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====== GPS Occultation ======

==== Explanation of Data ====

The idea of using radio occultations to obtain atmospheric profiles is a relatively new research endeavor.  Several missions have taken place throughout the last decade to obtain GPS occultation soundings:  GPS/Meteorology (GPS/MET), Satelite de Aplicaciones Cientificas-C (SAC-C), Challenging Mini Payload Satellite (CHAMP), Exploitation of Meteorological Satellites Polar System-Global Receiver for Atmospheric Sounding (EPS-GRAS), and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC).  One of the most recent missions is the FORMOSAT-3/COSMIC project which is a joint venture between the Taiwan National Space Organization and UCAR.  The website for this project is: www.cosmic.ucar.edu.

The COSMIC project began in April 2006 with the lanch of six GPS satellite receivers.  These GPS receivers pick up the satellite signal, then from the occulation (bend) of the signal, information about the atmosphere (such as temperature and water vapor) can be calculated. Therefore, this technique is a way to get atmospheric profiles using satellites instead of weather ballons. 

These atmospheric profiles created from the GPS receivers have shown to be very accurate in the upper atmosphere.  However, they are less accurate in the lower atmosphere, mainly from the ground to about 3-4 km.  Since this data is still new, it is still being tested and checked for accuracy. 

==== References ==== 

**[[http://www.cosmic.ucar.edu/related_papers/Anthes364.pdf|Applications of COSMIC to Meteorology and Climate]]**\\ 
R. A. Anthes, C. Rocken, and Y.-H. Kuo\\  
//Terrestrial, Atmospheric, and Oceanic Sciences//\\ 
2000

**[[http://www.cosmic.ucar.edu/launch/COSMIC_BAMS_2460_accepted.pdf|The COSMIC/FORMOSAT-3 Mission:  Early Results]]**\\ 
R. A. Anthes, P. A. Bernhardt, Y. Chen, L. Cucurull, K. F. Dymond, D. Ector, S. B. Healy, S.-P. Ho, D. C. Hunt, Y.-H. Kuo, H. Liu, K. Manning, C. McCormick, T. K. Meehan, W. J. Randel, C. Rocken, W. S. Schreiner, S. V. Sokolovskiy, S. Syndergaard, D. C. Thompson, K. E. Trenberth, T.-K. Wee, N. L. Yen, and Z. Zeng\\ 
//Bulletin of American Meteorological Society//\\ 
2007

**[[http://www.cosmic.ucar.edu/groupAct/references/2006GL027557.pdf|Estimates of the precision of GPS radio occultations from the COSMIC/FORMOSAT-3 mission]]**\\ 
W. Schreiner, C. Rocken, S. Sokolovskiy, S. Syndergaard, and D. Hunt\\ 
//Geophysical Research Letters//\\ 
2007

**[[http://ams.allenpress.com/archive/1520-0477/89/12/pdf/i1520-0477-89-12-1863.pdf|Prospects of the EPS GRAS Mission for Operational Atmospheric Applications]]**\\ 
Juha-Pekka Luntama, Gottfried Kirchengast, Micahel Borsche, Ulrich Foelsche, Andrea Steiner, Sean Healy, Axel von Engeln, Eoin O'Clerigh, and Christian Marquardt\\ 
//Bulletin of American Meteorological Society//\\ 
2008

**[[http://www.inscc.utah.edu/~pstaten/talks/Staten_Reichler_08_AGU_CA.pdf|Practical GPS RO Temperature Precision]]**\\ 
Paul W. Staten and T. Reichler\\ 
//AGU Fall Meeting//\\ 
2008

**[[http://www.inscc.utah.edu/~pstaten/talks/Staten_Reichler_08_SPARCGA_Bologna.pdf|Space-born detection of long-term tropopause trends]]**\\  
Paul W. Staten and T. Reichler\\ 
//SPARC General Assembly//\\ 
2008

**[[http://www.inscc.utah.edu/~pstaten/talks/Staten_Reichler_07_7thEMS_Spain.ppt|Can radio occultation be used to discern long-term tropopause trends?]]**\\  
Paul W. Staten and T. Reichler\\ 
//7th EMS Annual Meeting/8th ECAM//\\ 
2007

**{{:atmos:citation:research:nws_presentation.pdf|National Weather Service Perspective on Radio Occultation}}**\\
Lidia Cucurull\\
//NOAA Workshop on GNSS RO Improvements to Weather Forecasts//\\

**{{:atmos:citation:research:ro_papers.pdf|List of RO papers from 2009 Functional Review}}**\\
North Dakota Center of Excellence in Space Technology and Operations\\

**[[http://space.hsv.usra.edu/JCSDA/JCSDA%20Presentations/Cucurull.pdf|GPS/RO Data Assimilation]]**\\
Lidia Cucurull\\
//JCSDA DA Colloquium, Stevenson, WA//\\
2009

===== Getting Access to Data  =====


In order to gain access to the data, a username and password must be created.  The website to sign up for data access is: www.cosmic.ucar.edu.

  * Go to the left hand side where it says "CDAAC Data Access" and click the sign up link under COSMIC
  * Click on the Data User Agreement and click agree
  * Fill in the rest of the information
  * A user name and a password will be sent to the provided e-mail address.  
    This can take up to seven days, but will most likely arrive within a day or two.

===== Getting Data =====


==== Wget ====

Here is the command for Wget to download data from the COSMIC site:

''wget -nd -np -r -l 10 -w 2 --http-user=xxxx --http-passwd=xxxx\\ 
http://cosmic-io.cosmic.ucar.edu/cdaac/login/cosmicrt/level2/atmPrf/2006.207/ ''


  * Note:  There is a data format outline on the COSMIC website which provides information on the different types of data at http://cosmic-io.cosmic.ucar.edu/cdaac/fileFormats/index.html.

  * Note:  This command is to get the atmPrf data.  For this data, the syntax is: cosmic-io.cosmic.ucar.edu/cdaac/login/cosmicrt/level2/ncpPrf/2008 

  * Note: The ncpPrf data is data based from a model. 

==== Bash Script =====

A bash script was written to download this data.  This script will create a subdirectory for each day of the year and download all available data for that day into that subdirectory.  A sample of the code is listed below which will more than likely need modifications in order to work. 


<code>
X=1  # Initialize starting value for x
while [ $X -le 367 ]; do  # Loop over certain values of x.
  if [ $X -ge 1 ]; then   # For days 1 through 9
    if [ $X -lt 10 ]; then
      if [ ! -e "2008_00${X}" ]; then # If this directory doesn't exist
        mkdir "2008_00${X}"  # Make the directory
        cd 2008_00${X}       # Go into the directory
        # It is important to use -l 1 so that it doesn't go up and get other directories via ".." directory.
        wget -nd -np -l 1 -r -w 2 --http-user=xxxxx --http-passwd=xxxxx http://cosmic-io.cosmic.ucar.edu/cdaac/login/cosmic/level2/ncpPrf/2008.00${X}/  #get all the files for that day
        cd .. # Go down a level
      fi  # end if
    fi  # end if
  fi # end if
  if [ $X -ge 10 ]; then # same as above, just for days 10 through 99
    if [ $X -lt 100 ]; then
      if [ ! -e "2008_0${X}" ]; then
        mkdir "2008_0${X}"
           cd 2008_0${X}
        # It is important to use -l 1 so that it doesn't go up and get other directories via ".." directory.
        wget -nd -np -l 1 -r -w 2 --http-user=xxxxx --http-passwd=xxxxx http://cosmic-io.cosmic.ucar.edu/cdaac/login/cosmic/level2/ncpPrf/2008.0${X}/
        cd ..
      fi
    fi
  fi
  if [ $X -ge 100 ]; then # Same as above, just for days 100 to 366 (leap year)
    if [ $X -lt 367 ]; then
      if [ ! -e "2008_${X}" ]; then
        mkdir "2008_${X}"
        cd 2008_${X}
          # It is important to use -l 1 so that it doesn't go up and get other directories via ".." directory.
         wget -nd -np -l 1 -r -w 2 --http-user=xxxxx --http-passwd=xxxxxx http://cosmic-io.cosmic.ucar.edu/cdaac/login/cosmic/level2/ncpPrf/2008.${X}/
        cd ..
      fi
    fi
  fi
  X=$( (X+1) )
done  

</code>

===== Software =====


=== ncBrowse ===

So far, ncBrowse is the best program for plotting GPS data; however, a program for plotting aircraft data is still being searched for. (Note: The aircraft data works with ncBrowse but plotting vs time, not altitude.)

How to use:

  * Open ncBrowse using the command: ''ncBrowse''
  * Go to file, then open file
  * Find the file needed (must end be netcdf format)
  * After the file is loaded, click on New Map
  * Next, the x and y-axis variables can be chosen by dragging them to the proper spot
  * Name the plot and hit done
  * On the main screen, scroll to the bottom of the variable list to find the new plot
  * Double click to open plot and Parameter Editor
  * Can also just double click on a variable from the list to plot without making a new map. 

ncBrowse is also helpful when looking at the structure and organization of a netcdf file.  The structure of the data can be viewed as a tree or as a table for clarity.  


=== ncplot ===

ncplot will plot netCDF files

Depending on the version you have, things might be a little different.

ncplot will allow you to select certain variables to plot, it will usually be against a set variable which is defined in the netCDF file.

How to Use:

To start ncplot:

  ''ncplot <filename>.nc''

ncplot will then either open one or two windows, depending on the version. (If older version, one window will open with the different variables listed, if newer version, two windows will open, one is a graph and the other is where different variables are listed.).

Next, select the variable you want to plot versus the assigned variable that was defined in the netCDF file, usually time or distance.

There is basic modification allowed in ncplot, but if you need more specific modification, use a different program. This program also will not let you plot variables against each other easily.



=== ncdump ===

This command allows all the information to be seen that is in a netcdf file.  The command can be used as follows: ''ncdump <file name>''  Normally there is a lot of information in these files so consider piping it with less. So the command would be: ''ncdump <file name> | less''


It is always a good idea to look at the data first to be familiar with what kind of variables are available. ncdump will also list the variables with the units and the long names. 

=== ncview ===

This program also loads netcdf files and makes specified plots.  However, this software is not always the easiest to work with due to the limitations of manipulation.  Run the software by typing the following command in the command line: ''ncview <file name>''.  Be careful when closing windows because if the X button is clicked and not the close command, the program will terminate.