This Chapter provides some details about different data being used in VLBI analysis and how they are used in VieVS. For data used in VIE_GLOB, VIE_SCHED and VIE_SIM we refer to the according wiki pages.
Time-dependent correction and coefficients of periodic time dependencies are stored in the superstation file which can be found in VieVS/TRF/superstation.mat. Files used for the superstion file are stored in VieVS/TRF/create/superstation/neededFiles.
Corrections without periodic time dependencies are saved as time series. The corresponding files can be found in folders located the VieVS root directory (e.g. non-tidal atmosphere loading is saved at VieVS/ATM/ ).
Similar folders can still be found for corrections that are now saved in the superstation file. Those folders are used with older VieVS versions and are obsolete if the newest VieVS version is used.
For detailed information on the data being used in VLBI analysis we refer to the following subsections:
The observations are stored in the NGS file which is the main input file for VieVS.
The NGS header consists more or less of stations and sources of the session, including their rough coordinates - x,y,z for stations, right ascension and declination for sources. The mounting type of the telescope is given as well.
NGS data cards
This is the main part, all observation information is stored in here.
A full description of the NGS files can be found at http://lacerta.gsfc.nasa.gov/mk5/help/dbngs_format.txt
To select one or more sessions (NGS files) go to File - Set input files and click on Browse for sessions (see Fig. below). Multiple selections (press and hold down Ctrl) are possible. Process lists can be loaded using the button Browse for process lists. All selected sessions (single sessions and sessions from process lists) appear in the listbox in the GUI.
Tidal atmosphere loading is caused by diurnal heating of the atmosphere. Three models are currently implemented in VieVS:
The tidal atmospheric loading effect can be found in the superstation file and the corresponding source folder. We refer to the given references for more detail on the implemented models.
Non-tidal atmosphere loading is caused by pressure changes due to air mass movements. At mid-latitude stations experience a vertical crustal displacements of up to 25 mm. Two models are currently implemented in VieVS:
Files used for the non-tidal atmospheric loading effect can be found in the ATM folder. We refer to the given references for more detail on the implemented models.
Hydrology loading is caused by non ocean water bodies (e.g. canopy water, soil moisture, snow etc.) which cause a load on the crust. The corrections are provided by the NASA GSFC VLBI group (Eriksson & MacMillan, 2014). Files used for the hydrology loading can be found in the HYDLO folder. We refer to the given reference for more detail on the implemented models.
Ocean tide loading is caused by the redistribution of water mass due to tides. The water causes a load on the crust and therefor a deformation of up to 10 cm. 5 ocean tide loading models are implemented in VieVS:
The ocean tide loading effect can be found in the superstation file and the corresponding source folder. We refer to the given references for more detail on the implemented models.
Ocean pole tide loading is generated by the centrifugal effect of polar motion on the oceans. The IERS Conv. 2010 recommends the ocean pole tide model of (Desai, 2002) which provides the ocean pole load tide coefficients. The ocean pole tide loading effect can be found in the superstation file and the corresponding source folder. We refer to the given reference for more detail on the implemented models.
External tropospheric files contain the path delay through the troposphere and the partial derivatives needed for the estimation of VLBI observations in a standardized format. They provide an easy way of using different models for the delay for different stations.
The .trp files have to be created prior to VLBI analyses. The delays are read in during the calculation of the theoretical delay.
Two files have to exist. For all sessions these tropospheric files should be created:
A screenshot of the external tropospheric file is depicted in the Figure below:
The file contains the following information (sorted by column):
Open Models - Troposphere in the VieVS GUI. Then click the radio button with the caption from external file and push the button Create. A screenshot of this procedure is provided in the following Figure:
In the next Figure the GUI is depicted:
In the first (1.) part of the GUI the tropospheric parameter file has to be specified. See the according chapter if you want to create a tropospheric parameter file.
In a second step (2.) the sessions, for which .trp files should be created, have to be chosen. The first listbox from the left shows all years available in the folder VieVS/NGS. After selecting a year, all files in the folder are depicted in the second listbox. If a session is selected, it appears in the third listbox which specifies the used sessions (multiple selections are possible). There is also the option of using predefined process lists (fourth listbox).
A subdirectory where .trp files are saved can be selected in (3.). Pushing the Create Files button (4.) will create the .trp files.
Writing one file successfully takes a few seconds. If an error occurs, the error log message should appear in the matlab command window. Following errors messages can be found:
The tropospheric parameter file is a simple text file which contains the parametrization for all stations. A different parametrization for each station can be selected, for example:
Wettzell: Raytracing (only possible if available) Westford: p0+Saastamoinen, no Gradients All others: ECMWFh+VMF1h, LHG gradients
The tropospheric parameter file can be created using the VieVS GUI by pushing the button create using GUI depicted in the Figure above (1.). Another GUI should appear:
All ITRF stations should appear, their default parametrization is listed in the following Table:
|Parameter||A priori model|
|Hydrostatic zenith delay||Pressure at the site with model from Saastamoinen|
|Hydrostatic mapping function||$VMF1_h$|
|Direction of atmosph. asymmetry||$VMF1_w$|
A more detailed description of the columns is given in the next Figure:
In general, a 1 indicates that the specific model should be used for this station. If a 0 is written into the cell the program wont use the corresponding model. If there is more than one model available (e.g. $p_0$, ECMWFh and GPT for the hydrostatic zenith delay model), their sum has to be $\Leftarrow1$ (only one or no model can be used). Mapping functions are only used when there is a
main model chosen (for example: $ECMWF_w = 0$, $VMF1_w = 1$: The wet part of the atmosphere is NOT considered).
If all mapping functions are set to zero (e.g. $VMF1_h$ and $GMF_h$) but one
main model is chosen (e.g. $p0 = 1$), then the correction is NOT applied. Partial derivatives are mandatory which means that either the $VMF1_w$, $GMF_w$ of $1/sin(e)$ in the partials block has to be 1. Such problems can be prevented by selecting the check box check while editing table which checks the input for errors.
One can now edit the parametrization by hand. It is possible to set a whole column to one value by simply clicking into this column and pressing the button set column to (the desired value can be specified next to the button).
Stations can be deleted (button - delete station) or added (button + add station) to the list. An option to sort the list (button sort) is also included in the GUI.
A previously created tropospheric parameter file can be loaded into the GUI in the Load tropospheric-parameter-file section.
The output file can be specified in the Save tropospheric-parameter-file section. By clicking the save button the created/edited file is saved to the given location.
Azel files are text-files which are created automatically as part of the daily automatic processing. For each session one .azel file is created. It contains one line for each observation and each station. The parameters written to these files are:
On the basis of the azel-files the .trp (external tropospheric files) are created. The number of rows in the .azel file is therefore the same for the .trp file. This is important because the azel-files specifies if all observations are used (even those with a quality flag not equal 0).
Taken from: (Tierno Ros, 2011).
The ionosphere is a portion of the upper atmosphere; it is extended from about 60 km to 2000 km and has the characteristics that the particles there can be easily ionized by solar radiation resulting in a positive ion and a free electron. The ionospheric delay makes up a large fraction of observed VLBI group delays and it depends on the number of free electron (TEC) along the ray path. Usually, a correction can be applied by making simultaneous observations in both S- and X-band. However, this is not always possible. For such cases, there is the alternative of calculating the ionospheric correction from Global Navigation Satellite Systems (GNSS) TEC maps. (Please note that it is only recommended to use the GNSS corrections in case of not having observations in both S- and X-band, since the X/S ionosphere values are instantaneous direct measurements while the GNSS values suffer from low spatial and temporal resolution, Gordon, 2010).
GNSS TEC maps are representing the TEC values over the whole globe determined from GNSS observations. The routine generation of these Global Ionosphere maps (GIMs) is currently done at four analysis centers of the International GNSS Service (IGS), more details about the different techniques they use can be found in Schaer (1999); Feltens (1998); Mannucci et al. (1998) and Hernández-Pajares et al. (1999). The analysis centers transmit their products to the IGS Ionosphere Product Coordinator, who produces a weighted combined product (Hernández-Pajares et al.,2009). The accuracy of the maps is between 2 to 8 TECU. Such maps exist since 1998 and have a latitude/longitude resolution of 2.5/5.0 degrees.
The figure below shows how an external ionospheric file looks like. The header contains information about the creation date of the file, the VLBI-session which corresponds to the file, the type of GIM used and the coordinates of the stations participating in the session.
The file body is divided in columns:
In order to create an external ionospheric file you should follow these instructions: (Please note that before creating it, VIE_INIT and VIE_MOD should have been executed for the sessions for which you want to create the external files)
The files containing the ephemeridesae already described here.
The superstation file is a .mat file containing all static station-dependent data, e.g. TRF, loading data, discontinuities, eccentricities, furhter antenna information. The Table below provides a summary of all realizations/models which are included by default.
|Type of data||Included||Own|
|TRF||ITRF2005, ITRF2008, VTRF2008, VieTRF13, vievsTrf (Backup)||x|
|Tidal ocean loading||FES2004, GOT00, EOT08a, TPXO72, AG06||x|
|Ocean pole tide loading||by S. Desai||x|
|Tidal atmosphere loading||Vienna, GSFC, VanDam||x|
|Additional||Antenna information, eccentricities, equipment, horizon mask, discontinuities||-|
In Models - Reference frames there is a button
Create file. This button runs the createsuperstationsFile.m function which is a interface for creating the superstation file (Fig. below). This function as well as all needed data for the superstation file are located in /TRF/create/superstation/ .
The four menu buttons in the GUI let you specify filenames of the shown textfiles. In Main/Info files data like general antenna information, horizon masks and equipment data can be found. The menu button TRF lets the user specify up to six different terrestrial reference frames. The two most-right menu items ask the user to input loading correction data. If the files are located in /TRF/create/superstation/neededFiles/ , the button
Search for files on the right of the interface searches for those files and writes the correct file names (including paths) to the textboxes. The textbox at the bottom specifies the output file name (requires a .mat file), the button
Create runs the program and creates the superstation file.
This list is not complete ⇒ Please add information here!
If station coordinates are estimated, some conditions need to be applied in order to avoid singularity - e.g. No net translation (NNT) and No net rotation (NNR). If a station has bad a priori coordinates (or after an earthquake, when coordinates are wrong), this antenna should be removed from the NNT/NNR(/NNS) condition. The rest of the paragraph gives instructions on how to exclude a station from the given conditions. If an official TRF (i.e. all but vievsTrf) is chosen in Models - Reference frames and a station is found there (coordinates exist for an appropriate break), this station is taken as datum station. If the station does not exist or has no appropriate break (i.e. backup coordinates are taken from vievsTrf), the station is excluded from NNT/NNR. If vievsTrf is chosen as a priori reference frame (and only then), the ones and zeros in the vievsTrf-textfile indicate if a station is treated as datum station.
User can introduced an additional TRF. The text format of the TRF file is shown in the Fig. below:
Comment lines start with '%', data lines start with a station name, followed by x, y and z coordinates and the velocities in x, y and z direction. The three last columns are epoch, start- and end-break (this specifies the time for which the coordinates and velocities are valid). Each column has to be separated by at least one blank; the columns don't have to be aligned with each other.
The additional TRF can be a matlab structure file and has the next fields:
and extra (double, could not exist in structure):
The tidal ocean loading, ocean pole tide loading and tidal atmosphere loading can be added by user to the superstation file. Each file has to be specified in the proper panel.
The structure can be used for ocean loading and has to consist of:
|1×8 char||6x(ntides) double|
where coefficient order:
Usually, ntides = 11 as follows: M$_2$, S$_2$, N$_2$, K$_2$, K$_1$, O$_1$, P$_1$, Q$_1$, Mf, Mm, Ssa.
The user defined atmosphere loading file should have the same format as the 'original' files given by the corresponding models (see Fig. below). A data line starts with an eight-character station name. The required 12 values (in the same order as ocean loading for S$_1$ and then S$_2$) must start from position (column) 35 (more blank is allowed).
The user defined ocean pole tide loading file should have the same format as the 'original' file. The file provides all values on a longitude/latitude grid. A screenshot of this structure is shown in below:
Similar to the superstation file, there is also one file containing all static information about sources (i.e. name representations and catalog positions). The .mat file can be found in VieVS/CRF/supersource.mat. The most complete list of sources can be found in the ASCII file VieVS/CRF/create/neededFiles/vievsCrf.txt. There, the user can also add new/own sources.
To create a supersource file, go to Models - Reference frames and click on
Create file in the Celestial reference frame panel. In the appearing GUI set the paths to all required files (all but user crf are required) and save the supersource file.
VieVS automatically reads the default file (supersource.mat in VieVS/CRF/ ) at the start of VieVS. If you want to load another one, select
Chose file in Models - Reference frames in the CRF panel.