Global Navigation Satellite System (GNSS) consists of GPS, GLONASS and Galileo which is still under construction by the European Union. GPS is the most widespread GNSS in the world and applies successfully in so many fields such as positioning, navigation, geodesy, mapping, timing and so on. However, GLONASS has not done its work well for about ten years because of lack of funds. In summer of 2006, Russia’s GLONASS program continued its comeback and will have a full 24-satellite constellation by the end of 2009. Notably, China has a regional RDSS system using three geostationary satellites since 2000.
INS is a self-contained positioning and attitude device. In other words, it meets the all-environment requirement. The primary advantage of using INS is that velocity and position of the vehicle can be provided with abundant dynamic information and excellent short term performance. The main shortcoming is that the INS accuracy degrades greatly over time.