Tidal ocean loading
This page provides a summary of activities in modeling ocean loading
in Goddard VLBI group
The Earth as a whole responses to external forces as an elastic body.
Tides in the ocean
redistribute water mass with respect to land. Therefore, the weight of the
column of water above the given point periodically changes. These changes may
reach 40 mm vertical and 5 mm horizontal. Tidal ocean loading should be taken
into account in reduction of astronomical and space geodesy observations when
precision better than 5 nrad or 4 cm is required.
Ocean tide can be characterized be a sum of harmonic constituents.
For computing site displacements caused by tidal ocean loading a global
ocean tidal maps for major harmonic constituents should be used. A tidal map
represents the amplitude of the sea level due to harmonic constituent and
the phase with respect of the astronomical tide. The global map is convolved
with Green's functions which describes Earth's crust response on a loading
of the point mass. I used software
SPOTL developed by D. Agnew
for computation of the displacements due to ocean loading. The following ocean
tidal maps were used:
In total, ocean loading for 26 frequencies is modeled.
- Major diurnal and semidiurnal tides:
Q1 -- GOT00.2 model (R. Ray).
- Zonal tides:
Mtm, Mf, Msf,
Mm, Msm, Ssa, Sa
NAO99.b model (K. Matsumoto)
- The Chandler and annual components of polar tide, the tide with
period of 18.6 years -- equilibrium model.
- Minor tides with frequencies closed to the frequencies of major tides,
so-called companion tides:
Mf-a -- the ocean tide map obtained from the ocean
tide map by a multiplying by a complex scaling admittance factor.
It is assumed that the ratio of the ocean tide amplitudes for the
companion tide to the the major tide is the same as the ratio of
the amplitudes of the tide-generating potential.
Validation of the model
The model was extensively tested against the whole set of VLBI observations.
More details can be found here.
Files for downloading
3-D displacements of VLBI stations caused by ocean loading.
- Ocean tide loading
provider (Hans-George Scherneck)
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Web page was prepared by Leonid Petrov
Last update: 20-JAN-2006 20:47:53