Robust Positioning in the Presence of Multipath and NLOS GNSS Signals

GNSS signals can be blocked and reflected by nearby objects, such as buildings, walls, and vehicles. They can also be reflected by the ground and by water. These effects are the dominant source of GNSS positioning errors in dense urban environments, though they can have an impact almost anywhere. Nonline-of-sight (NLOS) reception occurs when the direct path from the transmitter to the receiver is blocked and signals are received only via a reflected path. Multipath interference occurs, as the name suggests, when a signal is received via multiple paths. This can be via the direct path and one or more reflected paths, or it can be via multiple reflected paths. Figure 1 illustrates this. Within the GNSS community, it is commonplace to classify NLOS reception as multipath. However, the two effects are not the same; their error characteristics are quite different. As a reflected path is always longer than the direct path, NLOS reception always results in a positive ranging error that is independent of the signal and receiver design. By contrast, the coherent nature of multipath interference can produce both positive and negative ranging errors and these vary with the signal and receiver designs [1].