Research School of Earth Sciences
Earth Physics
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Atmospheric Pressure Loading: Application in the GAMIT softwareFrom version stdrel.10.2 onwards it is possible to apply atmospheric pressure loading at the observation level within the GAMIT software. This is done in two stepsApplication of non-tidal deformationThe non-tidal deformation is computed and applied in the CM reference frame (centre of mass of the solid Earth plus fluid loads). Since the GPS satellites are sensitive to geocentre motion (the translations between the centre of mass of the solid Earth, CE, and CM), the CM frame is the most appropriate reference frame. All reductions of GPS observations are actually performed in the CM reference frame. Binary files are available from 1990 to 2005 (up to September only) in which the deformation in NEU is provided every 6 hours on a 2.5 x 2.5 degree grid. The values are interpolated (bi-linearly) to the actual coordinates of each station. A new program, GRDTAB, performs the interpolation and generates a file, atmlod.DDD, that is subsequently read by MODEL. The grid files are generated by Tonie van Dam (ECGS). Application of tidal deformationThe S1 (diurnal) and S2 (semi-diurnal) atmospheric "tides" also be applied. The amplitude and phase are extracted in program GRDTAB for each site from a (binary) global grid file for the north, east and up components. These values are appended to the u-file and are subsequently applied in MODEL. Comments on ATM LoadingTregoning and van Dam (2005) investigated the application of tidal and non-tidal atmospheric pressure loading (ATML) at the observation level or as a daily-averaged correction. They showed conclusively that applying ATML at the observation level (ie the way that we have set it up within GAMIT) produces significantly better reduction in height WRMS than applying daily-averaged corrections. Therefore, we advocate that users activate this new feature within GAMIT.Tregoning and van Dam (2005) also showed that it is not yet possible to produce a truly "non-tidal" ATML model by removing the diurnal and semi-diurnal atmospheric "tides" using the approach of Ponte and Ray (2002). In fact, applying the tidal models of Ponte and Ray (2002) and the associated "non-tidal" ATML yields a worse solution than just applying the ATML convolved from the raw NCEP pressure data - data that contains a partial (but not complete) sampling of the S1 and S2 tides. The global grids of ATML provided with stdrel.10.21 are therefore the "partial-tide" versions of ATML. That is, the raw NCEP pressure data have been convolved without any attempt to remove the partial tides. The convolved pressure data of the tidal models of Ponte and Ray (2002) are also provided. Users can choose to apply just the tidal deformation if they wish but - naturally - should not apply this as well as the "partial-tide" versions of the ATML because the tidal component would be applied twice. Figure 1a shows that station heights have smaller WRMS values when ATML is applied at the observation level. This provides the justification for making the effort to apply it! Figure 1b shows that solutions using just the raw NCEP pressure data (ie make no attempt to remove the partial tides) yields station heights with smaller WRMS values than from the approach of applying separately the tidal and non-tidal components. While the latter is intuitively the better approach, at present the limitations of the tidal models seem to make this worse than one would expect. All of the above is explained in Tregoning and van Dam (2005). Please contact
Paul Tregoning if you have any specific
questions.
Tregoning, P. and T. van Dam.
(top of page)
The non-tidal ATML is activated with a single line in sestbl. :
Apply Atm loading = Y ; Y/N for atmospheric loading; need atmdisp.YYYY grid file
The tidal component of the modelling is turned on by activating the 6th bit of the
bit-mapping for the tides applied :
Tides applied = 47 ; Binary coded: 1 earth 2 freq-dep 4 pole 8 ocean 16 pole tide 32 atmosphere (not yet coded) default = 15
You will need the non-tidal loading grid files as well as the grid file for the tidal
loading. The (binary!) files available at present are listed below. Note: each atmdisp file is ~180Mb.
For the moment, the files are accessible through the GAMIT/GLOBK distribution
web page:
(select ATM loading grid files as the "desired product for download")
The files available are:
Each of the atmasc_c?.YYYY files needs to be uncompressed then expanded into a binary atmdisp file
by running the program ATMCOMP. There is a loss of accuracy of < 1 mm in the atm loading
computations in this process. If this is an acceptable level, downloading the compressed
ascii versions will be around 3 times faster than the full binary versions of the files.
atl.grid (this is only provided in the CM frame)
In addition, you will need updated files in gg/com that are provided
in stdrel.10.2 and subsequent versions (currently stdrel.10.21)
The following recent modifications can be invoked in GAMIT processing:
Lunar Eclipse Modelling:
IERS2003 solid Earth tide model:
Ambiguity resolution by autcln:
NOTE: you will also need to add a line " lc_autcln" to your autcln.cmd file
SV Antenna Model = ELEV ; turns on sv antenna phase centre patterns
NOTE: you MUST have the igs_05.atx file (or IGS_test05.atx) for this to work
1. The effects of atmospheric pressure loading and 7-parameter transformations
on estimates of geocenter motion and station heights from space-geodetic observations.
2. Atmospheric pressure loading corrections applied to GPS data at the observation level
1. The impact of tropospheric mapping functions based on numerical weather models on the
determination of geodetic parameters,
2. Deformation in the Jura Mountains (France): First results from GPS measurements,
3. Effect of post-seismic deformation on Earth Orientation Parameter estimates
from VLBI observations: a case study at Gilcreek, Alaska.,
4. Tectonic interpretation of aftershock relocations in eastern Papua New Guinea
using teleseismic data and the Arrival Pattern method.
A full list of publications from geodetic and tectonic studies can be found
here
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Page last updated: 2005 November 9 Please direct all enquiries to: pault@rses.anu.edu.au u Page authorised by: Coordinator, Earth Physics, RSES |
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