Theory of GNSS Radio Occultation - GNSS Remote Sensing: Theory, Methods and Applications - page 118

Deeper into the lower atmosphere (e.g., below 10 km), the L2 signal becomes
too weak for robust tracking due to the low L2 signal power, incomplete knowledge
of P-code modulation as well as large a tmosphe ri c defocusing effects. In such case,
the ionospheric correction term (i.e., ˛ L1 .a/ ˛ L2 .a/), is linear extrapolated from
higher altitudes downward to the surface for continuing ionospheric correction
in the lower altitudes with the absence of L2 measurement. Also note that the
ionospheric calibration should not be applied above a certain height, when the
neutral atmosphere signature on the occulted signal is comparable to residual
ionospheric effects or the receiver's thermal noise. This tends to occur at the altitude
of 50-90 km, depending on the ionospheric conditions (Hajj et al. 2002 ).
The ionospheric calibration process described above effectively removes the first
order ionospheric term (1/ f 2 )inEq.( 5.1 ). Higher order contributions constitute
the major source of error during day-time solar maximum at high altitudes (e.g.,
Kursinskietal. 1997 ) will require further calibration (e.g., Bassiri and Hajj 1993 ).

5.3.4.2 Refractivity Retrieval from Abel Inversion

Under the assumption of spherically symmetric atmosphere, the refractive index
profile can be derived from the neutral atmospheric bending through the Abel
transform in Eq. ( 5.13 ). As the upper limit of the Abel integral requires knowledge
of the bending angle as a function of impact parameter up to the top of the
atmosphere. However, the estimated bending is reasonable accurate only up to a
certain upper height (e.g., 50-90 km dependent on the ionospheric condition
as discussed in previous section). Therefore, the a-priori (or background) bending
angle is needed to extend the observational neutral bending angle at higher altitudes.
This a-priori bending is often referred as the upper boundary condition for Abel
inversion. In practice, the a-priori bending can be derived from weather or climate
models (e.g., Hedin 1991 ) or from a simple model (e.g., exponential extrapolation
of bending upward from a certain altitudes (Hajj et al. 2002 )). The uncertainty
in the a-priori bending model could introduce errors in the refractivity retrieval
from Abel integral. To reduce the effects of error propagation downward from the
upper stratosphere, the optimization technique that mixes the observational neutral
atmospheric bending with the a-priori model can be used (e.g., Lohmann 2005 ).
After the refractive index as a function of impact parameter n ( a ) at the tangent
point, is derived from the Abel inversion, the tangent point radius is obtained from
( 5.14 ), i.e., r D a / n . The radius in turn is converted into height above an ellipsoidal
fit to the mean sea-level geoid.

5.3.4.3 Quality Control

In the GNSS RO data processing centers, various quality control methods are
applied and they are used at different processing stages. For example, in the
early processing stage, the quality of the measured signal SNR, excess phase and