3D Indoor Velocity Vector Determination Using GNSS Based Repeaters

Marc Francois, Nel Samama and Alexandre Vervisch-Picois

Abstract:	The present paper will describe a new method for indoor relative displacement positioning based on the use of 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 then the experimental set-up and results are presented. Finally, the combined use of both an absolute indoor positioning system [1] and this new relative displacement system is discussed for a typical indoor environment, such as a conference centre, an office building or a shopping mall. 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). 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. Various papers have already described the theory and have also experimentally shown the performances of such a technique [1], [2]. The purpose of the present paper is to develop this further by thinking now in terms of velocity measurements, rather than location measurements. It is also well known that GNSS receivers are carrying out two kinds of measurements: time measurements for pseudo-ranges determination and frequency or Doppler measurements for velocity determination. When dealing with a repeater based architecture, as we did for the location navigation solution, it is possible to show that the velocity solution leads to the outdoor antenna velocities (vx, vy and vz), i.e. zero since the antenna is clearly static. The point of interest for us is the fourth co-ordinate of the velocity solution that gives the clock bias drift (in an independent manner rather than by derivation of the clock bias taken from the location solution). The clock drift can be seen as the short time oscillator stability. When the indoor receiver is static, then the resulting clock drift is the same as that which would have been obtained if the receiver had been outdoors. Things are totally different once the indoor receiver moves: in such a case, the resulting clock drift will include the indoor motion (as long as the calculated velocities are zero), leading to an artificially increased clock drift value. This variation in the slope of the curve representing the clock drift versus time can then be used to define the radial velocity of the indoor receiver motion. A previous paper [3] described experimental results for a 1-D motion of the indoor receiver. When dealing with the 2-D and the 3-D approaches, it is important to remember that the information given by the clock drift is almost equivalent to Doppler information (i.e. defining the radial part of the actual velocity). Thus, a second clock drift measurement (from a second repeater) will give us a second radial velocity in a different referential. It is then possible to combine the two results in order to evaluate a relative displacement velocity vector. Assuming firstly a uniform displacement on a given floor of a building (i.e. constant “altitude”), these two values can be used to evaluate the relative displacement by integrating the 2-D velocity vector. This can be carried out with 2 repeaters (compared with the need for 3 repeaters for a 2-D absolute positioning system). In that sense, such a system could be used in some building locations where a direct line of sight is no longer available from all the repeaters: this approach has to be seen somehow as "dead reckoning". In case we need a 3-D velocity vector, we will have to consider three measurements from three repeaters. Nevertheless, a difficulty arises from the fact that the repeater approach is based on a sequential system: then, during the time needed to switch from one repeater to another, the indoor receiver moves, leading to an error in the velocity vector reconstruction process. The interesting aspect of this GNSS based relative displacement positioning system is that it uses the positioning system, in a degraded version, to give partial information on the displacement. For instance, it is not possible to obtain any positioning with only one or two repeaters, but it is still possible to obtain relative displacement information (respectively in 1-D and 2-D). Similarly, with three repeaters, it is possible both to achieve 2-D absolute positioning and 3-D relative displacement estimation: the important point to understand is that this is the same system that allows both approaches simultaneously. Of course, the next steps are to actually implement the combination of positioning and dead reckoning methods into a single processing unit to evaluate the real advantage of such a global system in typical indoor environments.
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:	2811 - 2819
Cite this article:	Francois, Marc, Samama, Nel, Vervisch-Picois, Alexandre, "3D Indoor Velocity Vector Determination Using GNSS Based Repeaters," 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. 2811-2819.
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