A method of determining location of a user device includes receiving global navigation satellite system (GNSS) fix data that represents GNSS calculated position of the user device. The method further includes receiving signal strength data associated with each satellite communicating with the user device, and receiving map information regarding environment surrounding the user device. The received GNSS fix data and signal strength data is provided to a non-linear filter, wherein the non-linear filter fuses the GNSS fix data and signal strength data to generate an updated position estimate of the user device. In addition, the non-linear filter utilizes probabilistic shadow matching estimates that represent a likelihood of received signal strength data as a function of hypothesized user device locations within the environment described by the received map information.