fuseboards, cabling, and telephone lines were installed at each station, and new three- wire Radiobeacon antennas were installed at Tory Island and Loop Head. The Radiobeacon equipment at Tory Island had already been upgraded from 100 watt to 200 watt.
The dgps service commenced on a test basis from the three Irish Lights sites in June 1998. A Far Field Monitor to check the signals was installed in Rossaveal in September. Since then, a steady stream of inquiries has been received at Irish Lights Office from people wanting to use the service. The other nine GLA sites came on line during May and July 1998.
The General Lighthouse Authorities, being maritime organ-isations, have safety at sea as their prime concern. While the dgps service was installed and designed to give complete coverage over the coasts of Ireland and Great Britain, coverage is almost complete over the land area also. dgps is finding many uses other than for marine navi-gation. Hiking, in-car navigation, assistance for the blind, surveying, crop-yield analysis on farms, pipe laying, forest management, and dynamic movement measurements are some of the fields of application. While there iS no doubt that satellite radionavigation is a superb system and the addition of differential corrections to enhance its integrity and accuracy is very welcome, caution is advised. dgps relies inherently on gps, the operation of which is still controlled by the US Department of Defense. It is for this very reason that a number of European governments decided that Loran-C was necessary as an alternative radionavigation system, under the control of the participating nations, for civil use in north-west Europe.
gps uses wgs84, a datum which is not necessarily the regional datum of a chart. The GLAs strongly advise that no single system of navigation should be used in isolation and that dgps users should use alternative methods to cross-check the infor-mation received. Users should also ensure that they have a receiver that gives sufficient warning of the loss of the dgps signal and reversion to gps.
The position is clear: don't throw away your compass and sextant yet!
The General Lighthouse Authorities (GLAs) public marine Differential Global Positioning System (dgps) is a satellite-based navigation system. It is the newest element of the mix of visual, audible and electronic aids to navigation provided by the three General Lighthouse Authorities of the Republic of Ireland and the UK under their Marine Navigation Plan.
The dgps service is provided by a network of 12 ground-based reference stations providing transmissions with coverage up to 50 nautical miles around the coasts of Ireland and Great Britain. It is an open system--available to all mariners--and is financed from light dues charged on commercial shipping, supplemented by a contribution from the Irish Government 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 monitoring the integrity of the US navstar Global Positioning System, and improving accuracy for safe navigation in waters where the freedom to manoeuvre is restricted.
Further details of the GLAs' DGPS service can be obtained from Irish Lights Office, 16 Lower Pembroke Street, Dublin 2, or our Website.
Full particulars of dgps and other satellite navigation systems will be found in Admiralty List of Radio Signals, Volume 8, published by the UK Hydrographic Office, Taunton, Somerset, TA1 2DN, England.
DGPS antennas above the dome at Loophead Lighthouse.
Notices to Mariners are usually about alterations to light-houses or buoys--but this one...? Differential Global Positioning System...? Unencrypted...? What does it all mean?
The Global Positioning System, or gps, is a system developed by the US Department of Defense, primarily 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 at least four satellites, a gps receiver can calculate its latitude, longitude and altitude. A standard gps receiver has an accuracy of +/- 100 metres 95% of the time. The system is controlled by the US Military which has purposely introduced an error on the signals, referred to as Selective Availability, for non-military users.
A Differential gps Reference Station, which is at a known surveyed position, receives the signals and computes a calculated position. The calculated position and the known position are compared and the difference is computed for each satellite (hence differential). These difference corrections are broadcast by radio transmissions and can be received by a dgps receiver. The dgps receiver applies the corrections to achieve greater accuracy, in the order of +/- 5 metres or better.
The DGPS installation team at Tory Island Lighthouse. (LR) Colin Day, Electrical Engineer; Tony Reynolds, Coast General Operative; Benny Goodman, Trinity House Research and Development; Paul Barron, Electronic Technician; Mark Cazzini, Lighthouse Technician Electrical; Brian Kelly, Lighthouse Technician Mechanical; Seamus Barry, Coast Tradesman; Duncan Hawksbee, Trimble Navigation Europe Ltd; and Sean Doherty, Attendant of Tory Island Lighthouse.
The General Lighthouse Authorities' (GLAs') system transmits the corrections from Marine Radiobeacon stations. The transmitted corrections are unencrypted - not coded in any way - and are therefore freely available to anybody with an appropriate receiver. The other important feature of dgps is the integrity monitoring of the satellite signals. A Reference Station can detect whether any satellite is broadcasting unusable information and can alert dgps users not to use that particular satellite. The format of differential correction trans-missions has been internationally agreed and dgps corrections are transmitted on Marine Radio-beacons worldwide, especially in Europe.
Three of the twelve Reference Stations in the GLAs' system have been installed by the Commissioners of Irish Lights, at Tory Island, Loop Head, and Mizen Head. These three sites already had Marine Radiobeacons in operation.
The gps reference station and integrity monitoring equipment was supplied and installed by Trimble Navigation Europe Ltd. The equipment at each Reference Station comprises duplicated gps receivers, duplicated Integrity Monitors, computer, communications equip-ment, and the Radiobeacon. Monitoring, control, and data from all the GLA sites can be downloaded by the Commissioners of Irish Lights at Dun Laoghaire, Trinity House Lighthouse Service at Harwich, and the Northern Lighthouse Board at Edinburgh.
The back up equipment and facilities were installed by Irish Lights Technicians before the arrival of the team of Trimble personnel who travelled to each of the twelve GLA sites in turn to install the main equipment. Together with the Lighthouse Technicians they worked long into the evenings to ensure that the installation programme remained on schedule.
The Trimble team included two Englishmen, a New Zealander and an American. International co-operation continued with Irish Lights' own Norwegian Electronic Technician. The Trimble team travelled and transported the equipment in a large hired van. The van was brought by barge to Tory Island where it caused quite a stir.
It was not all work, however. The best of West Cork hospitality was enjoyed one evening in Skibbereen, while Duncan, one of the Trimble team, wants to spend his next summer holidays on Tory Island.
The installation of the equipment at the three sites presented new challenges for Irish Lights' engineering personnel. The gps antennas had to be installed at each site so that they had a clear view of the sky in every direction to receive all the available satellite signals. At Tory Island and Loop Head this meant building a platform above the dome of the lighthouse. The design, fabrication, transport, and installation of these platforms was completed in a few months and it is a credit to the mechanical design and installation teams that the appearance of the platforms is so unobtrusive.
The dgps tower at Mizen Head with Radiobeacon aerial masts in the background (photos: Colin Day)
At Mizen Head a different solution for the siting of the gps antennas had to be found as there is no conventional lighthouse tower at this station. A new concrete structure, variously referred to as the tower, the turret, or the folly, was designed and built by the civil engineering section, to a very tight time schedule, on the side of the hill overlooking the Radiobeacon antenna. Visitors to Mizen Vision wanted to know if it was to be a new observation point for the heritage centre.
Once the siting of the antennas had been decided the next technical problem to be overcome was the route of the antenna cables. These very high specification cables are similar in rigidity to hydrodare water pipes--not the ideal cable to pull through the existing ducts at each station. The cables had to be installed in protective sleeving and laid in trenches that were then backfilled. The four antenna cables at each site were installed in record time due to the terrific co-operation of all personnel on site, including Benny from Trinity House Research and Development who became intimately acquainted with Irish mud!
Before the Trimble personnel arrived new batteries, chargers, fuseboards, cabling, and telephone lines were installed at each station, and new three- wire Radiobeacon antennas were installed at Tory Island and Loop Head. The Radiobeacon equipment at Tory Island had already been upgraded from 100 watt to 200 watt.
The dgps service commenced on a test basis from the three Irish Lights sites in June 1998. A Far Field Monitor to check the signals was installed in Rossaveal in September. Since then, a steady stream of inquiries has been received at Irish Lights Office from people wanting to use the service. The other nine GLA sites came on line during May and July 1998.
The General Lighthouse Authorities, being maritime organ-isations, have safety at sea as their prime concern. While the dgps service was installed and designed to give complete coverage over the coasts of Ireland and Great Britain, coverage is almost complete over the land area also. dgps is finding many uses other than for marine navi-gation. Hiking, in-car navigation, assistance for the blind, surveying, crop-yield analysis on farms, pipe laying, forest management, and dynamic movement measurements are some of the fields of application. While there iS no doubt that satellite radionavigation is a superb system and the addition of differential corrections to enhance its integrity and accuracy is very welcome, caution is advised. dgps relies inherently on gps, the operation of which is still controlled by the US Department of Defense. It is for this very reason that a number of European governments decided that Loran-C was necessary as an alternative radionavigation system, under the control of the participating nations, for civil use in north-west Europe.
gps uses wgs84, a datum which is not necessarily the regional datum of a chart. The GLAs strongly advise that no single system of navigation should be used in isolation and that dgps users should use alternative methods to cross-check the infor-mation received. Users should also ensure that they have a receiver that gives sufficient warning of the loss of the dgps signal and reversion to gps.
The position is clear: don't throw away your compass and sextant yet!
The General Lighthouse Authorities (GLAs) public marine Differential Global Positioning System (dgps) is a satellite-based navigation system. It is the newest element of the mix of visual, audible and electronic aids to navigation provided by the three General Lighthouse Authorities of the Republic of Ireland and the UK under their Marine Navigation Plan.
The dgps service is provided by a network of 12 ground-based reference stations providing transmissions with coverage up to 50 nautical miles around the coasts of Ireland and Great Britain. It is an open system--available to all mariners--and is financed from light dues charged on commercial shipping, supplemented by a contribution from the Irish Government 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 monitoring the integrity of the US navstar Global Positioning System, and improving accuracy for safe navigation in waters where the freedom to manoeuvre is restricted.
Further details of the GLAs' DGPS service can be obtained from Irish Lights Office, 16 Lower Pembroke Street, Dublin 2, or our Website.
Full particulars of dgps and other satellite navigation systems will be found in Admiralty List of Radio Signals, Volume 8, published by the UK Hydrographic Office, Taunton, Somerset, TA1 2DN, England.