Simulating GNSS Position Accuracy using Non-line of Sight Reflected Signals
Résumé
This paper describes the development and use of a simulation tool, called QualiSIM. This tool simulates GNSS-based position accuracy under various operational and environmental conditions, in particular in environments with large amounts of signal reflections. The signal reflections are implemented using the geometric path length the signal travels when it gets reflected from the surrounding buildings. Environmental influences on GNSS-based positioning are obstacles, such as buildings along streets which can lead to blocking and reflection of satellite signals. Examples of operational conditions are atmospheric effects and changes in the satellite geometry. The tool has been developed to work either with Galileo or GPS or with both satellite constellations. Since Galileo is only preoperational, the simulator is based on the nominal 27 satellite Galileo constellation. Further, the simulator uses: atmospheric models to simulate the effect on satellite signals through Earth’s atmosphere, designs of representative environments e.g. urban and rural scenarios, and a method to simulate reflection of satellite signals within the close environment of the GNSS (Global Navigation Satellite System) antenna. The simulator thus models: the signal path from satellite to receiver, satellite availability, the extended pseudoranges caused by signal reflection, and the position accuracy based on a least squares adjustment of the extended pseudoranges. In order to handle temporal and spatial variations of the different errors sources in GNSSbased positioning, the simulator has been developed for observing variations in position accuracy over time, along a given test trajectory, at different latitudes, and with different azimuths of the environment around the receiver. The environments used by the simulator could be typical representative scenarios such as, a narrow city street, a t-cross or mountain landscape. The choice of not using specific streets or cities has been made in order to keep the simulator open for every kind of environment. In this paper a simulation example is illustrated and shows the possibilities for visualisation of a narrow street with buildings on each side over a simulation time period of 24 hours. The impact of using reflected satellites in the position domain is significant. The DOP values are in general lower, but the error ellipses can be up to 8 times larger and furthermore deviate in shape.
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