1. GLOBK Velocity and Coordinate Solutions

2. Overview
In this lecture we look in more detail at defining coordinate systems for position and velocity solutions with globk/glorg
Defining coordinate system for velocities
Features of glorg
Other utility programs
8-Apr-19
GAMIT/GLOBK Tutorial 10

3. Coordinate system realization
To define frame requires origin, orientation and scale
Origin can be determined with one fixed site but any error in this site will translate to other stations
Orientation defined by EOP values but errors again will propagate into all sites
Scale may or may not be a problem. GPS should have well defined scale.
8-Apr-19
GAMIT/GLOBK Tutorial 10

4. General comments
When minimum constraints are used to define the frame, then baseline lengths and rates of change of baseline length are not affected
If the system is over constrained (ie. two or more sites fixed) then baselines will be affected.
8-Apr-19
GAMIT/GLOBK Tutorial 10

5. Velocity frame Similar arguments apply to velocity
Velocity of one site to fix translation rate
Rates of change of EOP for orientation rate
Scale rate -- Not clear if should exist
8-Apr-19
GAMIT/GLOBK Tutorial 10

6. Alternative to fixing a site
Alternative to fixing one site is to use glorg's generalized coordinate system definition
invoked with pos_org and rate_org commands in glorg command file.
pos_org sets parameters for position origin
pos_org <arguments>
XTRAN, YTRAN and ZTRAN for translation
XROT, YROT and ZROT for rotations
SCALE for scale factor
8-Apr-19
GAMIT/GLOBK Tutorial 10

7. pos_org rate_org commands
Only some need to be used; e.g., if orientation defined by constraining wobble and ut1 then use pos_org xtran ytran ztran
Same arguments are used for rate_org in which case these are rates
If apr_wob and apr_ut1 are tightly constrained then xrot yrot zrot should not be used
Algorithm used based on the assumption that system is free to translate and/or rotate
8-Apr-19
GAMIT/GLOBK Tutorial 10

8. org commands
Associated with the use of the pos_org and rate_org commands is the selection of sites to be used
If nothing is added to command all sites are selected.
To use as subset of sites use:
use_site clear
use_site [list of site names]
command may be repeated.
The same site list is used for pos and rate_org
8-Apr-19
GAMIT/GLOBK Tutorial 10

9. Stabilization commands
In addition to pos_xxx and use_site there are other commands
stab_ite [# iterations] [Site Relative weight] [n-sigma]
# iterations is number of iterations (default 2)
Site relative weight varies between 0 and 1; with 0 all sites given the same weight, 1 each site is given a weight according to its sigma (default 0.5)
n-sigma is editing (default 4.0)
Normally change to # iterations to 4
8-Apr-19
GAMIT/GLOBK Tutorial 10

10. STAB_MIN option
Allows user to set criteria for a site to be used in the stabilization in terms of its height sigma and deviation from other sites
Defaults work well and normally are not changed
Basic notion is that if the data for a site is poor on a given day then you don't want to use it in the coordinate system definition
8-Apr-19
GAMIT/GLOBK Tutorial 10

11. CND_HGTV Height weight
Commands allows the relative weight of heights to be used in the stabilization to be set
Arguments can be given for position and velocity
Default is 10, meaning heights get a weight approximately proportional to their relative variance
Setting a large value, removes heights from being used in system definitions
8-Apr-19
GAMIT/GLOBK Tutorial 10

12. Glorg system features
Glorg has additional commands that can be used to improve solutions
equate : Allows parameter adjustments for collocated sites to be equated
constrain : Allows equates with a finite constraint rather than absolute
eq_dist : Method for equating parameter adjustments for sites that are close together
force : forces or constrains parameters to specific values
8-Apr-19
GAMIT/GLOBK Tutorial 10

13. GLORG equate Forces parameter adjustments to be the same
Must be sure that a priori values are consistent (properly offset for positions; equal for velocitities)
Form
equate n1 n2 n3 n4
where nx would be parameter number or definition of the form iisc_gps ndot lhas_ndot would force iisc and lhas to have the same adjustment to their north velocities
any number of these commands can be issued.
Output gives chi**2 increments as equates are made (should be near unity)
8-Apr-19
GAMIT/GLOBK Tutorial 10

14. CONSTRA (ain) command
Similar to force except that a sigma can be added to constrain the equate rather than force it absolutely
Form
constrai <sigma> n1 n2 ..
where <sigma> is constraint sigma
n1 n2 .. are same form as equate
8-Apr-19
GAMIT/GLOBK Tutorial 10

15. GLORG eq_dist
eq_dist is an automated method of specifying equates based on distance between sites
Form
eq_dist <dist (m)> <equate type>
where dist is site separation
equate type is ndot, edot, udot, npos, epos, upos
Used to make velocities in a region all the same
Can be followed by un_equate commands to exempt some sites
8-Apr-19
GAMIT/GLOBK Tutorial 10

16. FORCE command
Allows parameter adjustments to be forced or constrained to specific values (similar to putting tight constraints in globk command file but applied at end of run
Form
force n1 value <sigma>
where n1 is parameter definition for example
force iisc_gps ndot
would force north velocity of iisc_gps to its apiori m/yr with sigma mm/yr
last 2 values are optional and default to 0 and 0
8-Apr-19
GAMIT/GLOBK Tutorial 10

17. Summary of glorg commands
See help for more detail $HELP_DIR/glorg.hlp
Specifies how coordinate system should be realized
pos_org, rate_org, stab_ite, stab_min, cnd_hgtv use_site
Allows parameters to be constrained and forced
equate, eq_dist, constra, force
Other commands to estimate plate rotation poles
8-Apr-19
GAMIT/GLOBK Tutorial 10

18. Program plate for plate motions
program plate allows velocities due to plate motion to be computed
plate <plate file> <Input .apr file> <output .apr file> \
[velocity file] [Reference frame]
where plate_file is a file containing gps_site plate_name
Input .apr file is globk apr file
Output .apr file is output file with velocities by plate motion velocity
velocity file is lat/long formatted plate velocities
reference frame default is NUV_NNR (Nuvel1A no net rotation)
8-Apr-19
GAMIT/GLOBK Tutorial 10

19. sh_make_plate
sh_make_plate automates making plate files but using lat/long to decide on which plate a site resides on (needs GMT)
sh_make_plate -apr <file> -plate <frame> -override <plate file>
8-Apr-19
GAMIT/GLOBK Tutorial 10

20. Changing frames Other utilities include
cvframe -- allows velocities files to be rotated to be difference frames. Later today we will generate velocities in the standard ITRF00 frame, but results in an Indian or Eurasia frame are more interesting
corcom -- allows are variety of tasks associated with comparing coordinate systems and can be used to change the reference frame an apr file.
8-Apr-19
GAMIT/GLOBK Tutorial 10

21. Explicit translation and scale
The stabilization in glorg assumes the system is free to translate and rotate.
apr_tran in globk commands can explicitly allow this
apr_scale in globk commands explicitly allows scaling and should always be used if scale is selected in the pos_org command
Not clear if apr_tran should be used (results vary)
8-Apr-19
GAMIT/GLOBK Tutorial 10

22. Summary Generalized coordinate system definition with glorg
Features of glorg: very flexible coordinate system definition and parameter manipulation at the end of runs
8-Apr-19
GAMIT/GLOBK Tutorial 10