Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 295

quencies and multipath's frequency-selective characteristics. There are also oppor-
tunities to explore processing of multiple polarizations, although many antenna
designs exhibit predominantly linear polarization response at low elevation angles
of many multipath arrivals. Improved receiver processing techniques may still be
developed, as may better theoretical understanding of capabilities and limitations of
multipath mitigation. Most multipath mitigation techniques incur practical conse-
quences, such as increased receiver complexity and poorer performance in noise and
interference, that must be evaluated on a case-by-case basis.

6.4 Ionospheric Scintillation

Irregularities in the ionospheric layer of the Earth's atmosphere can at times lead to
rapid fading in received signal power levels [26-28]. This phenomenon, referred to
as ionospheric scintillation, can lead to a receiver being unable to track one or more
visible satellites for short periods of time. This section describes the causes of iono-
spheric scintillation, characterizes the fading associated with scintillation, and
details the effects of scintillation upon the performance of a receiver.
The ionosphere is a region of the Earth's atmosphere from roughly 50 km up to
several Earth radii where incident solar radiation separates a small fraction of the
normally neutral constituents into positively charged ions and free electrons. The
maximum density of free electrons occurs at an altitude of around 350 km above the
surface of the Earth in the daytime. Most of the time, the principal effect of the pres-
ence of free electrons in the ionosphere is to impart a delay on the signals (see Sec-
tion 7.2.4.1). However, irregularities in the electron density occasionally arise that
cause constructive and destructive interference among each signal. Such irregulari-
ties are most common and severe after sunset in the equatorial region (within 20º

from the geomagnetic equator). High-latitude regions also experience scintillation,
which is generally less severe than in the equatorial region, but may persist for long
periods of time. Scintillation is also more common and severe during the peak of the
11-year solar cycle.
In the absence of scintillation, a simplified model for one particular signal as
seen by a receiver is:

+ / −

() () ( ) ()

rt = 2 Pst cos ωφ t + + nt (6.50)

where P is the received signal power (in watts), is the carrier frequency (in radi-
ans/second), s ( t ) is the normalized transmitted signal, and n ( t ) is noise.
Scintillation causes a perturbation to both the received signal amplitude and
phase, and the received signal in the presence of scintillation may be modeled as
[29]:

() () ( ) ()

rt = 2 P ⋅ δ P st ⋅ cos ω t φ + δφ + nt + (6.51)

where P is a positive, unitless parameter that characterizes amplitude fading due

to scintillation, and

δ

δφ is a parameter with units of radians that represents phase