Global Positioning System Reference
In-Depth Information
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(
90°
)
. The multipath is largest for a satellite in the horizon (reflection on vertical
surface). In the case of reflection from a horizontal surface, the multipath has a reverse
dependency, i.e., it is largest for satellites at the zenith, as can readily be verified.
Fenton et al. (1991) discuss one of the early implementations of narrow correla-
tion in C/A-code receivers. Narrow correlator technology and on-receiver processing
methods to reduce carrier phase and pseudorange multipath effects are extensively
documented in the literature, e.g., van Dierendonck et al. (1992), Meehan and Young
(1992), Veitsel et al. (1998), and Zhdanov et al. (2001). If the phase shift
β =
changes
rapidly, one might even attempt to average the pseudorange measurements. In addi-
tion to sophisticated on-receiver signal processing, there are several external ways to
mitigate multipath.
θ
Since multipath can also arrive from below the antenna (due to edge diffraction),
a ground plate is helpful. The ground plate is usually a metallic surface of
circular or rectangular form.
•
[24
Partial multipath rejection can be achieved by shaping the gain pattern of the
antenna. Since a lot of multipath arrives from reflections near the horizon, mul-
tipath may be sharply reduced by using antennas having low gain in these direc-
tions.
•
Lin
—
2.0
——
Lon
PgE
Improved multipath resistance is achieved with choke rings. These are metallic
circular grooves with quarter-wavelength depth.
•
Highly reflective surfaces change the polarization from right-hand circular (sig-
nal received directly from the GPS satellite) to left-hand circular. GPS antennas
that are designed to receive right-hand polarized signals will attenuate signals
of opposite polarization.
•
[24
Arrays of antennas can also be used to mitigate multipath. Due to a different mul-
tipath geometry, each antenna sees the multipath effect differently. Combined
processing of signals from all antennas allows multipath mitigation (Fu et al.,
2003). In a design proposed by Counselman the antenna elements are arranged
along the vertical rather than the horizontal platter (Counselman, 1999).
•
Since the geometry between a GPS satellite and a receiver-reflector repeats ev-
ery sidereal day, multipath shows the same pattern between consecutive days.
Such repetition is useful to verify the presence of multipath by analyzing the
repeatability patterns and eventually model the multipath at the station. In rela-
tive positioning the double-difference observable is affected by multipath at both
stations.
•
In practical applications, of course, the various satellite signals are reflected at dif-
ferent objects. The attenuation properties of these objects generally vary; in some
cases attenuation might even depend on time. Since the angle of incident also affects
attenuation, it can readily be appreciated that the multipath is a difficult error source
to deal with. It is common practice not to observe satellites close to the horizon in
order reduce multipath.
Equations (6.100) and (6.101) are useful to gage the multipath, in particular the
multipath effect on the pseudoranges, if dual-frequency observations are available.
