Transcript PowerPoint Presentation - 12.215 Modern Navigation

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
16-Jul-15
GAMIT/GLOBK Tutorial 10
2
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.
16-Jul-15
GAMIT/GLOBK Tutorial 10
3
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.
16-Jul-15
GAMIT/GLOBK Tutorial 10
4
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
5
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
•
•
•
•
16-Jul-15
pos_org <arguments>
XTRAN, YTRAN and ZTRAN for translation
XROT, YROT and ZROT for rotations
SCALE for scale factor
GAMIT/GLOBK Tutorial 10
6
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
7
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
8
Stabilization commands
• In addition to pos_xxx and use_site there are other
commands
– stab_ite [# iterations] [Site Relative weight] [nsigma]
• # 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
16-Jul-15
GAMIT/GLOBK Tutorial 10
9
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
11
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
12
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)
16-Jul-15
GAMIT/GLOBK Tutorial 10
13
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
14
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
15
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
16
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
16-Jul-15
GAMIT/GLOBK Tutorial 10
17
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)
16-Jul-15
GAMIT/GLOBK Tutorial 10
18
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>
16-Jul-15
GAMIT/GLOBK Tutorial 10
19
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.
16-Jul-15
GAMIT/GLOBK Tutorial 10
20
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)
16-Jul-15
GAMIT/GLOBK Tutorial 10
21
Summary
• Generalized coordinate system definition with glorg
• Features of glorg: very flexible coordinate system
definition and parameter manipulation at the end of
runs
16-Jul-15
GAMIT/GLOBK Tutorial 10
22