GLOBK Velocity and Coordinate Solutions

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 0.010 0.001 would force north velocity of iisc_gps to its apiori + 0.010 m/yr with sigma 0.001 mm/yr last 2 values are optional and default to 0 and 0 8-Apr-19 GAMIT/GLOBK Tutorial 10

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

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

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

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

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

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