Bernese GPS Software Version 5.0 - Bernese GNSS Software

10. Station Coordinates and Velocities
10.1.1 Tectonic Plate Motion
Station coordinates are changing in time due to the steady movement of tectonic plates.
Figure 10.8 shows the present day major tectonic plates. This movement must be taken
into account in GNSS analyses. Station coordinates (especially of reference sites) should
therefore always be propagated from the reference epoch to the observation epoch based on
corresponding station velocities. This ensures consistency with the IGS satellite orbits and
prevents network deformations induced by moving plates.
If unavailable from other sources (e.g., the IERS), reasonable station velocities may be
derived from a model. One such model is the NNR-NUVEL-1A [DeMets et al., 1994], complying
with the IERS Conventions [McCarthy and Petit, 2004].
The Bernese GPS Software offers corresponding programs to compute model velocities
and to propagate coordinates, namely NUVELO and COOVEL. Both are described in Section
10.6.
10.1.2 Solid Earth Tides, Pole Tides, and Permanent Tides
Effects of solid Earth tides have to be taken into account because they are at least one order
of magnitude larger than the accuracies currently achieved for GNSS-derived coordinates.
The Bernese GPS Software Version 5.0 adheres to the IERS Conventions 2003 [McCarthy
and Petit, 2004] to model the solid Earth tides. The “step 1” and “step 2” corrections are
implemented.
The effects of the solid Earth pole tide are modeled according to the IERS Conventions
2003, too, adopting mean values of 0.033” and 0.331” for x and y pole, respectively.
In accordance with common geodetic practice, estimated coordinates are freed from all tidal
corrections including the permanent tide. To obtain coordinate values that correspond to
the mean over long periods the permanent tide must be added to the estimated coordinates.
Handling of the permanent tide can be changed in the subroutine ${LG}/TIDE2000.f. A
description of the “permanent problem of the permanent tide” is given, e.g., in [Ekman,
1995].
Note, that positions for all stations are corrected for tides. The only exception are stations
that are flagged with SPACEBORNE in Section TYPE 005: HANDLING STATION TYPES.
Typically these are Low Earth orbiters equipped with GNSS receivers.
10.1.3 Ocean Tidal Loading Effects
Another important site displacement effect is the crustal deformation caused by the changing
mass distribution due to ocean tides (ocean tidal loading). A file containing station-specific
coefficients for the magnitude of the ocean loading effect (amplitude and phase shift for the
eleven most important constituents) may be selected where necessary (programs GPSEST,
GPSSIM, and CLKEST). Although this file is not mandatory its use is strongly recommended
because otherwise vertical and horizontal corrections will not be applied. Nodal modulations
are not accounted for in Version 5.0 .
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