2D Indoor Dynamic Positioning Using GNSS Based Repeaters

A. Vervisch-Picois, A. Bideau, M. Jeannot, N. Samama

Abstract:	The present paper will describe a method for indoor dynamic positioning using a GNSS repeater based indoor positioning system. The theoretical approach is given first, showing how it is possible to make the best use of current GNSS receiver capabilities, and how to make the transition from the static to the dynamic scheme. Then, the experimental set-up and results are presented for different types of movements. The indoor positioning system is based on the use of socalled GNSS repeaters that have the role of collecting GNSS signals in the best possible environment (namely outdoors, on the roof of the building). 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. Various papers have already described the theory and have also experimentally shown the performances of such a technique [1], [2] but always for static positioning. The purpose of the present paper is to show that it is also possible to calculate the indoor location of a mobile through the use of a repeater based infrastructure, for typical pedestrian behaviour. One of the major issues in achieving such a dynamic scheme is the determination of the precise time of transitions, together with their identification (i.e. the “name” of the repeater that is currently emitting). Indeed the main idea of the switched GNSS repeater architecture is to use a sequential mode based on the following steps: i) turning on a repeater while turning off all the others simultaneously, ii) to obtain, through the reading of the induced skip of the code loop phase, the difference in distance from the turned on repeater to receiver’s antenna and the turned off repeater to receiver’s antenna. It has been shown in [1] that with a system composed of 3 repeaters, a two dimensional position can be calculated with only 2 such differences. But it is necessary to know precisely the instants of these 2 transitions to lead to the calculation, the works described in paper [2], show the difficulties of determining precisely these instants of transition. For dynamic positioning, it is critical to have real time knowledge of the transition corresponding to the value of the measured range difference to calculate each point of the trajectory rapidly. The solution tested was to synchronise the sequential switcher to GPS time with the PPS signal given by a GPS receiver linked to the outdoor antenna. Thus, the transitions of the sequence happen at whole and identified seconds of GPS time. Then the indoor receiver can easily identify the transitions and achieve the calculation of theposition. Another point is the refreshing rate of the calculation. In static mode this is not a problem because the position is always the same so a position was computed using a whole sequence of switches, but in dynamic mode a faster rate is required. So as not to have to wait for the end of a whole sequence, the position can be calculated each time there is a new transition which corresponds to a new measurement. This is called the “sliding mode”. Thanks to the synchronisation of the sequential switcher with PPS and the use of the “sliding mode” for positioning it is possible to calculate with a rate of one per second, which is typical and sufficient for a walker displacement. The final paper will present the experimental results for different displacements of a mobile which were tested using the Repeater technique with 4 repeaters. We consider two trajectories: straight line and circle for quite short distances corresponding to the typical evolution of a pedestrian in a building. The trajectories will be represented as the interpolation of the 1 Hz rate calculated points. The use of 4 repeaters for 2 dimensional positioning allows us to implement the “least square algorithm”. The comparison of performance between “least square” and “simple linearization” in terms of precision will be presented. A comparison between static and dynamic performance will conclude the paper including elements of a first approach to the question of the influence of multipath on the positioning in both cases. Dynamic mode experiments with GNSS based repeaters are in the natural continuity of the previous works of the Navigation Group ([1]) which have shown that very accurate indoor positioning (in the one meter range in 3D) can be obtained. Experimental results for dynamic remain in a very comparable accuracy of 1-2 meters range with some differences. These first works on dynamic positioning lead us to discussions about the difference of performance between dynamic and static positioning and what should be done to improve the system considering multipath for example. The final goal of all this is to bring the system closer to conditions which exist in reality for pedestrians in most typical indoor environments.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	2484 - 2493
Cite this article:	Vervisch-Picois, A., Bideau, A., Jeannot, M., Samama, N., "2D Indoor Dynamic Positioning Using GNSS Based Repeaters," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 2484-2493.
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