Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 280

objects, or even grazing multipaths reflected from distant objects, can arrive at short
delays (e.g., tens or hundreds of nanoseconds) after the arrival of the direct path. Such
multipaths distort the correlation function between the received composite (direct
path plus multipaths) signal and the locally generated reference in the receiver. They
also distort the composite phase of the received signal, introducing errors in
pseudorange and carrier phase measurements that are different among the signals
from different satellites, and thus produce errors in position, velocity, and time.
Shadowing is excess attenuation of the direct path, typically introduced when
the direct path propagates through foliage or a structure. When the multipath does
not experience the same excess attenuation, the received power of the multipath may
be even greater than the received power of the shadowed direct path. Such a phe-
nomenon can occur in outdoor situations as that portrayed in Figure 6.8, and also in
indoor situations, such as when the direct path passes through walls or ceiling and
roof, while the multipath is reflected from another building and arrives through a
window or other opening. Consequently, shadowing of the direct path and
multipath has combined effects on the relative amplitudes of direct path and
multipaths. In some cases, shadowing of the direct path may be so severe that the
receiver can only track the multipath(s).
The error introduced by multipaths depends upon their delays, but also their
power and carrier phase relative to those of the direct path. Multipaths with
received power much less than that of the direct path produce little distortion of the
received signal and consequently produce little error. The received carrier phase of
the multipath relative to that of the direct path also has a direct influence on the
degree and character of the distortion.
Typically, consideration of multipath in a GNSS context emphasizes its effect on
signal code and carrier tracking accuracies, since these receiver functions are more
sensitive to multipath degradation than signal acquisition or data demodula-
tion—under most situations, multipath conditions that would cause observable deg-
radation to acquisition or data demodulation also introduce large degradations to
pseudorange accuracy. Effects on acquisition and data demodulation are assessed
using techniques developed in digital communications [12], and so the remainder of
this discussion focuses on tracking performance.

Shadowing

Figure 6.8 Outdoor multipath situation.