An Indoor Positioning System Using GPS Repeaters and AOA Measurements

Jean-Christophe Cousin, Nel Samama and Alexandre Veivisch-Picois

Abstract:	The present paper will describe an approach to simplify the infrastructure required for a repeater based indoor positioning system. The theoretical approach is given first, showing how it is possible to make the best use of both the repeater signals and a local antenna that has the ability to calculate the direction of arrival of these repeater signals. Then the experimental set-up and results are presented. Finally, the new system thus obtained is discussed for a typical indoor environment such as a conference room. A repeater based indoor positioning system uses so-called GPS repeaters that have the role of collecting GPS signals in the best possible environment (namely outdoors, on the roof of the building). Then the second role is that of a transmitting system to forward these signals into the indoor environment, with no further treatment other than amplifying them. In such a case, it is well known that the indoor computed navigation solution with any current GNSS receiver is the location of the outdoor receiving antenna. Previous papers have described the way it is possible to achieve indoor positioning using such repeaters, the theory states that four repeaters are required in order to calculate a 3-D location. An antenna that allows the direction of arrival (DOA) of an incident signal to be determined is interesting in the way it can simplify the repeater based infrastructure. Previous works have shown the feasibility of such antennas, but not yet at the mobile terminal end. To illustrate this point, let us just imagine that such an antenna already exists: in such a case, as long as one could measure the distance separating the repeater indoor transmitting antenna and the receiver, the localisation of the receiver can be calculated in 3 dimensions. This statement assumes that the direction of arrival is defined by two spatial angles (as defined in a spherical co- ordinates reference system). The global system is then reduced to a single repeater that has already demonstrated its simplicity. Of course, there are a few technical issues to be addressed prior to implementing this ideal system. The first concerns the actual attitude of the mobile receiver: we decided to work on the realisation of this DOA antenna at the mobile terminal end. This direction of arrival is of no help unless one has an indication on the real 3-D orientation of the terminal. This could be achieved by the addition of a 3-D inertial system at the cost of an increased complexity of the terminal. The second issue is to be able to have a precise determination of the indoor distance between the repeater and the receiver. Until now, the repeater-based approaches have shown that only the difference of distances at the time of transition between two repeaters can be obtained accurately. Although solutions exist to find this distance directly, this is not the purpose of this paper: so we decided to use the DOA measurements together with a repeater system using two repeaters. In this case, taking into account that the DOA antenna is able to give the two directions of arrival (from the two repeaters), the constraint on the mobile attitude determination is much less. This paper makes a complete analysis of the underlying theory and gives the details of the implemented solution, namely two repeaters and a DOA antenna at the receiver's end. The inertial sensor that outputs the 2-D orientation of the gravitational force has not yet been deployed. Both theoretical and technical aspects of the realisation of our DOA antenna are described, together with details of the processing required to calculate the various angles. Final results show localization accuracy in a large open space environment that lies in the 1 to 2 meter range. This is obtained by combining the distance calculation, the calculations of the four angles (two angles for each repeater) and the inclination measurement of the terminal (that was kept horizontal for the purposes of this paper). Accuracy achieved for the global system is commented o n . It is obvious that the system is quite complex: our purposes are mainly to discuss such issues as the DAO antenna at the terminal end or the combined use of attitude measurements and distance measurements. The required antenna is rather big for mobile personal devices, but could have a real interest when thinking in terms of cars, trucks or trains in a tunnel, for the transportation of dangerous goods for example. Inertial sensors have already been used for pedestrian navigation modules and our approach is much simpler in terms of the sensors used (but this is not within the scope of this paper to describe this part). The next steps are to actually implement the system in a portable device and to evaluate the efficiency of the localisation achievable in a real indoor environment (such as an office building or a conference centre).
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2743 - 2749
Cite this article:	Cousin, Jean-Christophe, Samama, Nel, Veivisch-Picois, Alexandre, "An Indoor Positioning System Using GPS Repeaters and AOA Measurements," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2743-2749.
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