Satellites have been used for navigation since the 1970s. The US-controlled Global Positioning System (GPS) is available 24 hours per day free of charge. Other systems such as the Russia-built GLONASS, and the Euro Galileo system to be launched in 2004 are also described. For civilian applications, the GPS signal has been degraded to reduce its positioning accuracy. This paper identifies these limitations and outlines recent developments in overcoming them. The techniques of code-measuring and phase- measuring devices are described and their performance in either single-point or differential positioning is explained. Furthermore, the design and implementation of a network of GPS reference stations which will achieve an accuracy at the centimetre level and with a baseline of 20 km is described. GPS equipment has also been deployed in Intelligent Transport System measures. This technology should play an important role in applications such as fleet management, system performance monitoring and electronic tolling in future years. (a).
Samenvatting