Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 293

mitigation techniques have been incorporated into production receivers, while oth-
ers remain research topics.
A number of considerations arise in assessing multipath mitigation techniques.
Good performance in realistic multipath conditions must be provided. Robustness
is also important, ensuring that performance is satisfactory over the range of envi-
ronmental conditions (including noise and interference) in which the receiver must
operate. Implementation complexity is also a factor, as are any restrictions on how
the receiver would be employed (such as requirements for multipath characteristics
to be time-invariant over long periods of time or restrictions to use by fixed receiv-
ers.) While multipath mitigation techniques remain important research topics, this
section outlines some of the strategies that have been pursued and remain areas of
active work.
One important group of multipath mitigation techniques attempts to reduce the
reception of multipath signals, reducing the need to discriminate against these
multipaths by the receiver processing. Antenna siting, and even removal or modifi-
cation (e.g., coating with RF-absorptive materials) of reflective structures in the
vicinity of the antenna can produce significant benefits. In benign environments,
such as an open field, placing an antenna closer to the ground can decrease observed
multipath errors. The reason is that with the antenna closer to the ground, the
multipath reflections from the ground experience shorter excess path delays that
tend to produce smaller multipath errors, as shown in Figure 6.13. Conversely, in
environments with obstacles near the horizon, the opposite course of action is often
beneficial—raising the antenna decreases antenna gain toward dominant reflectors
that produce multipath.
Antennas can also be designed to attenuate multipath reflections, particularly
multipaths that arrive at elevation angles near or below the horizon, where desired
signals are not expected to arrive. Choke ring antennas have been particularly suc-
cessful for mitigating multipath arrivals from the ground or low-elevation scatter-
ers. In short-baseline differential systems, multipath errors can also be reduced
through calibrations that measure multipath error based on satellite position [20].
Techniques for multipath mitigation receiver processing can be divided into
nonparametric and parametric processing. Nonparametric processing employs
discriminator designs that are less sensitive to multipath-induced errors, while para-
metric processing attempts to estimate parameters associated with the multipath
and then correct for their effect on the estimate of the direct path's TOA.
Some nonparametric techniques, such as [21, 22], rely on precise prior knowl-
edge of the signal's correlation function and employ novel receiver processing
approaches that attempt to match the ideal correlation function to the observed cor-
relation function in multipath. Nonparametric techniques in most common use,
however, are based on variations of early-late processing described in Chapter 5.
They go beyond the narrow correlator approach considered in Section 6.3.2, how-
ever, by either time-gating the reference signal or by computing two pairs of early
and late correlations with different early-late spacings. A number of similar tech-
niques have been developed and implemented in different brands of receivers.
An excellent overview of these modified-reference techniques, their capabilities,
and their limitations is provided in [23]. One interpretation is that this processing is
equivalent to generating a modified locally generated reference signal that does not