FIG. 1 illustrates a typical ground-based positioning system 100 that includes terrestrial transmitters 111, 113, 115 and 117, each of which is located among various environmental objects 190. Such systems encounter various challenges in urban or indoor environments. One particular challenge relates to a “multipath” effect on signals that are received by receiver 120. Ideally, a signal transmitted by a transmitter would travel over the shortest distance separating the receiver and that transmitter. Such a signal is illustrated by signal 133, and is referred to as a “line-of-sight” (LOS) signal. In urban environments, however, many signals are reflected, diffracted and/or scattered by any number of objects before arriving at the receiver. These signals are illustrated by signals 131 and 135, and are referred to as “multipath” signals because they reach the receiver after traveling along multiple segments corresponding to difference reflections. Signal 137, by illustration, represents a blocked signal that never reaches receiver 120. It should be noted that the various LOS and multipath signals in FIG. 1 and FIG. 2 are shown are shown for exemplary purposes only. Many other multipath signal paths may exist in such environments and the paths shown may not be the most predominant ones possible in an actual environment.
Measurements of the travel time for a signal between a transmitter and a receiver may be used as a measure for the distance over which the signal traveled, but that distance is not always an accurate measurement of the shortest distance (or “range”) between the transmitter and the receiver when the signal is a multipath signal. To illustrate this, FIG. 1 shows LOS distance 141 that is shorter than the path distance traveled by signal 131. Unfortunately, the multipath signal 131 produces an inaccurate measurement of the distance between the transmitter and the receiver that, if used during trilateration processing, will impair the precision of any estimated position of the receiver. In extreme situations, such as in dense urban canyon or deep indoor locations, the signal is totally attenuated by the objects between the transmitter and the receiver so that it is impossible to obtain an accurate LOS distance measurement by investigating the received signal delay profile alone. In some cases, reflected paths due to the multipath effect may be a few hundred meters longer than the LOS distance.
Additional complications are encountered with signals that travel over both LOS paths and multipath paths, making it more difficult for the receiver to retrieve the earliest arriving LOS signal, as well as estimate the transmission time associated with the signal. This directly causes a range measurement error. As a result, the calculated trilateration positioning solution is erroneous as well.
When a receiver receives a combination of both LOS and multipath signals from a given transmitter, the receiver may be able to extract the LOS signal component, in which case the signal may be considered an LOS signal from the standpoint of position location. Alternatively, when the presence of the multipath component is so predominant that it measurably alters the time-of-arrival measurement, the signal may be considered a multipath signal.
Multipath associated problems can be combated at several levels. For example, transmitters can be placed at higher altitudes to increase the likelihood of LOS signal reception. Alternatively, super-resolution, trilateration or other algorithms may be modified to increase likelihood of seeing a LOS signal or to better handle multipath signals. However, improvements to algorithms may not sufficiently address these problems, and deployment of higher-altitude transmitters may not be feasible.
One solution to these and other problems is described in more detail below.