1. GLOBK Theory and Solution Types
Kalman filtering
Stochastic processes
Applications

2. Summary of Use of GLOBK
GAMIT output is an “h-” file of loosely constrained estimates and covariances, used as input (quasi-) observations to GLOBK
GAMIT ascii h-files converted to binary h-files for input to GLOBK
GLOBK combines h-files from several networks for a single day and h-files from multiple epochs to estimate positions in a time series or average velocities
31-Dec-18
GAMIT/GLOBK Tutorial 7

3. Motivation GLOBK is quicker than GAMIT
Typically perform many GLOBK runs to test different hypotheses and correct problems in original GAMIT analyses with out re-running GAMIT.
Ultimately once all is understood, GAMIT is rerun and the process repeated for the final results
31-Dec-18
GAMIT/GLOBK Tutorial 7

4. Kalman Filtering
Estimation method in which parameters can be random processes
Examples:
polar motion and UT1
radiation pressure effects on satellites
coordinates after EQ
Nearly all parameters in GLOBK can be stochastic or deterministic
Statistical character set by commands
apr_XXX
mar_XXX
31-Dec-18
GAMIT/GLOBK Tutorial 7

5. Stochastic processes
Stochastic models in globk are random walks characterized by variance of change in process over an interval of time
Variance grows linearly with time (standard deviation grows as square root of time)
If process noise is zero, then classed as deterministic
31-Dec-18
GAMIT/GLOBK Tutorial 7

6. Common applications of GLOBK
Repeatability analysis (glred)
individual sessions
combined files
Combine sessions to get average position over experiment
connects sites observed separately
Combine averaged files together to estimate velocities
and/or earthquake offsets
31-Dec-18
GAMIT/GLOBK Tutorial 7

7. Things GLOBK cannot do Remove non-linear effects
GLOBK assumes a linear model
Repair mistakes in original analysis
cycle slips
wrong antenna phase center models
resolve ambiguities
(would make files too large)
But GLOBK can delete sites
can help avoid contaminating solution
31-Dec-18
GAMIT/GLOBK Tutorial 7

8. Preparing to run GLOBK Shell script sh_glred
automates standard runs with globk
useful to start
assumes internet access
but can be run without internet
Globk needs:
Binary h-files
written by htoglb from solve’s output
A command file to tell it what to do
e.g., globk.cmd
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GAMIT/GLOBK Tutorial 7

9. Binary h-files Generated with htoglb
from the h<expt>a.<yy><doy> files
solve writes these in day dir
or from SINEX files
available from IGS
and soon for IVS for VLBI and ILS for SLR results
Program detects file type
Or written by globk itself
31-Dec-18
GAMIT/GLOBK Tutorial 7

10. running htoglb htoglb dir ephfile[options] file_list
dir is directory where binary files will be written
ephfile is satellite orbit file
not used any more, because of make_svs command
options are optional options
file_list a list of ascii files
usually with UNIX wild cards
or generated from ls
31-Dec-18
GAMIT/GLOBK Tutorial 7

11. running htoglb htoglb glbf glbf/india.svs ????_???/hgps?a.*
run this from processing directory
converts all ascii hfiles for our runs
puts the binary H-files in directory glbf
Some options:
-n sets name format
-m allocates more memory
(only needed for very large SINEX files)
-c replaces SINEX constraints
31-Dec-18
GAMIT/GLOBK Tutorial 7

12. GLOBK command files Example in ~/gg/templates/globk_comb.cmd
Basic features of command files
These are also common to other input files used by globk
Any non-blank character in column 1 denotes comment
Only minimum redundant part of command needed
In many cases, not all arguments need be given
Examine the command file from templates
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GAMIT/GLOBK Tutorial 7

13. Defining a coordinate system in GLOBK
To define a coordinate system need
origin (3 parameters)
Theoretically, origin is defined but mis-modeled perturbation on satellite orbits can cause shift in origin
orientation (3 parameters)
scale (1 parameter)
Theoretically, scale should be defined but systematic errors in the position of the phase center of satellites can cause a scale error
31-Dec-18
GAMIT/GLOBK Tutorial 7

14. Defining a coordinate system with glorg
The glorg module of GLOBK applies rotation, translation, and scale to best align estimated coordinates with prior values for a selected set of stations
this process is called “stabilization” (pos_org)
subset of stations is the “stabilization list” (use_site)
User chooses which parameters are free (estimated)
translation xrot yrot zrot
rotation xtran ytran ztran
scale
If scale is used, it should be explicitly estimated in globk
31-Dec-18
GAMIT/GLOBK Tutorial 7

15. Velocity origin
Same approach can be applied to site velocities, i.e. rates of
translation
rotation
scale
Use rate_org command with same key words
This will align the estimated station velocities with prior values for (the same) set of selected sites
No specific station coordinates are tightly constrained
We call this “velocity stabilization” in jargon.
In papers we write, “velocities with respect to a set of stations on stable North America”
31-Dec-18
GAMIT/GLOBK Tutorial 7

16. Structure of GLOBK GLOBK is a suite of modules
can be run end-to-end as the globk module
or run individually, e.g.,
glred (reduce one day at a time)
glorg (origin)
31-Dec-18
GAMIT/GLOBK Tutorial 7

17. Program sequence
globk Main controlling program that calls other modules
glinit Initialization program
reads the headers of all the binary h-files
to determine their contents
glfor Forward kalman filter
glbak Backards running filter and smoother
glsave Saves combined binary h-files
glout writes output to files and/or screen
glorg defines origin and coordinate system
31-Dec-18
GAMIT/GLOBK Tutorial 7

18. Stand-alone programs
glorg (and sometimes glsave) can be run as separate programs, typically after globk has finished
or they can be run via globk by placing their commands globk.cmd file
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GAMIT/GLOBK Tutorial 7

19. Associated programs glist lists the contents of a series of H-files
useful to check input before solution
glbtosnx generates SINEX files from binary H-files
only way to view header of binary H-files
hfupd updates binary h-files for changes in station info or SINEX header file
(distributed by IGS)
glred runs globk multiple times
31-Dec-18
GAMIT/GLOBK Tutorial 7

20. Running globk globk crt print_file log_file gdl_file cmd_file
crt = 6 for screen output
print_file output print file name (can be 6)
log_file output log file name (can be 6)
gdl_file input file listing H-file names to process
cmd_file input command file name
If you can’t remember this order, just type “globk” to get the help message.
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GAMIT/GLOBK Tutorial 7

21. globk files .gdl lists all H-file names to include in solution
run 'ls' on the directory containing the binary H-files
.cmd
contains commands
user modifies with a text editor
based on template
.prt and .log
print and log files written by globk
31-Dec-18
GAMIT/GLOBK Tutorial 7

22. Main globk commands
globk uses three "scratch" files which can be explicitly named
com_file -- common file contains information about run
srt_file -- binary file with time-sorted list of hfiles
sol_file -- binary file with solution and covariance matrix
To use glorg as stand-alone module, the same com_file command must appear in both the command file for globk and the command file for the (subsequent) glorg run
31-Dec-18
GAMIT/GLOBK Tutorial 7

23. apr_xxx This command will free (estimate) a parameter.
If this command is absent, the corresponding parameter will be held fixed to its apriori value
If zero is given as apriori sigma, then parameter is not estimated (effectively left unconstrained)
To force a parameter to its apriori value, use F as sigma
31-Dec-18
GAMIT/GLOBK Tutorial 7

24. Site position estimation apr_neu
apr_neu site sigN sigE sigU sigVN sigVE sigVU
site is site name; may be all to apply to all stations
sigN sigE sigU apriori sigmas on position in meters
sigVN sigVE sigVU apriori velocity sigmas in m/yr
apr_neu SITE F 0.1 0
set prior sigma for N, E, U positions to 10 meters each
force the north velocity to its apriori value
set the prior sigma for the east velocity to 0.1 m/yr
do not estimate a vertical velocity
31-Dec-18
GAMIT/GLOBK Tutorial 7

25. Estimating site position stochastically mar_neu
Markov random walk process
mar_neu site rwN rwE rwU rwVN rwVE rwVU
site station name, may be all
rwN rwE rwU random walk for positions (m^2/yr)
rwVN rwVE rwVU random walk for velocities
in (m/yr)^2/yr
usually use 0 0 0
31-Dec-18
GAMIT/GLOBK Tutorial 7

26. Estimating satellite orbit parameters apr_svs
Set apriori sigma for space vehicle parameters
apr_svs PRN X Y Z Vx Vy Vz rad_parms
PRN satellite identification PRN_nn or ALL
X Y Z Position sigma (3 values) (m)
Vx Vy Vz Velocity sigma (3 values) (mm/s)
rad_parms Radiation parameter sigmas
up to 11 values
fraction 0-1
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GAMIT/GLOBK Tutorial 7

27. Satellite orbit estimation apr_svs (continued)
Have global GPS sites to constrain orbits
apr_svs all R
Local data only, constrain orbit to apriori value
apr_svs all R
R at end of line means remaining radiation parameters
31-Dec-18
GAMIT/GLOBK Tutorial 7

28. EOP parameters Apriori uncertainties
apr_wob <X> <Y> <Xdot> <Ydot>
X Y pole position (mas)
Xdot Ydot rate of change (mas/day)
apr_ut1 <ut1> <ut1 dot>
<ut1> in mas, <ut1 dot> in (mas/day)
Process noise has same form
mar_wob <RWX> <RWY> <RWXd> <RWYd>
units are (mas2)/yr and (mas/day)2/yr
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GAMIT/GLOBK Tutorial 7

29. EOP parameters (continued)
mar_ut1 for for ut1 (mas2) and (mas/day)2/yr
Normally used in two forms:
Global network of stations
apr_wob
apr_ut1 10 1
Regional network (constrained)
apr_wob
apr_ut
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GAMIT/GLOBK Tutorial 7

30. UT1 issues
Some care needs to be used with UT1 because it cannot be separated from the nodes of the satellites orbits
UT1 tables and orbits must have used the same frame if constraints on UT1 are applied.
If local processing, then should be OK
If orbits were determined with different EOP tables, then UT1 should be loosened
31-Dec-18
GAMIT/GLOBK Tutorial 7

31. Frame estimation commands
apr_tran <X YZ> <Vx Vy Vz> -- estimate translation of coordinate system
apr_scale <ppb> <ppb/yr> -- estimate scale and scale rate
mar_tran and mar_scale specify the process noise
Normally only used in global networks
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GAMIT/GLOBK Tutorial 7

32. File commands
A number of commands are used to control the files read by globk
apr_file <name> lets a new apriori coordinate file be used
Format is:
Site_name X Y Z Vx Vy Vz epoch
Site names are 8 characters, for GPS ABCD_GPS
X Y Z are geocenter Cartesian (m)
Velocities are m/yr
Epoch is decimal years
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GAMIT/GLOBK Tutorial 7

33. Apriori position files
apr_files commands can be issued multiple times with the latest values taken
If not specified, GAMIT apriori coordinates are used (no velocities)
EXTENDED option allows more complex behaviors (see globk.hlp)
31-Dec-18
GAMIT/GLOBK Tutorial 7

34. Orbit files
When single days are processed, orbits from GAMIT solution will be used by default
For multiple days, with globk, the make_svs command is needed to generate an orbit file.
Form:
make_svs <file name>
<file name> will be overwritten
Command must be near top of command file.
31-Dec-18
GAMIT/GLOBK Tutorial 7

35. EOP files
GAMIT EOP are used by default, but can be changed with in_pmu command
Form:
in_pmu <file name>
Format of file is
yy mm dd hr min Xpole +- Ypole +- UT1-AT +-
Must be uniformly spaced
Pole in arc-seconds, UT1 in time-seconds
31-Dec-18
GAMIT/GLOBK Tutorial 7

36. Earthquake files
It is possible to automatically account for displacement caused by earthquakes with the earthquake file
Main functions:
specify earthquakes with 2-char code
rename sites
Rename command:
Rename <old> <new> [HFile code] [Epoch ranges] [Position shift]
31-Dec-18
GAMIT/GLOBK Tutorial 7

37. Earthquakes in eq_file
For earthquake:
eq_def <code> Lat Long Radius Depth Epoch
eq_cosei <code> <Static Variance> <distance dependent variances>
eq_post <code> <dur> <Static RW> <distance dependent RW>
eq_rename <code>
The rename option cause site to renamed from xxxx_GPS to xxxx_G[Code]
31-Dec-18
GAMIT/GLOBK Tutorial 7

38. GLORG
Glorg allows coordinate systems to be defined by applying explicitly rotations and translations rather than tightly constraining some stations
It can be run as a separate program or during the globk run itself
31-Dec-18
GAMIT/GLOBK Tutorial 7

39. Glorg imbedded commands
The globk command file imbedded glorg commands are:
org_cmd <glorg command file name>
org_opt <Options for output>
org_out <output file name>
If org_out is not given then the extent on the print file name is replaced with .org
Next lecture discussed glorg
31-Dec-18
GAMIT/GLOBK Tutorial 7

40. Miscellaneous commands
max_chi <max chi**2 Increment> <max prefit difference> <max rotation>
allows automatic deleting of bad h-files and bad coordinates
app_ptid allows applying the pole tide correction if not included in gamit run.
(Careful with SINEX files, since these don't specify if correction has been applied)
31-Dec-18
GAMIT/GLOBK Tutorial 7

41. Miscellaneous commands
crt_opt, prt_opt, org_opt specify output options for screen, print and org files
glorg help gives all options, main ones are:
ERAS -- erase file before writing (normally files appended)
NOPR -- Do not write output
BLEN -- Baseline lengths
BRAT -- baseline rates when velocities estimated
31-Dec-18
GAMIT/GLOBK Tutorial 7

42. PRT_OPT, ORG_OPT options
VSUM -- Lat/long summary of velocity (needed to plot velocities)
PSUM -- Lat/long position summary
GDLF --Include list of hfiles and chi**2 increments from run
CMDS -- Echo globk command file into output file
Main use of these options is to keep output files small
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GAMIT/GLOBK Tutorial 7

43. Command file layout
In general, the commands in globk can appear in any order
They can be repeated with later version replacing earlier ones
e.g. apr_neu all
apr_neu iisc_gps
would use 10 m constraints on all sites, except IISC which would be 10 mm
The following commands must appear before others: com_file, srt_file, make_svs, eq_file
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GAMIT/GLOBK Tutorial 7

44. Summary Basics of Kalman filtering Inputs to globk Control of globk
Basic programs in the globk suite
31-Dec-18
GAMIT/GLOBK Tutorial 7