Simulating Non-LOS GNSS Reflected Signals in Urban and Dense Urban Environments

The work described in this presentation is carried out within the frame of the QualiSaR project on development of a qualification procedure for the use of Galileo satellite receivers for safety relevant applications. The QualiSaR project is funded by the EU seventh framework programme and is lead by the Technical University Braunschweig. This presentation focuses on part of the project, namely development of a simulator to be used in the planning phase of the qualification procedure for Galileo receivers. The qualification procedure should verify measurement quality of the satellite-based localisation services as provided by Galileo receivers. The quality of Galileo-based localisation in for instance ground transportation highly depends on the environmental and operational conditions. Examples of environmental influences are obstacles, such as high buildings and trees along streets, as well as topographical objects, such as mountains which can lead to blocking and reflection of satellite signals. An important part of the QualiSaR qualification procedure is test measurements. Before test measurements are performed, design of the test, the test track, test environment etc. must be determined, and for that purpose the simulator QualiSIM has been developed. The simulator will be used as part of the QualiSaR project in determination of test settings and test trajectories. This presentation describes the simulator and results of its use in different environments. The purpose of the simulator is to provide a representative tool for simulating position accuracy under various GNSS conditions, on various trajectories, in different types of environment. Since Galileo is at present only pre-operational, the simulator is based on the nominal 27 satellite constellation for Galileo. Further, the simulator uses atmospheric models to simulate the effect on satellite signals through Earth’s atmosphere, it uses designs of typical environments e.g. urban and rural scenarios, and it uses windows of reflection points to simulate reflection of satellite signals within the environment around the Galileo receiver. The simulator thus models the signal path from satellite to receiver, it models satellite availability, the extended pseudoranges caused by one time reflected signals as multipath, and it models the position accuracy based on a least squares adjustment of the modelled pseudoranges. As an add-on the simulator can also include GPS, to simulate position accuracy for a multiconstellation scenario. The simulator is developed in Matlab and C, and it is controlled through a graphical users interface. QualiSIM is developed to simulate position accuracy obtainable with high sensitivity receivers and pseudorange observations. It can, however, be initialized to also illustrate performance with high accuracy carrier phase-based Galileo receivers. The position accuracies simulated by QualiSIM are provided as output along with error bounds normally used in positioning. This makes it possible to evaluate whether the position accuracy is within an error bound of for instance two times the standard deviation (95% significance in one dimension) which is often used for evaluation purposes. In areas with much multipath the position accuracy will more often exceed the error bound compared to open areas with no obstructions or reflecting objects. The simulator is able to simulate various environmental conditions, various locations, various time of day, various orientations of the test trajectory etc. in order to account for the various GNSS error sources. Also in order to handle temporal and spatial variation of the different errors sources, the simulator has been developed for simulating variation in position accuracy over time, along a given test trajectory, at different environment latitudes, and with different azimuths of the environment around the receiver. In short, QualiSIM has been developed to comply with the following three overall criteria: A tool for easy design of test runs, a tool for easy change of environmental settings, and a tool for visualisation of results. The most important and novel part of the simulator is the modelling of multipath which will be described in detail in the presentation. In urban areas, in mountain regions, and in forests reflection of satellite signals has a very important influence on position accuracy. Therefore, in order to simulate position accuracy, it is important to model the signal paths for non line of sight satellites. This provides a more realistic estimate of the position accuracy in the given environment compared to using only line of sight satellite signals. The environments used by the simulator are defined to be representative scenarios, representative of for instance a city or mountain landscape in general. Specific streets or cities have not been used. On the contrary the purpose is to keep the simulator open to be used with for instance a typical environment in a small mountain village as well as in an urban canyon of a big city. This also makes it possible in another context to design an environment which is similar to a specific street. This presentation and paper first describes the simulator and the implementation in detail. Then examples of simulation results obtained with QualiSIM for different environments and locations are discussed. The results illustrate how the simulator can be used for testing purposes in two different ways. One approach is to use the simulator to define test scenarios which provide the best possible opportunities for meeting requirements of a given test. This approach is illustrated in details. Another approach is to use the simulator for evaluating a given test scenario before the field test is actually carried out in order to find the most representative time and /or place for performing the field tests, or in order to evaluate which parameters are most relevant to vary and/or evaluate during the field test when performance of a GNSS receiver is to be validated.