Theory of GNSS Radio Occultation - GNSS Remote Sensing: Theory, Methods and Applications - page 111

correction can then be applied to achieve higher accuracy of positioning for the GPS
receiver of interest that collects the exact same data from the same GPS satellites
at the same time. The DGPS technique eliminates selective availability and other
clock errors and allows civilian receivers with broad-beam antennas to achiever
millimeter precision in phase measurements (e.g., Wu 1984 ). The widespread usage
of differential GPS services as well as the new technologies to deny GPS service to
potential adversaries on a regional basis by the US military eventually leads to the
termination of the SA service in 2 May, 2000.
The similar technique can be applied for deriving high-precision LEO satellite
obits. The two most widely applied procedures are called double-differencing and
single-differencing, and they differ in how the effect of the GPS satellite oscillator
fluctuations are removed from the LEO phase data (Kursinski et al. 1997 ;Wickert
et al. 2002 ;Hajjetal. 2002 ; Beyerle et al. 2005 ). Figure 5.5 shows a diagram that
illustrates differencing geometry.
The single-differencing procedure requires the LEO receiver viewing simultane-
ously an occulting transmitter (GPS_occ) and a non-occulting reference transmitter
(GPS_ref) during the occultation (Fig. 5.5 ). The occultation link (LEO-GPS_occ)
and reference link (LEO-GPS_ref) data are differenced to remove the effect of the
receiver clock errors, and the solved-for high-rate GPS clock offsets are interpolated
based on the IGS final clock products to remove the effects of the transmitter clock
errors (Wickert et al. 2002 ; Schreiner et al. 2010 ).
However, in some situations, especially before the deactivation of Selective
Availability (S/A), the GPS clock can be sufficiently unstable and need to be
calibrated. In such case, the double-differencing procedure is needed to remove
both the GPS and LEO clock errors (Hajj et al. 2002 ; Wickert et al. 2002 ;
Schreiner et al. 2010 ). Therefore, other than the two data links between LEO
and the occulting/reference GPS transmitters, two additional data links between
a ground reference station (GS_ref, precisely known position) and the same
occulting/reference transmitters need to be measured (Fig. 5.5 ). The similar single-
differencing technique needs to be applied twice to eliminate the clock errors of
both the GPS transmitters and LEO receiver.
One significant disadvantage of double-difference processing is its susceptibility
to availability of ground fiducial network station data (Galas et al. 2001 ;Wickert
et al. 2001 ) as well as error sources including multipath, residual atmospheric
and ionospheric noise, data interpolation, and thermal noise. Since deactivation
of Selective Availability (S/A), the GPS clock errors are reduced by orders of
magnitude. Without S/A GPS clocks are sufficiently stable, therefore, double
differencing can be replaced by the single difference technique to eliminate the need
for concurrent high-rate ground station observations (Wickert et al. 2002 ).
After the POD and differencing process, the L1 and L2 atmospheric excess
phases can be derived along with the precise orbits (positions and velocities) of both
the LEO and occulting GPS satellites. The further derivation of the atmospheric
properties based on these occultation measurements will be explored in the next
section.