Global Positioning System Reference
In-Depth Information
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The brightness temperatures are substituted into (6.57) to get the opacity. If the
linear regression line through the computed opacities does not pass through the
origin, the gain factor
G
is adjusted until it passes though the origin. If the regression
coefficient of the linear fit is better than a threshold value, typically
r
0
.
99, the
tip curve calibration is accepted. The time series in Figure 6.11 show the history of
passed tip curve calibrations at the various microwave frequencies. Additional details
on radiometer calibration are best obtained from manufacturers.
The tipping curve calibration assumes that we know the microwave cosmic back-
ground brightness temperature
T
cosmic
=
2
.
7 K. Arno Penzias and Robert Wilson
received the Nobel Prize for physics in 1978 for their discovery of the cosmic back-
ground radiation. Conducting their radio astronomy experiments, they realized a
residual radiation that was characteristically independent of the orientation of the
antenna.
=
[21
6.4 IONOSPHERIC REFRACTION
Lin
—
0.0
——
No
PgE
Coronal mass ejections (CMEs) and extreme ultraviolet (EUV) solar radiation (solar
flux) are the primary cause of the ionization (Webb and Howard, 1994). A CME is
a major solar eruption. When passing the earth it causes at times sudden and large
geomagnetic storms, which generate convection motions within the ionosphere, as
well as enhanced localized currents. The phenomena can produce large spatial and
temporal variation in the TEC and increased scintillation in phase and amplitude.
Complicating matters are coronal holes, which are pathways of low density through
which high-speed solar wind can escape the sun. Coronal holes and CME are the two
major drivers of magnetic activities on the earth. Larger magnetic storms are rare but
may occur at any time.
Solar flux originates high in the sun's chromosphere and low in its corona. Even a
quiet sun emits radio energy across a broad frequency spectrum, with slowly varying
intensity. EUV radiation is absorbed by the neutral atmosphere and therefore cannot
be measured accurately from ground-based instrumentation. Accurate determination
of the EUV flux requires observations from space-based platforms above the iono-
sphere. A popular surrogate measure to the EUV radiation is the widely observed flux
at 2800 MHz (10.7 cm). The 10.7 cm flux is useful for studying the ozone layer and
global warming. However, Doherty et al. (2000) point out that predicting the TEC by
using the daily values of solar 10.7 cm radio flux is not useful due to the irregular,
and sometimes very poor, correlation between the TEC and the flux. The TEC at any
given place and time is not a simple function of the amount of solar ionizing flux.
The transition from a gas to an ionized gas, i.e., plasma, occurs gradually. During
the process, a molecular gas dissociates first into an atomic gas that, with increasing
temperature, ionizes as the collisions between atoms break up the outermost orbital
electrons. The resulting plasma consists of a mixture of neutral particles, positive ions
(atoms or molecules that have lost one or more electrons), and negative electrons.
Once produced, the free electron and the ions tend to recombine, and a balance is
established between the electron-ion production and loss. The net concentration of
[21
