Differential GPS - Understanding GPS: Principles and Applications - page 450

• Phase wind-up: Relative rotation between a GPS satellite and the user antenna
can cause carrier-phase measurements to change by up to one cycle. This effect
is referred to as phase wind-up. A correction for this effect is provided in [70].

Solid Earth tides and ocean loading: The Earth's surface is not rigid, but rather
somewhat pliable. Its shape varies with time, dominated by diurnal and
semidiurnal components, in response to gravitational forces. These Earth sur-
face movements are referred to as solid Earth tides. Additional motion of the
Earth's surface, especially in coastal locations, due to ocean tides is referred to
as ocean loading. Solid Earth tides and ocean loading site displacements can be
as large as 30 cm and a few centimeters, respectively. By convention, ECEF
coordinate systems such as ITRF are explicitly defined to not include solid
Earth tides and ocean loading effects. Thus, these effects should be removed
for applications where the user position in ECEF coordinates are desired.
Accurate models for the Earth's deformation due to solid Earth tides and
ocean loading can be found in [71].

•

Using these techniques and models, the positions of a number of sites were deter-
mined in [69] with an accuracy often better than 5 cm in each coordinate. Approxi-
mately 30 minutes of data were required to be processed before the obtained
accuracy converged to these levels.
An important example of an operational PPP system is the JPL's Internet-Based
Global DGPS (IGDG) system [72], which provides clock and ephemeris data over
the Internet using dedicated frame relay lines. In collaboration with JPL, NavCom
Technology (a subsidiary of John Deere) also distributes the data globally via
L-band (1,525-1,565 MHz) geostationary satellite links as part of their StarFire net-
work DGPS service [73].

8.6.2 Carrier Based
In the past, geodetic positioning required LOS connections to a network of
monumented points in the ground. This geodetic network defined a consistent refer-
ence frame and helped control measurement error. Now, a network of continuously
operating GPS receivers may replace the traditional geodetic network of monumented
points. The network of receivers has an authoritative set of coordinates and supplies
base station carrier-phase and code range data for accurate differential processing.
Continuously operating networks are popular, and numerous examples exist. We
shall focus on two, the U.S. Continuously Operating Reference Stations (CORS) and
the global IGS system.

8.6.2.1 CORS
The National Geodetic Survey (NGS) of the NOAA manages a CORS system to sup-
port nonnavigation, post-processing applications of GPS. GPS receiver data are col-
lected throughout the country and are archived at the main site in Silver Spring,
Maryland, and at a parallel facility in Boulder, Colorado. The U.S. CORS system
provides code range and carrier-phase data from a nationwide network of GPS sta-
tions through the Internet. The CORS Web site is http://www.ngs.noaa.gov/CORS.