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1.Creating A Work Area
2.Downloading GPS Data
3.Downloading an Ephemeris
4.Selecting Sites To Hold Fixed
5.Creating The POM Files
6.Running run_survey
7.Manually Modifying The Processing

Description

The programs of this software suite can only read RINEX data files. This format is well documented and has become a de facto standard for distributing high accuracy GPS data. All data distribution centers mentioned here use this format although the files themselves may be compressed to speed transmission. Most GPS receiver manufacturers and some third party development groups provide convertors to RINEX so the requirement for this format should not be an obstacle.

Data from a receiver is commonly converted into four RINEX files, each with a specialized task. The formats for these files are described in the RINEX standards. The naming convention for these files is:

  SSSSDDD#.YYC
where
  SSSS is the four character site ID
  DDD is the three digit day-of-year
  # is a session code
    0 = zero = all data for that site for that day
    a-x = hourly subset: a = 00:00 - 01:00 UT, b = 01:00 - 02:00 UT, ...
There is no redundant, overlap data between these consecutive hourly data sets.
    1-9 = session (although, historically, other characters have been used)
  YY is the two digit year
  C is the file type designation
    d = Hatanaka compressed observations
    m = surface meteorological data
    n = broadcast navigation message
    o = lower case letter O = observations
    s = data status and quality control information
Only the observation files are required for these tutorials with one exception; one broadcast navigation file is also necessary. The navigation file, also known as the nav, broadcast or RINEX "n" file, from any one of the sites to be processed could be used. Many data centers also generate a global navigation created from many nav files from a globally distributed set of sites and suitable for use with any data set. If multiple broadcast navigation files are present at the start of processing, run_survey will select one, based on the number of satellites included, and continue normally.

Retrieving data taken locally, by one's own group or institution, should be straight forward and may be as simple as copying the file into the appropriate location from some other directory or uploading the data from a PC. Such cases will not be discussed here. Downloading data from a remote repository via the internet is only slightly more difficult. In this tutorial, we'll download data from three sites from three different data centers, not because it is necessary but, rather, to sample some of the institutions redistributing data as a service to the community. Every data center has a policy about how long recent data are kept on-line, on reloading historical data and/or transfer via other media. Contact the data center directly to inquire about those policies.

This begs the question of why the GPS data available through these centers are of interest beyond the project for which they were collected? Often it is desirable to include data from one or more sites with accurately known coordinates in the processing. Holding these sites' coordinates to their known values, along with the GPS satellites' coordinates, provides a connection to an accepted, common reference frame, such as the NAD83, WGS84 or ITRF2000, and forces the site coordinates computed as part of the processing to be consistent with that frame. Obviously, sites with well determined coordinates and long histories are most desirable for the purpose of being a reference site. In turn, collecting, distributing and archiving these data becomes a valuable service provided to the GPS community.

Three sites, GODE, SOL1 and USNA will be used in these tutorials. These sites were selected to be representative of a small network and to be readily available from any of the three data centers mentioned. GODE will act as our connection to the ITRF reference frame; SOL1 and USNA will act as our unknown sites although they too have well determined coordinates. As mentioned, a nav file will also be needed and downloaded.

Tutorial

At the end of the last tutorial, the current directory was "~/tutorial/".

Change to the working directory, "01_033/".
cd 01_033
The first stop will be the CORS data center, operated by the NGS, which redistributes GPS data from government and private sites predominately in North America. CORS, by the way, stands for Continuously Operating Reference Station. The site to be downloaded, SOL1, is located in Solomons Island, MD. CORS data from this center can be selected and downloaded via ones web browser from the CORS data center web page along with maps, site logs and a wealth of other useful information. Unfortunately, only one file at a time can be retrieved using this method from this or any of the data centers. Using FTP will almost certainly be quicker for downloading several files. Note that scripts facilitating downloading data files from the CORS site are available.

Retrieve SOL1 data from the CORS anonymous FTP site:
 
  ftp cors.ngs.noaa.gov
  Connected to www.ngs.noaa.gov.

    ... welcome message ...

  220 NGS FTP server ready.
  Name (...): anonymous
  331 Guest login ok, send your complete e-mail address as password.
  Password: me@my.home.address
  ftp> bin
  200 Type set to I.
  ftp> prompt
  Interactive mode off.
  ftp> cd rinex/2001/033/sol1
  250 /cors/rinex/2001/033/sol1" is new cwd.
  ftp> get sol10330.01o.gz
  local: sol10330.01o.gz remote: sol10330.01o.gz

    ... transfer size and time info ...

  226 Transfer completed.
  ftp> bye
  221 Goodbye.
   
  run_survey recognizes GNU gzip compressed (indicated by the .gz extension) and Unix compressed files (.Z extension) as valid file types so no additional actions need to be taken. The CORS FTP site also contains site logs, ASCII files which give information about a site and its hardware. These are stored in the station_log subdirectory.
USNA is at the U. S. Naval Academy in Annapolis, MD. This time, let's retrieve the data from the Scripps Orbit and Permanent Array Center, SOPAC, The Scripps Institute of Oceanography acts as one of the International GNSS Service's (IGS) global data center. In that capacity, SOPAC provides access to GPS tracking sites distributed around the world, site logs, IGS precise ephemerides and other products. In addition, SOPAC redistributes data from many national and international projects.

Retrieve the USNA data from the SOPAC anonymous FTP site:
 
  ftp garner.ucsd.edu
  Connected to garner.ucsd.edu.

    ... welcome message ...

  Name (...): anonymous
  331 Guest login ok, send your complete e-mail address as password.
  Password: me@my.home.address
  ftp> bin
  200 Type set to I.
  ftp> prompt
  Interactive mode off.
  ftp> cd pub/rinex/2001/033
  250 "/pub/rinex/2001/033" is new cwd.
  ftp> get usna0330.01d.Z
  local: usna0330.01d.Z remote: usna0330.01d.Z

    ... transfer size and time info ...

  226 Transfer completed.
  ftp> bye
  221 Goodbye.
  The final example will retrieve data for GODE, the Goddard Optical Research Facility east pier in Greenbelt, MD, and a broadcast nav file from the NASA/Goddard Crustal Dynamics Data Information System, CDDIS. The CDDIS also serves as an IGS global data center providing GPS data from the global network, IGS products, site logs. The CDDIS also acts as a clearinghouse for space geodesy data from all NASA and associated institutions regardless of the observation technique. The CDDIS also generates a global nav file identified by the four character ID BRDC. This global nav file will downloaded too. Retrieve the data from the CDDIS anonymous FTP site:
 
  ftp cddis.gsfc.nasa.gov
  Connected to cddis.gsfc.nasa.gov.

    ... welcome message ...

  Name (...): anonymous
  331 Guest login ok, send your complete e-mail address as password.
  Password: me@my.home.address
  ftp> bin
  200 Type I ok.
  ftp> prompt
  Interactive mode off.
  ftp> cd gps/data/2001/033/01d
  250 "/gps/data/2001/033/01d" is new cwd.
  ftp> get gode0330.01d.Z
  local: gode0330.01d.Z remote: gode0330.01d.Z

    ... transfer size and time info ...

  226 Transfer completed.
  While at this FTP site, retrieve the global broadcast navigation message file.
 
  ftp> cd ..
  250 "/gps/data/2001/033" is new cwd.                              
  331 Guest login ok, send your complete e-mail address as password.
  cd 01n
  250 "/gps/data/2001/033/01n" is new cwd.
  ftp> get brdc0330.01n.Z
  local: brdc0330.01n.Z remote: brdc0330.01n.Z

    ... transfer size and time info ...

  226 Transfer completed.
  ftp> bye
  221 Goodbye.
 

Notes

The directory "~/tutorial/01_033/" should now contain four files:
ls
brdc0330.01n.Z gode0330.01o.Z sol10330.01o.gz usna0330.01d.Z
 
 
2009-02-15 18:41 UTC