Global Positioning System Reference
In-Depth Information
for increased interoperability between GALILEO L1F and GPS modernized L1C
(GPS III) open-access signal in the L1 band has been made, for the time being, since a
common BOC(1,1) modulation was adopted out of the EU-U.S. negotiations on
GALILEO and GPS. Starting from this baseline assumption, a process that is
allowed by the EU-U.S. agreement is being put in place for further cooperation
between both parties for a joint or isolated optimization of their respective signals
modulation.
Another area of interest for interoperability at signal level is the selection of the
PRN code families. Currently, GALILEO E5a and L1F signals have the same modu-
lation and carrier frequency as the GPS L5 and modernized L1C signals (on GPS III),
respectively. Having common code families could bring some more benefit to the
user community.
10.4.2 Geodetic Coordinate Reference Frame
International civilian standards will be adopted for the GALILEO coordinate refer-
ence system. The realization of the GALILEO coordinate and time reference frame
will be based on stations and clocks different from those of GPS.
The GALILEO Terrestrial Reference Frame (GTRF) shall be an independent
realization of the International Terrestrial Reference System (ITRS). The ITRS is
defined and monitored by the Central Bureau of the IERS.
The ITRF is based on a set of station coordinates and velocities derived from
observations of very long baseline interferometry, satellite laser ranging, lunar laser
ranging, GPS, and DORIS (the French Doppler system). A reduction of the individ-
ual station coordinates to a common reference epoch considering their station veloc-
ity models is performed using fixed plate motion models or estimated velocity fields.
WGS-84 is the coordinate reference frame for GPS. WGS-84 is also a realization of
the ITRS. Coordinates of both the GPS CS and NGA monitoring stations serve as a
reference for WGS-84. The differences between WGS-84 and the GTRF are
expected to be on the order of a few centimeters [8]. This implies that the WGS-84
and GTRF will be identical within the accuracy of both realizations (i.e., the coordi-
nate reference frames are compatible). This accuracy is sufficient for navigation and
most other user requirements. The remaining discrepancies are in the 2-cm level and
are only of interest for very special research in the geosciences.
Coordinate reference frames often include Earth gravity models. For example,
the WGS-84 uses a spherical harmonic expansion of the gravity potential up to the
order and degree 360 [9]. For GALILEO, a similar model must be considered. In
that context, the European satellite gravity missions Gravity Field and Steady-State
Ocean Circulation Explorer and Challenging Mini-Satellite Payload for Geo-Scien-
tific Research and Applications Program are of importance. The American Gravity
Recovery and Climate Experiment will also provide data. The findings from these
missions will serve as a basis for the GTRF Earth gravity model.
10.4.3 Time Reference Frame
GALILEO System Time (GST), modulo 1 second, is planned to be steered to a pre-
diction taken from a number of UTC laboratories obtained through an external
 
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