Introduction to GAMIT/GLOBK

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Introduction to GAMIT/GLOBK
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Basic framework GAMIT

• GAMIT Series of programs that analyze GPS phase
  data to estimate parameters
• Station positions (coordinates)
• assumed constant at epoch of data
• Satellite orbital parameters
• Initial conditions
• Radiation parameters
• Phase center offsets
• Earth orientation parameters (EOP)
• Atmospheric delay parameters
• Time-dependent Zenith delays and gradients
• Carrier phase ambiguities

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Basic framework GLOBK

• GLOBK combines solutions from GPS, SLR, VLBI
  data via their parameter estimates and full
  covariance matrices to estimate other parameters
• Site velocities
• temporal derivative of position in mm/yr
• requires a time series of measurements
• Satellite orbit adjustments
• EOP parameters
• Others parameters, e.g., co-seismic displacements

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GAMIT Structure

• History
• Data types
• Steps needed in processing
• Files needed for processing

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GAMIT History

• Development started in late 1970s when MIT was
  building GPS receivers
• Code derived from 1960-1970 planetary ephemeris
  and VLBI software
• Ported to Unix in 1987
• Start of IGS in 1992 prompted development of
  automatic processing schemes
• Fully automatic processing mid-1990s including
  continuous stations and campaign GPS measurements

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Current GAMIT Status

• Automatic features
• FTP'ing of data and other files
• Extraction of information from RINEX files
• Automatic creation of directories
• Status/Warning files to monitor process
• Largest automatic run
• Southern California Earthquake Center
  campaign-style data set from 1992-2002

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Data Types

• L1 and L2 phase
• used for parameter estimation
• P1 and P2 range measurements
• used to help clean phase data
• used to estimate receiver clock offset with
  tolerance of 1 microsecond ( 300 m)
• LC phase
• proportional to geometric range
• sensitive insensitive to ionospheric delay
• linear combination of L1 and L2

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Data Types (contd)

• LG phase
• measures ionospheric delay only
• insensitive to geomery
• Widelane WL
• linear combination of L1, L2, P1 and P2
• Melbourne-Wubbena
• Should be constant (/- noise)

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Major steps for GAMIT

• arc integrates GPS satellite orbits
• model
• calculates theoretical (modeled) phase
• partial derivatives of phase w.r.t. parameters
• autcln
• cleans GPS data and repairs cycle slips
• solve
• estimates parameters via least squares
• calculates goodness of fit

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Minor steps for GAMIT

• Generate yaw table for GPS satellites
• Ocean tide loading coefficients
• Explicit site values
• Interpolation of gridded values
• Estimation clock models
• satellite frequency standard
• ground receiver clock
• Display phase residuals
• time series
• sky plot

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Files needed for processing

• Control files for GAMIT (Lecture 03)
• Initial orbit information from
• Broadcast ephemeris from receivers
• SP3 orbit files from IGS
• GAMIT g-files
• Earth Orientation Parameters (EOP) files
• arc integrates in inertial frame
• Satellite clock files (J-files)
• Ocean tide files

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Files (continued)

• Moon and Sun ephemerides
• orbit integration
• solid Earth tides
• leap-second file (UTC versus GPST or TAI)
• Satellite information files
• Information about stations
• prior (approximate) estimates of coordinates
• receiver and antenna types
• antenna heights
• models for GPS antenna phase center

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Data Files

• Raw receiver files
• UNAVCO teqc program translates to RINEX
• RINEX files
• GAMIT converts to own format x-files
• Data files
• local, or
• ftp'd from international archives

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GAMIT File summary

• GAMIT uses many files
• Many of these files are standard and
  automatically created
• Automatic processing scripts either link or ftp
  these files.
• Some files should be updated from MIT ftp site
  (bowie.mit.edu)
• For example, the satellite information files need
  to be updated when new satellites are launched

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GAMIT documentation

• GAMIT documentation available as PDF, Postscript
  and RTF files
• Each chapter as separate document
• Chapter 1 Overview
• Chapter 2 Theory
• Chapter 3 File structure and names
• Chapter 4 Creating input files
• Chapter 5 Batch Processing
• Chapter 6 Data Editing
• Chapter 7 Running modules

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Chapters continued

• Chapter 8 Atmospheric delay models
• Chapter 9 Utility programs
• Chapter 10 Automatic Batch Processing
• Appendices
• Shell scripts document themselves

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GLOBK Structure

• History
• Steps in processing
• Files in processing

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GLOBK History

• Suite of programs
• Written in mid-1980s to handle combination of
  VLBI data
• Extended in 1989 to handle GPS results GAMIT
  modified to output needed files
• 1990's Extensions to handle SLR data and SINEX
  files

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GLOBK philosophy

• Driven by command files
• Flexible format and file-naming
• Accepts solutions from upstream analysis
• Input upstream solutions as binary h-files
• globk (combination)
• glred (smaller sessions, one at a time)
• glorg (generalized constraints)
• Analysis interpretation

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Required files

• binary h-files generated by htoglb from
• GAMIT h-files (ASCII),
• SINEX
• other
• command files
• very flexible, key-word structure

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Optional files

• Orbit files (svs format)
• EOP (pmu files)
• Earthquake/rename definitions
• "Source" files with subsets of commands
• apriori coordinate and motion files
• Information not specified by user files is
  generated by various techniques

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GLOBK documentation

• Available as PDF, PS and RTF files
• Nearly all modules in GLOBK suite have up-to-date
  documentation on-line
• To get help for a command, type its name alone on
  the command line.

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Differences between GAMIT and GLOBK

• GAMIT operations and files are rigid GLOBK uses
  flexible formats and is used in many different
  ways
• GAMIT tends to stop if a problem is encountered
  (less so these days) GLOBK tries to continue as
  far as it can by defaulting values
• Pros/Cons GAMIT stops can be frustrating GLOBK
  may appear to work but might generate erroneous
  results.

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Differences (continued)

• Basic aim of GAMIT is to generate
  positions/orbits for one day of data. Standard
  product of GAMIT is a minimally constrained
  solution
• GLOBK flexibly combines minimally constrained
  parameter estimates from one or more measurement
  types (GPS, SLR, VLBI, DORIS)

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Getting GAMIT/GLOBK

• Free for research/non-commercial license
• Fortran Source code
• Executables for HP, Solaris and Linux

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Getting GAMIT/GLOBK (2)

• Point browser to
• http//www-gpsg.mit.edu/simon/gtgk/index.html
• Click on icon to download software
• Complete the license information, and send via
  e-mail to rwk_at_chandler.mit.edu
• Password changes with every new release.
• Registered users will receive an email message
  when new passwords are set

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Getting GAMIT/GLOBK (3)

• Enter password
• Your working directory is now
• pub/gps/updates/source
• contains source code and other files
• Copy all .tar files and install_software to your
  machine
• etopo5 and maps not strictly needed
• Scherneck ocean grid file needed for ocean
  loading corrections.

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Getting GAMIT/GLOBK (4)

• To get documentation or executables, go up one
  directory (cd ../)
• Remember to read the file release_notes to learn
  about important changes to the software.
• If you already have a previous release,
• the incremental_updates directory contains the
  latest changes since the previous release.
• Then use install_updates

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Installing GAMIT/GLOBK

• In the directory on your machine containing the
  tar files, simply type install_software
• Wait a while.

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After installation script finishes

• Use ln -s to make link in home directory called
  gg (for gamit/globk). This should link points to
  the same directory as install_software was done.
• Add to path in /.cshrc file
• /gg/com /gg/gamit/bin /gg/kf/bin
• Also add to your /.cshrc file
• setenv HELP_DIR /gg/help
• setenv INSTITUTE lt3-char codegt
• We use the tcsh or csh shells

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Summary

• GAMIT Phase data processing
• Suite of programs needed for processing
• Files needed from various sources
• Automatic script sh_gamit generates most files
• GLOBK Combination program
• Also a suite of programs
• Files from GAMIT and other sources
• Contact person for GAMIT/GLOBK
  rwk_at_chandler.mit.edu