Indoor Code and Carrier Phase Positioning with Pseudolites and Multiple GPS Repeaters

I. Petrovski, K. Okano, S. Kawaguchi, H. Torimoto, K. Suzuki, M. Toda, J. Akita

Abstract:	Indoor positioning is of great interest for a number of applications. Our research was initially aimed to support the Pedestrian Intelligent Transportation Systems (PITS) project. This project was organized as part of a nationwide ITS program by the Ministry of Land, Infrastructure and Transport of Japan through National Institute for Land and Infrastructure. Our company, GNSS Technologies Inc. together with Hitachi Ltd. Was in charge for pseudolite implementation in the NTT Communications Consortium. This project required seamless indoor-outdoor positioning so that, for example, a person can go through a tunnel or building or along urban canyon without interruption in the positioning information. On the other hand our research is also conducted outside of the PITS project and apart from seamless indoor-outdoor positioning for ITS aiming at applications in robotics and automation. Indoor positioning comes up against number of obstacles. In many respects it is different from typical GPS positioning outdoors. For instance, although the error budget indoor doesn't include atmospheric and orbital errors, it suffers from a potentially high multipath component. However, the main challenge in indoor positioning is to find an appropriate algorithm for positioning. Even in the case of pseudorange positioning, a solution for a classic system of nonlinear equations can present a significant challenge. In the conventional approach, such as least squares, the initial position error plays a critical role, because nonlinear pseudorange equations require linearization close to the true position. In the case of closely located pseudolites the error in directional cosines matrix due to initial position error is much more significant than for distant GPS satellites. Therefore, directional cosines matrix varies substantially as a function of the initial position. A few meters in case of pseudolites will cause a far more significant error in the matrix than thousands of kilometers for GPS. The positioning with carrier phase data encounters even more obstacles. First of all it requires a rather good initial position estimation, delivered usually by pseudorange positioning. The other problem is that pseudolites are fixed in the space, which doesn't allow to implement conventional carrier phase technique without imposing some conditions on the rover receiver movement, which is not always acceptable from the practical point of view. The other way to implement the conventional technique is to start from a known location. The special procedure and algorithm had been developed to overcome the above obstacles, which allowed us to achieve decimeter-level positioning indoors in low multipath environment. The developed procedure doesn't require knowledge of the initial rover receiver position, and doesn't impose any restrictions on rover receiver dynamics. The proposed technique uses specially developed multiple free reference station approach. This approach uses a pseudorange positioning at the first stage to define the initial position guess. The code-based algorithm we have developed looked at the variation in behavior of the estimated position in cases of error in initial position and in cases without error. Such variations give one a way to find the correct initial position with a search algorithm, much the same way as for carrier phase ambiguity resolution. Our indoor positioning software can be represented by two main blocks, one is initial position estimation, based on pseudorange observations and the other is carrierphase positioning component. The algorithm doesn't require knowledge of the initial position and use pseudorange and carrier-phase observations. During the indoor test we translated GPS signals received at the roof antenna and retransmitted them into the premises. Indoors, one can clearly use only a single GPS satellite out of all those signals received at the roof antenna. This may serve as a separate signal source. However, using multiple antennas with obstructed view and FDMA technique, we have constructed an indoor positioning system based on multiple GPS repeaters and have had it successfully tested. The test, which was conducted in Future University - Hakodate University, has demonstrated centimeter-level indoor initialization accuracy in low multipath environment and the possibility to build indoor positioning systems based on multiple GPS repeaters as the signal sources.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1135 - 1143
Cite this article:	Petrovski, I., Okano, K., Kawaguchi, S., Torimoto, H., Suzuki, K., Toda, M., Akita, J., "Indoor Code and Carrier Phase Positioning with Pseudolites and Multiple GPS Repeaters," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1135-1143.
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