Global Navigation Satellite Systems are the primary means of obtaining position, navigation and timing information at sea.
The main systems available today or being developed are discussed below. Where possible relevant links for further information have been included, however visitors are also invited to refer to the International Committee on Global Navigation Satellite Systems for more information.
Global Positioning System (GPS)
GPS Constellation
(Courtesy of US NEC for PNT)
20,200km Altitudes, 55 degree inclination
GPS Block II-F satellite, image courtesy
of NASA
The Global Positioning System (GPS) is operated by the U.S. Government and was designed as a dual-use system for military and civilian use. The primary purpose of GPS is to enhance the effectiveness of U.S. and allied military forces. However, GPS provides positioning, navigation, and timing services to civilian users worldwide, free of any direct user charge.
GPS consists of a nominal 24-satellite constellation with satellites positioned across six medium-Earth orbital planes, with a period of approximately 12 hours. These satellites emit continuous navigation signals on two different L-band frequencies, L1 and L2. Civilian users are able to calculate their position to within 7.8m 95% of the time when using a single frequency solution.
The GPS constellation is evolving with new civilian signals planned on the L1, L2 and L5 frequencies. These signals are aimed to improve the positional accuracy afforded to civilian users, while providing signals with greater resilience to interference that are compatible with other GNSS. The anticipated schedule shows that these new signals should be available across the constellation by 2016 (L2C), 2019 (L5) and 2021 (L1C).
For further information, please refer to the US Coast Guard NAVCEN website.
GLObal NAvigation Satellite System (GLONASS)
The Russian Federation’s Global Navigation Satellite System (GLONASS) has a design constellation of 24 satellites. Like GPS, it provides positioning, navigation and timing information and is free from any direct user charge.
The GLONASS constellation suffered from low satellite availability in the 1990’s however since 2000 there have been regular satellite launches to bring the constellation back to 24 satellites. Civilian users are able to use two frequencies when calculating their position, enabling them to calculate their accuracy in the region of 10-14m (95% of the time).
GLONASS is also evolving with plans to introduce additional Code Division Multiple Access (CDMA) signals by 2019. The broadcast of CDMA signals will enable greater compatibility with other GNSS.
For further information on GLONASS please refer to the Russian Federal Space Agency website.
Galileo
Artist Interpretation of Galileo,
image courtesy of ESA- J. Huart
Europe is developing Galileo as its own GNSS. The Galileo system is operated by the European Commission as a civilian system and will provide five services, each with its own performance levels. These services are the Open Service (OS), Safety-of-Life service (SoL), Commercial Service (CS), Public Regulated Service (PRS) and Search and Rescue service (SAR).
Galileo has a design constellation of 30 satellites, of which 27 will be operational and 3 will be in-orbit spares.
The system is being developed with the Initial Operational Capability, consisting of 18 satellites broadcasting the Open Service, Public Regulated Service and the Search and Rescue service, planned by 2014. Full Operational Capability is expected by 2016.
For further information on Galileo, please refer to the European Space Agency and the European Commission websites.
Compass/Beidou
The Compass Navigation Satellite System (CNSS), or BeiDou 2, is China's second-generation satellite navigation system capable of providing continuous, real-time passive 3D geo-spatial positioning and speed measurement. It has a design of 5 geostationary satellites with 30 medium-Earth orbit satellites.
The system will broadcast data over three frequencies and will provide civilian users with two usable signals as part of the Open Service (OS). These signals will be compatible with other GNSS, and it is anticipated that users will be able to calculate their position to within 10m, (95% of the time).
The first geostationary satellite was launched in 2009 and the system is scheduled for completion by 2020.
For further information on Compass/Beidou, please refer to the Compass website (In Chinese but webpage translators are available).
