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Introduction to GAMIT/GLOBK

Introduction to GAMIT/GLOBK. 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

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Introduction to GAMIT/GLOBK

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  1. Introduction to GAMIT/GLOBK

  2. 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 GAMIT/GLOBK Tutorial 1

  3. 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 GAMIT/GLOBK Tutorial 1

  4. GAMIT Structure • History • Data types • Steps needed in processing • Files needed for processing GAMIT/GLOBK Tutorial 1

  5. 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 GAMIT/GLOBK Tutorial 1

  6. 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 GAMIT/GLOBK Tutorial 1

  7. 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 GAMIT/GLOBK Tutorial 1

  8. Data Types (cont’d) • 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) GAMIT/GLOBK Tutorial 1

  9. 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 GAMIT/GLOBK Tutorial 1

  10. 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 GAMIT/GLOBK Tutorial 1

  11. 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 GAMIT/GLOBK Tutorial 1

  12. 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 GAMIT/GLOBK Tutorial 1

  13. 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 GAMIT/GLOBK Tutorial 1

  14. 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 GAMIT/GLOBK Tutorial 1

  15. 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 GAMIT/GLOBK Tutorial 1

  16. Chapters continued • Chapter 8: Atmospheric delay models • Chapter 9: Utility programs • Chapter 10: Automatic Batch Processing • Appendices • Shell scripts document themselves GAMIT/GLOBK Tutorial 1

  17. GLOBK Structure • History • Steps in processing • Files in processing GAMIT/GLOBK Tutorial 1

  18. 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 GAMIT/GLOBK Tutorial 1

  19. 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 GAMIT/GLOBK Tutorial 1

  20. Required files • binary h-files generated by htoglb from: • GAMIT h-files (ASCII), • SINEX • other • command files • very flexible, key-word structure GAMIT/GLOBK Tutorial 1

  21. 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 GAMIT/GLOBK Tutorial 1

  22. 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. GAMIT/GLOBK Tutorial 1

  23. 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. GAMIT/GLOBK Tutorial 1

  24. 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) GAMIT/GLOBK Tutorial 1

  25. Getting GAMIT/GLOBK • Free for research/non-commercial license • Fortran Source code • Executables for HP, Solaris and Linux GAMIT/GLOBK Tutorial 1

  26. 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@chandler.mit.edu • Password changes with every new release. • Registered users will receive an email message when new passwords are set GAMIT/GLOBK Tutorial 1

  27. 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. GAMIT/GLOBK Tutorial 1

  28. 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 GAMIT/GLOBK Tutorial 1

  29. Installing GAMIT/GLOBK • In the directory on your machine containing the tar files, simply type install_software • Wait a while. GAMIT/GLOBK Tutorial 1

  30. 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 <3-char code> • We use the tcsh or csh shells GAMIT/GLOBK Tutorial 1

  31. 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@chandler.mit.edu GAMIT/GLOBK Tutorial 1

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