Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 284

Near echoes Far echoes

Delay

Figure 6.9 Canonical power-delay-profile for land-mobile satellite channel.

each Poisson distributed, described by different Poisson parameters. Multipath
phases are modeled as independent and identically distributed over 360º. Extensive
tables of statistical parameters for these components are provided in [16] for many
different environments (e.g., open, rural, urban, highway) and satellite elevations.
Time variation of the multipath characteristics is described in [16] using sec-
ond-order statistics based on Doppler spectra, with bandwidth established by the
movement of satellites and the receiver.
As noted in [16], the LOS does not always exist between the receiver and trans-
mitter, particularly for low-elevation angles. For instance, trees or buildings along a
road may block signals from below a certain elevation angle. In urban environ-
ments, 97% of signals were blocked when the transmitter was at an elevation angle
of 15º, and blockage of lower-elevation satellites was also not uncommon even in
rural environments, due to shadowing by trees. In these circumstances, it is entirely
possible for a receiver to track a reflection rather than the direct signal, causing large
pseudorange errors.
Over the range of environments and elevation angles considered in [16], the
average power of the near echoes never exceeds 16.5 dB relative to the average

power of the direct path. The mean power levels of the near echoes fall off at a wide
variety of rates, ranging from 1 to 37 dB/

−

s depending on the elevation angle
and environment. The range of delays associated with the near echoes is from 0 to
0.6

µ

µ s. No significant far echoes occur beyond 5 to 15 µ s, and the mean power levels

of the far echoes are within the range of 30 dB (relative to an unshadowed

direct path). Doppler spreads are dominated either by the satellite motion or the
receiver motion. Delay spreads are often multiple microseconds, while Doppler
spreads for a stationary receiver can be tenths of hertz—but for a receiver in a vehi-
cle, it can be many hertz, particularly for multipaths with small excess delay.
Indoor multipath has very different characteristics depending on the placement
of the building relative to other buildings, satellite elevation, whether the receiver is
in an interior area deep within the building or near a window, what floor the
receiver is on, and the building materials. Except in cases where the direct path is
shadowed, multipath with significant values of MDR typically arises from reflec-
tions near the receive antenna, thus having small excess delay. Indoor data discussed

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