Theory of GNSS Radio Occultation - GNSS Remote Sensing: Theory, Methods and Applications - page 108

O( f 2 ) (Bassiri and Hajj 1993 ) is normally small and can be ignored, but it becomes a
dominant error term at high altitudes (>40-60 km) during solar-maximum day-time
conditions (Kursinski et al. 1997 ).
In Eq. ( 5.15 ), the dominant term on the right-hand-side is the geometrical range
between the transmitter and receiver. Whereas the excess atmospheric phase delay
accumulated in the GPS L1 and L2 phase measurements in the neutral atmosphere
( k neutral ) and ionosphere ( k iono ) is of primary interest. By differentiating the
excess phase delay, the excess Doppler due to the atmosphere can be derived, which
is the fundamental building blocks for retrieving the vertical profile of bending angle
and refractive index of the atmosphere.
The process of extracting the excess phase delay due to the atmosphere is
generally referred as the calibration processes, which consists of two major steps.
Firstly, Precision-Orbit-Determination (POD) is performed to derive the precise
orbits (positions and velocities) for both the transmitter (e.g., GPS) and LEO RO
receiver satellites on the occultation link. This process will allow removal of the
dominant geometrical range term ()inEq.( 5.15 ) between the transmitting and
receiving satellites. Secondly, through the differential technique with simple linear
combination, the clock errors of both GPS transmitter and LEO receiver can be
removed. Consequently, the sum of the neutral and the ionospheric delays is isolated
(up to a constant). The details of the calibration process is presented in the following
two sessions.

5.3.1.1 Precision Orbit Determination (POD) Method

In order to derive useful atmospheric profiles from radio occultations, the velocities
of both GPS and LEO satellites need to be estimated with high accuracy within
sub-mm/s level, as it is directly related to the excess Doppler due to the atmosphere.
Given the fast movement of GPS ( 3.8 km/s) and LEO ( 7 km/s) satellites, this is a
daunting challenge. The way to achieve highly accurate orbit information is referred
as Precision Orbit Determination (POD) process.
The objective of POD is to obtain an accurate orbit (position and velocity vectors)
that accounts for the dynamical environment in which the motion occurs, including
all relevant forces affecting the satellite's motion. In practice, each LEO satellite
is generally equipped with at least two upward-looking antennas, with one to track
a high-elevation GPS satellite for POD purpose and another to track a reference
GPS satellite. In addition one or two downward-looking (forward or rear-looking
along satellite trajectory) are to the limb of the Earth's atmosphere for occultation
measurements (Fig. 5.5 ). For example, each COSMIC spacecraft is equipped with
four antennas, with two upward-looking antennas and two limb viewing occultation
antennas.
Since the beginning of the space age the POD technique has been used in the
geodesy research community as a means to improve or even determine geodetic
models. As a major legacy of GPS geodesy, the International GPS Service for