GPS Selective Availability ends
'Along with our commitment to enhance GPS for peaceful
applications, my administration is committed to preserving fully
the military utility of GPS'
- President Clinton.
- President Clinton.
ON 1 MAY 2000 President Clinton announced the
immediate ending of the selective availability of GPS, the United
States' military controlled satellite navigation system. Selective
availability is the name for the USA's policy of deliberately
degrading the satellite signals available to civilian users,
reducing their accuracy to within 100 metres. The application of
selective availability was completely random, so civilian users
were never sure how accurate the signal they were receiving
was.
To counteract the effects of selective availability Differential GPS (DGPS) was developed. DGPS works by a land-based reference station collecting signals from all the available GPS satellites and calculating the difference (hence differential) between these signals and the reference station's known position. The station calculates correction factors and broadcasts these to DGPS users on the medium frequency marine radiobeacon wave band. DGPS signals also include integrity messages which warn users if any satellite is transmitting erroneous information.
The General Lighthouse Authorities provide a DGPS system in Ireland and Great Britain, three of the stations-at Tory Island, Loophead, and Mizen Head-being operated by the Commissioners of Irish Lights. The ending of selective availability has improved the accuracy of GPS to within 20 metres and makes DGPS even more accurate, potentially within one metre.
IALA, the International Association of Marine Aids to Navigation and Lighthouse Authorities, has welcomed the ending of selective availability but has emphasised the continuing need to augment GPS to meet the requirements for navigation in harbour entrances and approaches, and other waters in which the freedom to manoeuvre is limited, pointing out that the International Maritime Organisation (a United Nations agency) specifies that positional information with an error not greater than 10 metres, with a probability of 95% should be provided by a radionavigation system used to assist in the navigation of ships in such waters. Iala urges lighthouse administrations to continue to maintain and develop DGPS services to provide the necessary degree of accuracy and improved integrity of the system.
The Commissioners of Irish Lights have welcomed the ending of selective availability but point out that the US government continues to control GPS without regard to the civil navigation requirements of other states. GPS was developed by the US Department of Defense as a military system and that role continues. The re-imposition of selective availability at any time, or jamming on a regional basis, remains an option for the US government in its own national interest.
The Commissioners have confirmed the validity of the General Lighthouse Authorities' DGPS system which will continue to provide the necessary integrity and accuracy for the safety of navigation. The provision of DGPS, using the maritime radiobeacon frequency band, is consistent with standards in the European and international maritime communities. The system will help to provide an accurate position-fixing interface with future developments in electronic charting, vessel traffic services, and transponder systems. It will also assist in the natural transition to a civil global navigation satellite system for all modes of transport in the 21st century.
To counteract the effects of selective availability Differential GPS (DGPS) was developed. DGPS works by a land-based reference station collecting signals from all the available GPS satellites and calculating the difference (hence differential) between these signals and the reference station's known position. The station calculates correction factors and broadcasts these to DGPS users on the medium frequency marine radiobeacon wave band. DGPS signals also include integrity messages which warn users if any satellite is transmitting erroneous information.
The General Lighthouse Authorities provide a DGPS system in Ireland and Great Britain, three of the stations-at Tory Island, Loophead, and Mizen Head-being operated by the Commissioners of Irish Lights. The ending of selective availability has improved the accuracy of GPS to within 20 metres and makes DGPS even more accurate, potentially within one metre.
IALA, the International Association of Marine Aids to Navigation and Lighthouse Authorities, has welcomed the ending of selective availability but has emphasised the continuing need to augment GPS to meet the requirements for navigation in harbour entrances and approaches, and other waters in which the freedom to manoeuvre is limited, pointing out that the International Maritime Organisation (a United Nations agency) specifies that positional information with an error not greater than 10 metres, with a probability of 95% should be provided by a radionavigation system used to assist in the navigation of ships in such waters. Iala urges lighthouse administrations to continue to maintain and develop DGPS services to provide the necessary degree of accuracy and improved integrity of the system.
The Commissioners of Irish Lights have welcomed the ending of selective availability but point out that the US government continues to control GPS without regard to the civil navigation requirements of other states. GPS was developed by the US Department of Defense as a military system and that role continues. The re-imposition of selective availability at any time, or jamming on a regional basis, remains an option for the US government in its own national interest.
The Commissioners have confirmed the validity of the General Lighthouse Authorities' DGPS system which will continue to provide the necessary integrity and accuracy for the safety of navigation. The provision of DGPS, using the maritime radiobeacon frequency band, is consistent with standards in the European and international maritime communities. The system will help to provide an accurate position-fixing interface with future developments in electronic charting, vessel traffic services, and transponder systems. It will also assist in the natural transition to a civil global navigation satellite system for all modes of transport in the 21st century.