Multiple GNSS Constellations – A New Era of Satellite Navigation
The US GPS (Global Positioning System) system is a well-known technology to the information society as it is now available to most cars and smart phones. With GPS, drivers and pedestrians can now easily find his/her ways to the destinations. Global Satellite Navigation System (GNSS) is now becoming not only essential to the modern information society, but also attractive to regional politics. In addition to the current US GPS system and the Russian GLONASS (GLObalnaja NAvigatsionnaja Sputnikovaja Sistema) system, Europe is now developing its own GNSS system called Galileo, while China is also doing the same thing by developing a similar system called Beidou or COMPASS. Both the European system Galileo and the Chinese system COMPASS will be fully operational by 2015 according to the current plans. Furthermore, US is also upgrading their current system to a modern system called GPS III that will introduce new signals e.g. L2C, L5 and L1C that will be transmitted by the new generations of satellites. There will be in total 125 GNSS satellites orbiting the Earth on the sky, of which each 30 from GPS, GLONASS and Galileo and 35 from COMPASS. Therefore the sky will be full of GNSS satellites.We are entering into a new era of satellite navigation with rich satellites available on the sky. However, what does this mean to the end-users? What kind of opportunities will we have with multiple GNSS constellations? The most direct and significant change is the number of visible satellites that will be increased. This implies that more measurements will be available for the purpose of positioning. This change will bring benefits of improving the positioning availability, accuracy and integrity.
Significant improvement of availability will occur in GNSS degraded environments such as urban canyon where an open sky is typically not available. The narrow view to the sky will limit the visibility to four or more satellites that is required for locating a mobile user. However, when the number of satellites on the sky will be increased, the possibility of having lines-of-sight to more than four satellites will be increased dramatically. This means the position availability will be improved significantly especially in urban areas.
Although the number of satellites visible from the sky will be increased dramatically, the additional satellites will increase the GDOP (Geometric Dilution of Precision) slightly only as the openness of the sky view is still the dominated factor of GDOP. Therefore, additional ranging measurements will not make any significant improvements directly to positioning accuracy. However, the redundant measurements provide us with a possibility of detecting the possible large errors in some of the measurements and therefore excluding those measurements from position computation. This is an indirect way to improve the positioning accuracy.
Integrity has not been considered in the current GPS system. With the future multiple GNSS constellations, a large amount of redundant measurements will be available. Therefore more reliable RAIM (Receiver Autonomous Integrity Monitoring) algorithm can be implemented inside the receiver for improving the efficiency and reliability of integrity monitoring.
Summary: Multiple GNSS constellations including GPS, GLONASS, Galileo and COMPASS will be operational by 2015. The most direct benefit to the end users is the increasing number of visible satellites. This will increase the positioning availability in GNSS degraded environments such as urban canyon. The large amount of redundant measurements will also improve the positioning accuracy and the efficiency of integrity monitoring.
Prof. Dr. Ruizhi Chen
Department of Navigation and Positioning
Finnish Geodetic Institute
08.Dec.2009.
Masala.