GNSS Overview with Emphasis on Propagation Issues

Kurzfassung

Today, satellite navigation with GPS is well established and widely used. The European satellite navigation system Galileo is under development and shall become operational in 2013. The current status and planning of both Global Navigations Satellite Systems (GNSS) will be outlined where the emphasis will be on the different signals and services available with both systems. Since satellite navigation is based on measuring the signal delay between transmission at the satellite and reception by the user, the modelling and correction of the additional delay due to the propagation phenomena plays an important role for the accuracy of the derived position solution. The largest signal delay occurs within the ionosphere. The ionosphere delay can be precisely determined and nearly completed eliminated by dual frequency measurements. However, for single frequency receivers, like most commercially available GPS-receivers for the mass market, it must be corrected by modelling, and a significant error can remain. While atmospheric attenuation is negligible at the navigation frequency bands in L-band, occasionally, fast amplitude and phase scintillations can occur due to fast variations of the total electron content in the ionosphere or atmospheric turbulences. Strong scintillations occur only rarely, but then they are critical and can even lead to complete loss of the navigation signals by the receiver. The troposphere delay can be separated in a wet component due to water vapour and a dry component due to other atmospheric gases. While the dry delay can be modelled with high accuracy, the wet delay is a crucial component if accuracies in the decimetre or centimetre range are required, although it contributes normally only with 10%-20% to the total delay, because of the high temporal and spatial variability of the water vapour content in the troposphere. Due to the extreme low signal power of the satellite navigation signals when arriving at the Earth, the signals can be easily attenuated and shadowed by buildings or vegetation, e.g. in urban or rural environments or for indoor applications. In these environments multipath propagation by reflexions of the signals by buildings and other obstacles before they arrive at the user antenna can significantly degrade the ranging and positioning accuracy. Multipath propagation is difficult to correct by models, because it depends strongly on the local user environment. Different techniques exist to mitigate the effect of multipath signals partly in the signal processing of the receiver, but multipath mitigation is still the main error source in navigation besides the ionosphere error.