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public:vievs_manual:exercises_scheduling_simulating_global_solution

Scheduling, simulating and global solution

:!: Screenshots are from VieVS version 2.1.

Tasks:

  • Create observation schedules for given stations for two consecutive days in September 2011.
  • According to these schedules, simulate the time delays which would be measured in such a network and create in this way NGS files with artificial observations.
  • Take these two NGS files as input for an analysis of VLBI observations with VieVS. Plot the calculated baseline length repeatabilities with the VieVS plotting tool.
  • Compute the station coordinates in a global solution (of both sessions).

Scheduling

A Continuous VLBI Campaign CONT11 was scheduled with the NASA sked program to be observed in the second half of September 2011. The plan for the CONT11 campaign was to acquire state-of-the-art VLBI data over a time period of about two weeks to demonstrate the highest accuracy of which the current VLBI system is capable. The stations participated in the CONT11 are shown in the Figure below (WARK12M had to cancel participation due to technical problems).

Stations of the original CONT11 campaign

Scheduling with VieVS

In our exercise you should make 24-hour schedules for the first two days of the original CONT11 campaign, i.e. September 15 and 16, 2011 starting at 5pm. In the scheduled network following 10 stations should be included (we take fewer stations to speed up the processing):

  • BADARY
  • FORTLEZA
  • HARTRAO
  • HOBART12
  • KOKEE
  • NYALES20
  • TIGOCONC
  • TSUKUB32
  • WESTFORD
  • WETTZELL

Signal-to-Noise Ratio

All baselines (except for baselines with station TIGOCONC) should have a minimum signal-to-noise ratio of 20 in the X-band and 15 for the S-band. For station TIGOCONC, which is a small 6 meter transportable telescope, the desirable SNR is lower, namely 15 for X-band and 12 for S-band. This exception for station TIGOCONC has to be set in an external file: snrmin.txt stored in VieVS/CATALOGS/ .

When the snrmin.txt is changed and stored according to our demands, the VieVS software can be started.

  1. Open Matlab, change the current folder to VieVS/WORK and type vievs
  2. In the GUI go to Scheduling - Parameters
    1. Select the 10 stations which will participate in the session.
    2. Choose the start of the session (2011/9/15, 5 pm) and the duration (24 hours) (Each session has to be planned separately).
    3. Check the settings for the minimum SNR for the X-Band und S-Band (20, 15).
    4. You can change the number of radio sources which will be observed simultaneously (1, 2 or 4).
    5. In the GUI menu bar change to Run - Run options and choose the module VIE_SCHED only. The scheduling software can be now started with the Save + Run button.

Here are screenshots of the used GUI menus Scheduling - Parameters and Run - Run options:

VieVS Scheduling GUI

VieVS Run GUI scheduling options

Repeat the scheduling for the second session taking place on September 16, 2011. You have to change only the datum in the menu bar Scheduling.

The created schedules and the scheduling protocol can be found in VIEVS/DATA/SCHED/2011/ .

Simulation of the VLBI observations

You have now created two schedules - instructions for the radio telescopes which source will be observed at which time. Because there do not exist any real observations which would follow your instructions, we will simulate artificial time delays.

In VieVS the simulated VLBI observables are generated taking into account the three most important stochastic error sources in VLBI, i.e. tropospheric wet delay, station clock, and measurement error.

  1. With the simulation tool we want to simulate all observations which were scheduled. Therefore do not choose any directory with outliers (GUI menu bar File - Set input files) and as the directory with specific session options choose an empty directory. If you do not have it in your computer, create an empty directory in VIEVS/DATA/OPT/ and name it e.g. EMPTY.
  2. In the menu bar File - Set input files select sessions (i.e. the prepared schedules in NGS format) which you want to simulate. You find them in VIEVS/DATA/SCHED/2011/ .
  3. In the menu bar Simulation you can choose the error sources which will be taken into account. They can be defined for all stations identically in the GUI by the click on specify now. You do not need to change the default values.
  4. The simulation software can be started in the menu bar Run - Run options. For the creation of the simulated time delays the modules VIE_SIM, VIE_INIT and VIE_MOD have to be run. You can specify a name of the subdirectory where the interim results and data will be stored in VIEVS/DATA/LEVEL0/ and LEVEL1 directories. For the run press the button Save + Run.

Screenshots of the discussed GUI menues (File - Set input files, Simulation and Run - Run options):

VieVS File GUI selection for simulation

VieVS Simulation GUI

VieVS Run GUI simulation option

The simulated NGS files will be stored in VIEVS/DATA/SIM/2011/ .

Analysis of the artificial VLBI observations

The analysis of the artificial observations will be done in the same way as it would be done for the real time delays. It is only recommended not to use any outliers and option directories.

In the analysis of a 24 hour session we usually estimated: clock parameters, zenith wet delays, troposphere gradients, EOP and station coordinates. For the parameterization of these parameters use the default settings of the GUI.

  1. In the menu bar File - Set input files select the NGS files with observations which you want to analyze. The simulated files can be found in VIEVS/DATA/SIM/2011/ .
  2. Do not select any outlier and OPT-directories (in Files).
  3. For a further processing of the data with VIE_GLOB click on the button for preparation of the normal equation matrices which will be used as input for VIE_GLOB. This has to be done in the menu bar Estimation - Global parameters. You can also add partial derivatives of source coordinates into the NEQ system.
  4. In the menu bar Run - Run options select the modules VIE_INIT, VIE_MOD and VIE_LSM. Choose a name for the subdirectory where the results will be saved (VIEVS/DATA/LEVEL3/ subfolder). The analysis can be started with the Save + Run button.

Below you find the related GUI screenshots (File - Set input files, Estimation and Run - Set input files):

VieVS File GUI selection for analysis

VieVS Estimation GUI selection for analysis

VieVS Run GUI analysis option

Use the GUI plotting tool and plot the baseline length repeatabilities computed from these two sessions.

In the menu bar Plotting - Session analysis load your subfolder with the results and click on the button Basel. len. rep.:

VieVS Plotting GUI baseline length repeatability

Global solution

With the module VIE_GLOB one can estimate station coordinates from both sessions in a common adjustment.

Before selecting the options in the GUI, an external .txt file with names of the stations which will be used for datum definition (NNT+NNR) has to be created. The recommended stations for datum definition are listed in the Figure below. Save this file in VIEVS/DATA/GLOB/TRF/DATUM/ . (In case you want to estimate the source coordinates, another datum file for the sources has to be created.)

List of stations suitable for datum definition

  1. Parameters which will be estimated in the global solution can be selected in the menu bar Global solution - Select parameters. In this exercise we will estimate the station coordinates.
  2. In Global solution - TRF/CRF parameterization select for the NNT/NNR condition the file with the names of the stations which you have previously created.
  3. In the menu bar Run - Run options select only the module VIE_GLOB. In the window LEVEL2 subdirectory the name of your subfolder (for the global solution) where you stored the NEQ has to be written. In the window Output directory for VIE_GLOB you can write an arbitrary name of a folder, where the results from VIE_GLOB will be saved.
  4. Start the global solution with the Save + Run button.

Below you find screenshots of the discussed setup in the GUI (Global solution - Select parameters, Global solution - TRF/CRF parameterization and Run):

VieVS Global solution - Select parameters GUI

VieVS Global solution - TRF/CRF parameterization GUI

VieVS Run GUI global solution option

The results are stored in a .txt file in VIEVS/OUT/GLOB/_ESTIMATES/ subfolder and the plots in VIEVS/OUT/GLOB/_PLOTS/subfolder.


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public/vievs_manual/exercises_scheduling_simulating_global_solution.txt · Last modified: 2014/10/21 08:03 by dlandskr