Theory of GNSS Radio Occultation - GNSS Remote Sensing: Theory, Methods and Applications - page 110

There are various software packages for POD process in the radio occultation
research community. For example, the GFZ group (Geoforschungszentrum, German
Research Center for Geosciences) use the EPOS-OC; NASA Jet Propulsion Labora-
tory (JPL) utilize the GIPSY-OASIS (GNSS-Inferred Positioning System and Orbit
Analysis) software and apply the reduced-dynamic strategy; The UCAR COSMIC
group use the Bernese GPS data processing package to solve for satellite orbits with
a reduced-dynamic approach (e.g., Ho et al. 2009 ).

5.3.1.2 Differencing Technique to Remove Clock Errors

Once the effect of satellite motion is removed, the GPS and LEO satellite clock
errors need to be eliminated through the differencing technique to derive the
atmospheric excess phase of the occultation link.
In the ideal case, when both GPS and LEO satellite clocks are sufficiently
stable and require no calibrations, i.e., the LEO and GPS clock offset are zero or
known, the total excess phase delay due to the atmosphere can be directly resolved
from Eq. ( 5.15 ) after removing the satellite geometric term. This is referred as
zero-difference (or un-difference), i.e., no differencing is needed. Beyerle et al.
( 2005 ) demonstrate the zero-difference processing can produce highly accurate
excess phase data by applying prior estimated LEO and GPS clocks in GRACE-
B occultation measurements. Note that the twin GRACE (Gravity Recovery and
Climate Experiment A&B) spacecrafts are equipped with an ultra-stable-oscillator
(USO), which allows highly accurate clock measurements without need for clock
calibration.
However, for most of the other GPS occultation missions, the less stable LEO
receiver's clocks generally require calibration. Sometimes the GPS clocks are also
need to be calibrated. A commonly known calibration process is the differencing
technique. Differential GPS/GNSS (DGPS/DGNSS) is a technique for reducing the
error in GPS-derived positions by using additional data from a reference GNSS
receiver at a known position. The technique was originally developed by GPS
geodesists to significantly improve the precision of ground GPS measurement, in
the presence of large errors due to the selective availability (SA) process. The SA
was introduced by the U.S. Department of Defense to degrade the performance of
GPS. The intentional, time varying errors of up to 100 m were intentionally added
to the L1 publicly available navigation signals to destabilize GPS signals for “unau-
thorized” users. This significantly limited the usage of the civilian GPS application
that requires much higher precision of measurements without a costly classified
receiver. In the early and mid 1980s, the pioneering work of GPS geodesists leads
to a DGPS technique. Generally, the DGPS involves determining the combined
effects of navigation message ephemeris and satellite clock errors (including the
effects of propagation) at a reference station and transmitting corrections, in real
time, to a user's receiver. Given a reference receiver with known exact position, the
vector displacement or differential, between the known position and the position the
reference receiver get from the GPS satellites, can be calculated. This differential or