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 Earth orientation parameters (EOP) Atmospheric delay parameters Time-dependent Zenith delays and gradients Carrier phase ambiguities 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

GAMIT Structure History Data types Steps needed in processing Files needed for processing 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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) 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

Chapters continued Chapter 8: Atmospheric delay models Chapter 9: Utility programs Chapter 10: Automatic Batch Processing Appendices Shell scripts document themselves 27-Nov-18 GAMIT/GLOBK Tutorial 1

GLOBK Structure History Steps in processing Files in processing 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

Required files binary h-files generated by htoglb from: GAMIT h-files (ASCII), SINEX other command files very flexible, key-word structure 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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. 27-Nov-18 GAMIT/GLOBK Tutorial 1

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. 27-Nov-18 GAMIT/GLOBK Tutorial 1

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) 27-Nov-18 GAMIT/GLOBK Tutorial 1

Getting GAMIT/GLOBK Free for research/non-commercial license Fortran Source code Executables for HP, Solaris and Linux 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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. 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

Installing GAMIT/GLOBK In the directory on your machine containing the tar files, simply type install_software Wait a while. 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1

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 27-Nov-18 GAMIT/GLOBK Tutorial 1