Global Positioning System Reference
In-Depth Information
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LEO
GPS
earth
atmosphere
Figure 6.4 Schematic view of an LEO
sa
tellite and a GPS satellite configuration.
contributes. Figure 6.4 shows a schematic view of a low earth orbiter (LEO) and a
GPS satellite. As viewed from the LEO, an occultation takes place when the GPS
satellite rises or sets behind the earth's ionosphere and troposphere. When the sig-
nals pass through the media they experience tropospheric delays, ionospheric code
delays, and phase advances. If the accurate position of the LEO is known and if the
LEO carries a GPS receiver, one can estimate atmospheric parameters by comparing
the travel time of the signal and the geometric distance between both satellites. Since
the modeling associated with GPS occultations is still evolving, as is accurate orbit
determination of LEOs, the reader should consult the current literature for details; a
recommended start is Kursinski et al. (1997). One often assumes in these computa-
tions that the travel path through the media is symmetric and considers the tangent
point as the point of measurement.
We present two figures showing typical products that can be derived from GPS
occultation. Figures 6.5 and 6.6 indicate results from the GPS/MET experiment that
was managed by the University Corporation for Atmospheric Research (UCAR) and
lasted from April 1995 to March 1997. A 2 kg TurboRogue receiver modified for
use in space was piggybacked on a LEO with a 730 km circular orbit and 60° in-
clination. Figure 6.5 shows a temperature profile as determined by GPS occultation,
direct radiosonde measurements, and an atmospheric weather model. The occulta-
tion occurred at 1:33 UT on May 5, 1995, over Hall Beach, Northwest Territory,
Canada. The radiosonde at 0:00 UT was 85 km from the occultation location and
spatially interpolated to the occultation location. The surface temperature was below
freezing with a sharply defined tropopause near 8 km. The good agreement with the
radiosonde in resolving the sharp tropopause and the change below 3 km illustrates
the high sensitivity and vertical resolution of the occultation technique. Figure 6.6
shows an electron density profile of the ionosphere as a function of height derived
from GPS/MET occultation, May 5, 1995, 3:20 UT. The figure also shows another
independent determination of the electron density at 3:40 UT using incoherent scatter
radar with a 320
[19
Lin
—
1.4
——
Nor
PgE
[19
s pulse mode located at Millstone Hill (Massachusetts). Some of
the discrepancies seen in the figure are a result of spatial and temporal mismatch of the
observations and the spherical symmetry assumption for the signal path. The latter
assumption can be a problem because the signal travels through a large portion of
the ionosphere, in particular, the upper ionospheric region. Such a profile, of course,
changes with time and location.
µ
