Global navigation satellite systems (GNSSs) can provide reliable positioning information under optimum conditions, where at least\r\nfour satellites can be accessed with sufficient quality. In critical situations, for example, urban canyons or indoor, due to blocking\r\nof satellites by buildings and severe multipath effects, the GNSS performance can be decreased substantially. To overcome this\r\nlimitation, we propose to exploit additionally information from communications systems for positioning purposes, for example,\r\nby using time difference of arrival (TDOA) information. To optimize the performance, hybrid data fusion and tracking algorithms\r\ncan combine both types of sources and further exploit the mobility of the user. Simulation results for different filter types show\r\nthe ability of this approach to compensate the lack of satellites by additional TDOA measurements from a future 3GPP-LTE\r\ncommunications system. This paper analyzes the performance in a fairly realistic manner by taking into account ray-tracing\r\nsimulations to generate a coherent environment for GNSS and 3GPP-LTE.
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