Optimised tracking loop for multipath mitigation case of repeater based indoor positioning system

The present paper will describe a new code tracking loop architecture for multipath mitigation. This architecture is well suited for the so-called "repeater based indoor positioning system". The problem of multipath mitigation is of uppermost importance in time measurement system, and specifically indoors [1] where the antenna is surrounded by many reflectors. The new proposed loop is designed in order to cope with short delay multipath, which is typical of an indoor environment. The theory of the GNSS repeater based approach, together with experimental results validating its principle, was presented in previous paper [2]-[3]. The main idea of this method consists in using GNSS repeaters in a sequential mode in order to carry out measurements of code phase jumps at the instant of transition from one repeater to the next. Once obtained, three such jumps allow the indoor location calculation, through a classical GNSS computation. Electromagnetic simulations of the indoor propagation [1] have shown that in a typical environment, the effect of multipath could largely decrease the positioning accuracy. On the other hand, these simulations have also shown that, usually, multipath rays of more than few tens of meters are of reduced amplitude. In such a case, the resulting effect on positioning is negligible. Thus our main gaol was to find a way to remove "small delay" multipath. The paper will present the simulated performances of the loop, showing that the direct signal can be found even in very severe configurations where, for example, the reflected path is twice or three times the power of the direct one. These results will also show that the proposed architecture is no longer acceptable for multipath having relative delays of more than 150 meters. Experimental results are also available in two different cases: • the guided mode, where the receiver (a NordNav R-30) input is directly connected to the output of a wired system simulating the multipath signals, • the open space mode where free propagation is considered, in a repeater based approach (including cycling repeaters). The new code loop architecture has been implemented in the NordNav R-30 in order to be tested in real experiments. The NordNav allows for user specific designs since it is a software receiver that incorporates a software development kit and the corresponding library of functions. The modular structure of the NordNav software receiver allows relatively simple modifications of the tracking loops. C++ adaptation of the functions that manage operations performed by the GPS loops is allowed. One of these functions is more specifically used for the closing loop operation. Thus, it is at this level that the modifications are carried out. This approach is bound to allow for better indoor positioning using a repeater system, but also outdoors for standard GNSS receivers. The main limitation for outdoors is that the proposed loop does not mitigate multipath rays having relative delays more than 150 meters.