Atmospheric Sensing Using GNSS RO - GNSS Remote Sensing: Theory, Methods and Applications - page 151

Fig. 6.6 LEO to LEO occultation geometry

On the other hand, the horizontal drift of the airborne/mountain-top RO tangent
point is generally larger for an airborne case than for a space based receiver because
the receiver is moving at a much slower speed than the transmitter. The errors due
to the spherical symmetry assumption in the presence of significant horizontal gra-
dients are compounded by the horizontal drift of the tangent point. Xie et al. ( 2008 )
demonstrates significant representative errors can be induced by the combination
of limb sounding geometry and the horizontal tangent point drifting. Therefore,
it is important to note that the airborne/mountain-top RO bending or refractivity
profile derived from an occultation measurement should not be treated as local or
in-situ measurement. Various data assimilation technique used in the spaceborne RO
community could be adapted to maximize the benefit of airborne/mountain-top RO
measurements for regional numerical weather forecasting.

6.6.3 LEO-to-LEO Occultation

Note the GNSS radio occultation sounding technique makes use of the L-band dual
frequency signals and only measures the real part of the atmospheric refractivity, i.e.,
neglecting the absorption effect of the atmosphere. Since the late 1990s, many other
frequencies that are significantly higher than GNSS signals have also been explored
with similar types of limb sounding technique. By mounting the transmitter and
receiver on two separated LEO satellites that are moving away or toward to each
other (Fig. 6.6 ), the LEO to LEO occultation can be recorded. The LEO to LEO
occultation measuring both refraction and absorption of coherent microwave and
infrared signals could provide a much more complete set of atmospheric variables
that includes thermodynamics (temperature, pressure, water vapor), dynamics (line-
of-sight winds), climate/chemistry (ozone, carbon dioxide, methane, and other
greenhouse gases and trace species) as well as cloud, aerosol and turbulence
information (Kursinski et al. 2002 ; Kirchengast and Hoeg 2004 ; Kursinski et al.
2009 ; Kirchengast and Schweitzer 2011 ; Schweitzer et al. 2011 ).
The measurement of LEO-LEO occultation signals phase and amplitude in
the microwave bands (MW occultation) near 22 GHz ( 1.36 cm) and 183 GHz
( 1.6 mm) water vapor absorption lines and 184 and 195 GHz ( 1.5 mm) ozone