Global Positioning System Reference
In-Depth Information
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free electrons is what impacts electromagnetic waves passing through the ionosphere.
In order for gases to be ionized, a certain amount of radiated energy must be absorbed.
Hargreaves (1992, p. 223) gives maximum wavelengths for radiation needed to ionize
various gases. The average wavelength is about 900 Å (1 Å equals 0.1 nm). The
primary gases available at the upper atmosphere for ionization are oxygen, ozone,
nitrogen, and nitrous oxide.
Because the ionosphere contains particles that are electrically charged and capable
of creating and interacting with electromagnetic fields, there are many phenomena in
the ionosphere that are not present in ordinary fluids and solids. For example, the
degree of ionization does not uniformly increase with the distance from the earth's
surface. Instead, there are regions of ionization, historically labeled
D
,
E
, and
F
,
that have special characteristics as a result of variation in the EUV absorption, the
predominant type of ions present, or pathways generated by the electromagnetic field.
The electron density is not constant within such a region and the transition to another
region is continuous. Whereas the TEC determines the amount of pseudorange delays
and carrier phase advances, it is the layering that is relevant to radio communication
in terms of signal reflection and distance that can be bridged at a given time of the
day. In the lowest
D
region, approximately 60-90 km above the earth, the atmosphere
is still dense and atoms that have been broken up into ions recombine quickly. The
level of ionization is directly related to radiation that begins at sunrise, disappears at
sunset, and generally varies with the sun's elevation angle. There is still some residual
ionization left at local midnight. The
E
region extends from about 90-150 km and
peaks around 105-110 km. In the
F
region, the electrons and ions recombine slowly
due to low pressure. The observable effect of the solar radiation develops more slowly
and peaks after noon. During daytime this region separates into the
F
1 and
F
2 layers.
The
F
2 layer (upper layer) is the region of highest electron density. The top part of
the ionosphere reaches up to 1000 to 1500 km. There is no real boundary between
the ionosphere and the outer magnetosphere.
Ionospheric convection is the main result of the coupling between the magneto-
sphere and ionosphere. While in low altitudes the ionospheric plasma co-rotates with
the earth, at higher latitudes it is convecting under the influence of the large-scale
magnetospheric electric field. Electrons and protons that speed along the magnetic
field lines until they strike the atmosphere not only generate the spectacular lights of
the aurora in higher latitudes, but they also cause additional ionization. Peaks of elec-
tron densities are also found at lower latitudes on both sides of the magnetic equator.
The electric field and the horizontal magnetic field interact at the magnetic equator
to raise ionization from the magnetic equator to greater heights, where it diffuses
along magnetic field lines to latitudes approximately
[21
Lin
—
0.0
——
Lon
PgE
[21
15° to 20° on either side of
the magnetic equator. The largest TEC values in the world typically occur at these
so-called equatorial anomaly latitudes.
There are local disturbances of electron density in the ionosphere. On a small
scale, irregularities of a few hundred meters in size can cause amplitude fading and
phase scintillation of GPS signals. Larger disturbances of the size of a few kilome-
ters can significantly impact the TEC. Amplitude fading and scintillation can cause
receivers to lose lock, or receivers may not be able to maintain lock for a prolonged
period of time. Scintillation on GPS frequencies is rare in the midlatitudes, and am-
±
