Carrier and Code Tracking - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 97

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Time delay [chip] Time delay [chip]

FIGURE 7.16. Constructive and destructive multipath interference. The solid thin line rep-
resents the original triangular correlation function; the dashed line the multipath correlation
function, and the third line is the sum of the two.

7.6 Multipath

Of the multiple error sources associated with GNSS signal processing, multipath
directly impacts the code tracking performance. With the prior description of how
code tracking is implemented, it makes sense to investigate how multipath impacts
the code tracking loop.
The signal observed at the receiver is a distorted version of the one transmitted.
One distortion effect is called multipath propagation or, for short, multipath .
If the receiver can directly see the satellite, a part of the received signal has
propagated via the direct path from the satellite to the receiver. This signal com-
ponent is delayed according to the distance between the satellite transmitter and
the receiver. The signal component propagated via the direct path usually is by far
the strongest part of the received signal.
In addition to the direct signal, the receiver may observe other signals propa-
gating via other and longer paths. This can happen if the radio wave reaches the
receiver after interaction with one or more objects/obstacles in the environment.
Different kinds of interactions between radio waves and objects exist, but in the
context of GNSS it suffices to think of an interaction as a reflection altering the
direction of propagation, amplitude, polarity, and phase of the radio wave.
First we consider the simple case where the transmitted signal reaches the re-
ceiver via two paths. Thus, the received signal consists of two components: a
direct component and a signal component reaching the receiver via a reflection on
a nearby building. In this case the reflected signal component is a delayed, phase-
shifted, and attenuated version of the line-of-sight signal component. Due to the
change in phase and delay, the two signal components interfere. In case the re-
flected and the direct signals are in phase, the amplitude of the sum is larger than
the amplitude of each of the components. This we call constructive interference ;
see Figure 7.16. On the other hand, if the direct and reflected signals are out of
phase, the amplitude of the sum signal decreases and we talk about destructive
interference . In case the relative phase between the direct and reflected signals